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General Category => Amstrad CPC hardware => Topic started by: Cwiiis on 17:44, 20 August 21

Title: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 17:44, 20 August 21
I'm posting in the hope that someone has successfully done this, or knows how to go about it. I want to stream development from my Plus and still use the original monitor. I happen to have a Plus -> SCART adapter and a SCART -> CM14 adapter and going straight through works fine, as you might expect (the cables are passive, so this seems like an obvious outcome). Unfortunately, the signal coming out of the Amstrad is really far out of spec it seems... I got a SCART distribution amplifier from JS Technology (https://www.js-technology.com/store/index.php?id_product=59&controller=product) but the Amstrad monitor doesn't like the signal coming out of it. Blues are much darker, white screens go grey and bright screens start losing sync. I ran through several steps debugging it with them and at the end of it, though I've made some mods to the amplifier that make the situation a little better (sync is better, white is white), it's still not good enough (blue is still almost black, sync still starts skewing on bright screens).

So, has anyone managed to successfully duplicate video output from a Plus? Ideally, I'd want one output to go to a CM14 and then another output (really of any kind, analog or digital) that I can use with my capture equipment. If anyone knows what kind of modification or circuit would allow me to do this, I'd be extremely grateful!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 21:52, 20 August 21
There are quite a few schematics online for active VGA splitters. Most of the circuits will work fine to duplicate the CPC output for 2 or more monitors.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 00:58, 21 August 21
So, ignoring the ID pins and figuring something out for audio, I suppose this would work? It seemed that the impedance of the RGB channels was 100 ohms on the Amstrad rather than 75, would the resistance values need to differ?
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 04:00, 21 August 21
Yes correct, the RGB needs to be terminated by 100 Ohms for correct Colour "Mid Level Colour Intensity".

Attached is a modified Schematic, with two styles of RGB buffers - (one using diodes and the other using transistors connected as diodes) - the later offering better buffered Voltage offset accuracy.

Also, a slightly differant circuit for the Video Sync buffer circuit..


I've not tested the circuit but should be ok... (hopefully).



Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 21:37, 21 August 21
Quote from: RetroCPC on 04:00, 21 August 21
Yes correct, the RGB needs to be terminated by 100 Ohms for correct Colour "Mid Level Colour Intensity".

Attached is a modified Schematic, with two styles of RGB buffers - (one using diodes and the other using transistors connected as diodes) - the later offering better buffered Voltage offset accuracy.

Also, a slightly differant circuit for the Video Sync buffer circuit..


I've not tested the circuit but should be ok... (hopefully).

Expanding your circuit, I have this so far (I've added the luma line, but I suppose that's not necessary for a CM14). I was wondering why you have a different circuit for the sync buffer, and if you have any ideas about the audio? I'm looking at audio splitter circuits and they're all surprisingly complicated... Most seem to use a particular opamp and require 12V input... Ideally, I'd like to keep to just 5V input and using basic components... Any suggestions?
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 00:35, 22 August 21
Hi Chris,


I'd also add some High Frequency decoupling on the 5V Power Rail to Ground.... say 4x 100nF ceramic (spread between the buffer stages) + say 100uF Electrylitic "Bulk" Capacitor.


I tired to keep the Sync Signal output impedance the same as the CPC output (after buffering) - it might not be needed, but as I have no moniter to test, I wanted to play safe.


Audio is more difficult as you require signals swinging around 0V - (symetrical Voltage swing around Zero volts). Obvusally you can AC couple - but with 5V rails you really require Rail to Rail circuitry... Sure it can be done, but my heart aches to design something of poor quality....

Give me 30 minutes and I'll design something semi-decent for you....
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 01:57, 22 August 21

Hi Chris,

Heres a descrete Output Audio Buffer, you will need one "Pair" for each audio channel (L/R)

It might be too much for your needs, but will work well, and cheap components... however you will have fun designing the PCB...

Clean 5V power would be very helpful, as Power supply rejection is rather limited (40dB at 50Hz, 66dB at 1KHz & 92dB @ 20KHz)... if its a problem, then C6 can be increased to improve the noise rejection...

The Electrolytic Capacitors need to be 6.3V or greater...
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 21:23, 22 August 21
Quote from: RetroCPC on 01:57, 22 August 21
Hi Chris,

Heres a descrete Output Audio Buffer, you will need one "Pair" for each audio channel (L/R)

It might be too much for your needs, but will work well, and cheap components... however you will have fun designing the PCB...

Clean 5V power would be very helpful, as Power supply rejection is rather limited (40dB at 50Hz, 66dB at 1KHz & 92dB @ 20KHz)... if its a problem, then C6 can be increased to improve the noise rejection...

The Electrolytic Capacitors need to be 6.3V or greater...

Seems the audio part of this equations is oddly much more complicated than the video part :) I've attached the circuit I have so far, but I'm not sure I really understood your suggestion with the decoupling capacitors - I have the power input smoothing capacitor there (100u seems high?), but I don't know that I've put the 4 decoupling capacitors you suggested in the right place... That said, that's the only place where there are 4 components that match up, so maybe?

Re the audio, I found this nice, simple audio splitter circuit: https://www.learningelectronics.net/circuits/laptop-audio-out-splitter-circuit.html - it uses a couple of TDA2822M ICs, but I think the trade-off is very much worth it... I suppose using this circuit, we could replace P1/P2 with resistors (though not sure what value... I suppose I could find this out by using a potentiometer and comparing with straight-through, or perhaps just providing voltage to it and seeing what comes out?)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 21:43, 22 August 21
Video signals are ground referenced and vary between 0V up to 2 or 3V. Audio signals have the ground reference in the middle, ie: they vary from a +V to -V which makes the electronics slightly more complicated. RetroCPC's circuit looks complicated because it's a purely discrete circuit, ie: made up of transistors and passive parts. If this is too complicated, you could always use the circuit that Amstrad used for the internal audio, however, RetroCPC'S circuit will give you a much (much, much....) better audio quality than the Amstrad circuit.

Bryce. 
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 23:44, 22 August 21
Quote from: Cwiiis on 21:23, 22 August 21I've attached the circuit I have so far, but I'm not sure I really understood your suggestion with the decoupling capacitors - I have the power input smoothing capacitor there (100u seems high?), but I don't know that I've put the 4 decoupling capacitors you suggested in the right place... That said, that's the only place where there are 4 components that match up, so maybe?

Chris,

As you suspected, the 100nF capacitors I mentioed are in the incorrect location on your schematic, they need to be connected between the +5V rail and Ground - the same as you have the 100uF Cap connected.

I was imagining them evenly spread across the PCB between sets of the video buffer transistors. There function is to offer high frequency PSU decoupling... I'm sure the circuit would work without them, but better with them.

The TDA2822M circuit you linked to is a mini power amplifer (to drive small speakers - think those cheap USB powered speakers) and headphones rather then a audio splitter for sure it could still work as a splitter... (but you dont really want any Gain) - and typically power amplifers have 25dB to 30dB Gain (the TDA2822M has 39dB fixed Gain!!)...

You mnentioned you wanted to use "using basic components" so I tock this as being "basic" discrete components...
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 00:48, 23 August 21
Quote from: RetroCPC on 23:44, 22 August 21
Chris,

As you suspected, the 100nF capacitors I mentioed are in the incorrect location on your schematic, they need to be connected between the +5V rail and Ground - the same as you have the 100uF Cap connected.

I was imagining them evenly spread across the PCB between sets of the video buffer transistors. There function is to offer high frequency PSU decoupling... I'm sure the circuit would work without them, but better with them.

The TDA2822M circuit you linked to is a mini power amplifer (to drive small speakers - think those cheap USB powered speakers) and headphones rather then a audio splitter for sure it could still work as a splitter... (but you dont really want any Gain) - and typically power amplifers have 25dB to 30dB Gain (the TDA2822M has 39dB fixed Gain!!)...

You mnentioned you wanted to use "using basic components" so I tock this as being "basic" discrete components...

Ah, I think I might understand what you mean with the capacitors - I've moved them to the video lines coming off Q2, Q4, Q6 and Q8.

Your assumption of what I meant was correct, I just didn't imagine it to end up being quite so densely populated :) I suppose this is what it takes though. Please do excuse my naivete, hardware/electronics really isn't my field... I can't seem to find a simple IC that takes a line-level audio input and gives 2 line-level audio outputs (assuming the Amstrad is even line level? Or somewhere close?), which would be the ideal (well, 2 in 4 out would be the ideal I suppose), so I'll go ahead and integrate your design. I'll test the video circuit first as I have most of those parts already, the audio may have to wait a bit.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 01:48, 23 August 21
Hi Chris,


Can you forward your updated schematic just to confirm you have the capacitors connected correctly, they should be connected between +5V and Gnd - not to any of the Video / Sync signals.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 03:00, 23 August 21
Quote from: RetroCPC on 01:48, 23 August 21
Hi Chris,


Can you forward your updated schematic just to confirm you have the capacitors connected correctly, they should be connected between +5V and Gnd - not to any of the Video / Sync signals.

I had it one way, but I think maybe I realise what you were saying now and have it like this. Close?
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 03:27, 23 August 21
Hi Chris,


Yes, looks good now - you can drop C6 as you have plenty of decoupling caps now :)

I'm kind of hoping you will use Ground plane :) (Copper Fill for the GND signal)


If you use skype then I can talk you though the PCB layout - I'd do it for you but I'm fighting my own PCB Layout ATM.... or I should say more correctly its fighting me a WINNING!!!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 13:10, 23 August 21
Quote from: RetroCPC on 03:27, 23 August 21
Hi Chris,


Yes, looks good now - you can drop C6 as you have plenty of decoupling caps now :)

I'm kind of hoping you will use Ground plane :) (Copper Fill for the GND signal)


If you use skype then I can talk you though the PCB layout - I'd do it for you but I'm fighting my own PCB Layout ATM.... or I should say more correctly its fighting me a WINNING!!!

I very much appreciate the offer! I'm going to test on breadboard first, see if the concept will work - if it does, maybe I'll make a video-only version (well, one output will have audio!) on veroboard or something. I've done PCB layout before, so I can give it a go, but I'll definitely be consulting with you if you have the time :)

I like your audio circuit, but I'm considering 4xLME49721 as buffers, mainly to make layout a bit easier. Either way, first things first, I'll think about audio when video is working. Missing parts should be arriving tomorrow, so I'll update once I get the time to try it out. Thanks for all your help so far!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 20:35, 31 August 21
I've tested the circuit out on breadboard now (took a while to get the parts I didn't have) and it appears to work! Just thought I'd update here in case this it's useful for anyone else - I'm going to go ahead and design a PCB now. I haven't gotten the parts to test audio splitting with the opamp yet, so I'm going to rough out a circuit and if it's wrong I can just break out audio separately (or patch it through temporarily to just one of the outputs). I'll update again once I have a design.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 13:09, 01 September 21
Quote from: RetroCPC on 03:27, 23 August 21Hi Chris, Yes, looks good now - you can drop C6 as you have plenty of decoupling caps now :) I'm kind of hoping you will use Ground plane :) (Copper Fill for the GND signal) If you use skype then I can talk you though the PCB layout - I'd do it for you but I'm fighting my own PCB Layout ATM.... or I should say more correctly its fighting me a WINNING!!!

Attached is the updated circuit with the audio splitter and preliminary PCB layout.I've used a ground plane, but only on the top layer as everything is through-hole except for the two op-amps which are soldered on the top of the board. This could be a much smaller board but I figure it's nice to have a bit of space to work with and possibly do any necessary bodging :) Any thoughts/feedback? I'm very uncertain of the values around the audio circuitry, but I guess that's what I get by diverging from your design!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 13:55, 01 September 21
Quote from: Cwiiis on 13:09, 01 September 21
Attached is the updated circuit with the audio splitter and preliminary PCB layout.I've used a ground plane, but only on the top layer as everything is through-hole except for the two op-amps which are soldered on the top of the board. This could be a much smaller board but I figure it's nice to have a bit of space to work with and possibly do any necessary bodging :) Any thoughts/feedback? I'm very uncertain of the values around the audio circuitry, but I guess that's what I get by diverging from your design!

Looks good, but PCB space costs money, you could easily half the size and still have room for bodges etc.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 15:22, 01 September 21
Quote from: Bryce on 13:55, 01 September 21Looks good, but PCB space costs money, you could easily half the size and still have room for bodges etc. Bryce.
You're right - I've reduced the empty space somewhat to bring it down to an outline of 126x90mm. It could still be much smaller, but I think this is a reasonable compromise - I'm optimising more for putting it together than I am for size/cost, I can't imagine there are too many people out there looking to split their analog Plus output :)

If I was planning on making a few, or I wanted to make a miniature/cheaper version, I'd replace all of these components with their 0603 surface mount equivalents and have the board be barely bigger than the connectors - but I'll leave that to someone else for now!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 15:56, 01 September 21
You could also put dual footprints, for the equally low number of people who would want to split VGA signals?  :D

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 05:16, 02 September 21
Quote from: Cwiiis on 13:09, 01 September 21Any thoughts/feedback? I'm very uncertain of the values around the audio circuitry, but I guess that's what I get by diverging from your design!

Hi Chris,

You need to AC couple and bias the input of the Opamp buffers, I've attached a quick schematic (you need to power the opamp from the 5V rails - the Power supply and Ground connections to the opamp are not indicated on my schematic).

I've also added R1, R9, R11, R7, R8, R10 (these need to be located as close as possible to the opamp) - there function is to insure the opamp remains stable (does not oscillate) and also some input RF filtering (C4).

I'd Ground fill both layers - and liberally add Vias spread around the PCB to connect the Top and Bottom Ground planes...
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 11:49, 02 September 21
Quote from: RetroCPC on 05:16, 02 September 21
Hi Chris,

You need to AC couple and bias the input of the Opamp buffers, I've attached a quick schematic (you need to power the opamp from the 5V rails - the Power supply and Ground connections to the opamp are not indicated on my schematic).

I've also added R1, R9, R11, R7, R8, R10 (these need to be located as close as possible to the opamp) - there function is to insure the opamp remains stable (does not oscillate) and also some input RF filtering (C4).

I'd Ground fill both layers - and liberally add Vias spread around the PCB to connect the Top and Bottom Ground planes...


Thanks again, this kind of circuit is definitely beyond me right now... I guess the extra resistors essentially drive the signal? Is C3 definitely the right way round? It looks like it would just block input, but maybe there's something I don't understand there... Obviously, any explanation is a huge courtesy, but I'd very much appreciate it :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 12:33, 02 September 21
Chris,

Your power connector is rotated the wrong way around - the PCB mounting pads are at the rear of the connector (not the front) - as you have it currently, the power input (Hole) is facing towards the center of the PCB - not the edge of the board....

The Opamp needs to be biased to half of your PSU supply (so 2.5V) - ideadly the audio input should swing about 0V (but the Amstrad is not AC coupled) - anyway, the Capacitor C3 has 2.5Vdc on the Opamp side (biased by R2/R3/R5 - with C1 offering some PSU noise filtering) and hopefully something less (idealy swinging about 0V) on the Audio input side...

The circuit uses the same LME49721 you use - but with a few extra components :) I can confirm your PCB layout if you give it a try.


Also, I'd move your Audio input signals away from the Video pin connector as the you will pickup a "Buzz/ hum" from the Sync signals / Video data...
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 12:49, 02 September 21
Quote from: RetroCPC on 12:33, 02 September 21
Chris,

Your power connector is rotated the wrong way around - the PCB mounting pads are at the rear of the connector (not the front) - as you have it currently, the power input (Hole) is facing towards the center of the PCB - not the edge of the board....

The Opamp needs to be biased to half of your PSU supply (so 2.5V) - ideadly the audio input should swing about 0V (but the Amstrad is not AC coupled) - anyway, the Capacitor C3 has 2.5Vdc on the Opamp side (biased by R2/R3/R5 - with C1 offering some PSU noise filtering) and hopefully something less (idealy swinging about 0V) on the Audio input side...

The circuit uses the same LME49721 you use - but with a few extra components :) I can confirm your PCB layout if you give it a try.


Also, I'd move your Audio input signals away from the Video pin connector as the you will pickup a "Buzz/ hum" from the Sync signals / Video data...


God, I can't believe I didn't notice the power connector, thanks for that!  :picard:




Thanks for the explanation and more tips, I'm revising the schematic and PCB now, I'll update again with the next revision - and hopefully the correct one :) Re the video pins causing audio interference, I suppose the individual cables coming off the DIN connector ought to be shielded in that case?
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 13:35, 02 September 21
Quote from: RetroCPC on 05:16, 02 September 21
Hi Chris,

You need to AC couple and bias the input of the Opamp buffers, I've attached a quick schematic (you need to power the opamp from the 5V rails - the Power supply and Ground connections to the opamp are not indicated on my schematic).

I've also added R1, R9, R11, R7, R8, R10 (these need to be located as close as possible to the opamp) - there function is to insure the opamp remains stable (does not oscillate) and also some input RF filtering (C4).

I'd Ground fill both layers - and liberally add Vias spread around the PCB to connect the Top and Bottom Ground planes...


Something I'm uncertain about in this circuit (amongst other things :)) is how to integrate C5 and C6 with the larger circuit - I suppose this is a combination of decoupling/smoothing(?), but I already have C1 (though it's a much smaller value) for smoothing and C10 and C11 for decoupling, and I'm not sure how to go about duplicating this for two sources, or its intention. I've attached what I have so far, which I think is the whole circuit minus C5 and C6 from your diagram and with power input/grounds for the opamps with decoupling on C10 and C11.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 13:44, 02 September 21
The 100µf C1 and RetroCPC's C6 perform the same function. The position on the layout is good too, so just decide which value you want to use (I think 100µf would be sufficient). The smaller C5 should be in parallel to your C1 and also close to the power socket.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 15:20, 02 September 21
Chris,

To make your life easier, (and if you cared about interchannel Crosstalk - not so much an issue with the "Lo-Fi" CPC audio quality...)  then I suggest having one Dual opamp for the Left Channel and the second Dual Oamp for the Right Channel - this will make your Schematic, PCB layout & Life :) much easier.

When you layout the PCB, you want the PCB tracks between (R22,R30,R31,R23) & (R24, R25, R32, R35) and there connections paths to there respective +in / -in on the opamp as as short as possible (so the shortest PCB track from this end of the resistors to the opamp (+in / -in inputs).

"C6 postion" on my schematic can be place in parrellel with your C11 & C10...

WRT the potentional "Hum / Buzz" issue - I was refering to the fact you have the Left Audio input physically next to the Video Sync pin on the input header, as they are so physically close to each other, the high impedance audio input from the CPC could pick up the Sync signal which is physically one pin over (which could result in Hum / Buzz)... So my suggestion is to have the Audio inputs on a seperate pin header, physically spaced away from the Video pin header...

Also, good practice to layout the Audio input header "Left input, Ground, Right input"- with the center Ground pin now offering some isolation between the Left & Right channels (improving interchannel Crosstalk).

A lot of this is "Too much quality" for the CPC, but its more about learning - if your going to the effort :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 15:52, 02 September 21
Yup, you're giving some really excellent best practice audio design tips, but for a very Lo-Fi signal source :D It's a bit like giving tips on what 8K 75in OLED TV would be best for watching VHS video on. But it's all good information which can (and should) be used on "real" audio projects.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 18:49, 02 September 21
Quote from: RetroCPC on 15:20, 02 September 21Chris,


To make your life easier, (and if you cared about interchannel Crosstalk - not so much an issue with the "Lo-Fi" CPC audio quality...)  then I suggest having one Dual opamp for the Left Channel and the second Dual Oamp for the Right Channel - this will make your Schematic, PCB layout & Life :) much easier.


When you layout the PCB, you want the PCB tracks between (R22,R30,R31,R23) & (R24, R25, R32, R35) and there connections paths to there respective +in / -in on the opamp as as short as possible (so the shortest PCB track from this end of the resistors to the opamp (+in / -in inputs).


"C6 postion" on my schematic can be place in parrellel with your C11 & C10...


WRT the potentional "Hum / Buzz" issue - I was refering to the fact you have the Left Audio input physically next to the Video Sync pin on the input header, as they are so physically close to each other, the high impedance audio input from the CPC could pick up the Sync signal which is physically one pin over (which could result in Hum / Buzz)... So my suggestion is to have the Audio inputs on a seperate pin header, physically spaced away from the Video pin header...


Also, good practice to layout the Audio input header "Left input, Ground, Right input"- with the center Ground pin now offering some isolation between the Left & Right channels (improving interchannel Crosstalk).


A lot of this is "Too much quality" for the CPC, but its more about learning - if your going to the effort :)

Ok, here we go, let's hope this is a revision good enough to go to the printers! I'll wait for feedback, as it seems pretty likely I may have made mistakes... I've attached the updated circuit diagram and PCB layouts. My previous layout really didn't have enough room left for all the audio circuitry, so I started from scratch and its ended up quite a bit smaller (90x108mm now). Once again, thanks for all of this guidance, there is zero chance I could have done any of this without help...
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 07:25, 03 September 21
Chris,

WOW your quick (That's what she said)... to spin a PCB! your welcome to came and help me out with my PCB designs!!!

Almost there:

Shield should be connected to Ground - I'd just just bringing the Ground plane all the way to the bottom of the PCB (on both layer) - also, your Video input header should have a Video Ground signal - so you will have two Grounds coming from the CPC - one for Audio, and one for Video... (is the CPC+ Shield connected to Ground on the CPC side? I'm presuming it is...)

Also, I'd add some PCB vias evenly distributed across the PCB to tie the top and bottom Ground planes together for lowest Ground plane impedance - see attached image you can see the vias evenly spread around the Solid Ground plane to connect to different Ground layers  (Top / Bottom in your case).

Also, for shortest PCB track from Resistor Pad to Opamp +IN pad, I'd move resistors R30, R31, R32, R33 upwards towards the Top of the PCB, this will result in the short connection track to the +IN's on the Opamp.

Its always a good idea to add a HF bypass capacitor as close as possible to your power input jack - so I'd add an extra 100n Ceramic capacitor in the space in-between the DC input Jack and C1.

Last point, I'd thicken the PCB tracks where possible - with the power tracks as thick as you can go without going stupid (I typically use 40th to 100th for "thinner" power tracks) - with signal tracks from 10th to 30th depending (30th for audio / 10th for Video) :) thin PCB tracks are easy to break - ESPICALLY to connector pads - and larger unsupported stand up components (such as the transistors) - its common practice to use "tear-droping" to go from say a connector pad to a thinner PCB signal track (such as the video signal lines) - attached is an image of a pcb teardrop... (You just want to added extra Copper support to the thinner PCB tracks from component Pads that can be stressed).

My PCB software is rather old and does not automatically create teardrops - so l just manually step down the PCB track as it progresses from the component pad e.g.:

Connecter pad is 80th diameter, so I start from the connector pad with a short length of 80th PCB track (to create the "teardrop"), and then step down to say 10th track (for signals such as video) for the "proper track" that goes along the PCB...

Dont forget to add your name to your PCB :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 12:35, 03 September 21
Quote from: RetroCPC on 07:25, 03 September 21
Chris,

WOW your quick (That's what she said)... to spin a PCB! your welcome to came and help me out with my PCB designs!!!

Almost there:

Shield should be connected to Ground - I'd just just bringing the Ground plane all the way to the bottom of the PCB (on both layer) - also, your Video input header should have a Video Ground signal - so you will have two Grounds coming from the CPC - one for Audio, and one for Video... (is the CPC+ Shield connected to Ground on the CPC side? I'm presuming it is...)

Also, I'd add some PCB vias evenly distributed across the PCB to tie the top and bottom Ground planes together for lowest Ground plane impedance - see attached image you can see the vias evenly spread around the Solid Ground plane to connect to different Ground layers  (Top / Bottom in your case).

Also, for shortest PCB track from Resistor Pad to Opamp +IN pad, I'd move resistors R30, R31, R32, R33 upwards towards the Top of the PCB, this will result in the short connection track to the +IN's on the Opamp.

Its always a good idea to add a HF bypass capacitor as close as possible to your power input jack - so I'd add an extra 100n Ceramic capacitor in the space in-between the DC input Jack and C1.

Last point, I'd thicken the PCB tracks where possible - with the power tracks as thick as you can go without going stupid (I typically use 40th to 100th for "thinner" power tracks) - with signal tracks from 10th to 30th depending (30th for audio / 10th for Video) :) thin PCB tracks are easy to break - ESPICALLY to connector pads - and larger unsupported stand up components (such as the transistors) - its common practice to use "tear-droping" to go from say a connector pad to a thinner PCB signal track (such as the video signal lines) - attached is an image of a pcb teardrop... (You just want to added extra Copper support to the thinner PCB tracks from component Pads that can be stressed).

My PCB software is rather old and does not automatically create teardrops - so l just manually step down the PCB track as it progresses from the component pad e.g.:

Connecter pad is 80th diameter, so I start from the connector pad with a short length of 80th PCB track (to create the "teardrop"), and then step down to say 10th track (for signals such as video) for the "proper track" that goes along the PCB...

Dont forget to add your name to your PCB :)
Thanks, and so close now! Unfortunately I'm out for the day, but I got a little bit of time this morning to start on some of those changes - attaching a screenshot in case there was any misunderstanding; all my traces were previously 10mil, I've now added a bypass cap to the power input and bumped up those traces to 30mil, after C1 all power lines are now 20mil and I've started making all the audio traces 20mil too. When I get home later I'll finish off and teardrop all the through-hole pads, as well as add a bunch of vias :)

Re the shield/ground, I've tested a few cables and machines in the past and none seem to connect them to the same ground source... I'll avoid that in this board, with the presumption that if either side ties them together then that will do (unless you think otherwise?) - I'll double-check this specifically on the Plus when I get back later too :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 22:34, 03 September 21
And hopefully done! All the suggested changes made I think, bar the shield/ground tie. I'll let it cool off for a day or so, so any comments, please do let me know, but otherwise I'll take this to the printers and cross my fingers :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 06:45, 04 September 21
Chris,

:) Its getting there (have I said this before)? ...

You added extra Vias :) , but they are isolated from the Grounds - so apart from adding extra holes to the PCB they currently serve no useful function :)

Also, for High Frequency signals, its recommended to avoid 90Deg sharp track angles - also avoid unnecessary track "details / undulations" - some examples:

The PCB track from the Emitter of Q6 to R10 has both (90Deg angles + some weird "pip undulation" on the track) - not good for HF signal intergrity nor my OCD...

The PCB track from the Emitter of Q10 to R17 could avoid the 90Deg transition if you just continued the 45Deg section.

The Transistor Base connections (middle pin) could just be run though the first transistor to the second transistor Base pad - no need for the "twisted" track length to the second transistor Base's....

I'd also move the whole transistor block (with input termination Resistors) down the PCB - as close as possible to the output DIN sockets - this reduces the PCB track lengths (Video is High Frequency signals).

Also, rotate R1, R2, R3, R4, R13 - so that the Resistor end that connects to the transistor Base is as close as possible to the transistor Base - also the Ground end of the Resistor is now as near as possible to input Ground (Best for signal termination).

Last point, if you move the Via closer to the Opamp Power pin (Pin 8) - this will allow a wider ground path under both Opamps - at the moment, the Ground path flowing under the Opamp is unnecessary restricted by the Power Via on Opamp Pin 8 placed so far away from the pin / pad.

When I design PCB's, I always tray to move tracks to minimise Ground plane restriction if possible (Such as keeping the Ground plane as wide as possible under the Opamp).

I'm still freaked out that there is only one Input Ground connection (shared fro both Video and Audio)... This would trigger my OCD and prevent me from sleeping at night!!! I'm dont recall if you mentioned you already have done this, but just confirm if the DIN shell is not connected to Ground at the CPC side.. if it is connected, then no need to seperate them on the Buffer PCB.

Personnaly, I would connect them together and have full Ground fill... The more solid Low impedance Ground - the better...


You make a better PCB designer then I could ever be a Musician - I'm so envious!!  :P
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: pelrun on 07:43, 04 September 21
The 90 degree track thing is a complete myth; it has nothing to do with EMI and everything to do with manufacturing process issues from decades ago that no longer apply. If you're etching your own boards it might help, but if you're getting a proper fab to make them it's irrelevant.
https://resources.altium.com/p/pcb-routing-angle-myths-45-degree-angle-versus-90-degree-angle
I'm still going to use them but only for aesthetic reasons :D
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 11:13, 04 September 21
For sure too much information for a our humble CPC designs, but as Chris is teaching himself PCB design (and doing very well), I'm just offering advice from my own 40+ years of experance of designing electronics & pcbs (I admittelty suffer from OCD).. :)

WRT the Altuim artical, I have to disagree as its very easy to see the reflections of 90Deg PCB traces with a TDR system - I'm afraid the Altuim article is too simplistic and therefore incorrect.

Using a TDR system, any variation along the signal path can clearly been seen, with a sharp 90Deg PCB edge being really messy.... A decent engineer never wants to needlessly compromise there design - so after ones first encounter with the effects observed on a TDR system - avoiding 90 Deg PCB angles becomes matter of fact...

In my lab I use TDR heads on extender cables to bring the TDR sampling system as close as possible to the PCB to aid measurement (probing) fidelity - when those idiots at Altuim show respective TDR measurements then things will become apparent to them.

Do 90deg tracks work - for sure yes, BUT they demonstrably impact HF signal integrity (confirmed by anyone using TDR) so WHY use them if they can be avoided?

Also - anyone who has to worked with Altuim PCB software will attest, Altuim software is truly terrible to use, PCB software designed by software engineers not working PCB designers !!! I have several Altuim perpetual license's as sometimes I have to work with Altuim files, but NOBODY in the lab wants to work with it - I'd rather shoot myself in the head, so I have laugh when I see such article's from Altuim!

As a side note, when I design High speed PCB's I embed TDR test tracks - where I place a rectangle kink (2x 90Deg transition) in the open-ended track length - (with a normally unpopulated SMA connector footprint at one end of the track to allow a controlled connection to the TDR system when needed for QC) - using this imbedded TDR test track(s) allows me to Quality control multilayer PCBs (inner layer stackup, PCB dielectric etc.)

I've attached a picture a 20GHz TDR head I use to measure signal integrity of the critical PCB tracks...
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 12:28, 04 September 21
Quote from: RetroCPC on 06:45, 04 September 21
Chris,

:) Its getting there (have I said this before)? ...

You added extra Vias :) , but they are isolated from the Grounds - so apart from adding extra holes to the PCB they currently serve no useful function :)

Also, for High Frequency signals, its recommended to avoid 90Deg sharp track angles - also avoid unnecessary track "details / undulations" - some examples:

The PCB track from the Emitter of Q6 to R10 has both (90Deg angles + some weird "pip undulation" on the track) - not good for HF signal intergrity nor my OCD...

The PCB track from the Emitter of Q10 to R17 could avoid the 90Deg transition if you just continued the 45Deg section.

The Transistor Base connections (middle pin) could just be run though the first transistor to the second transistor Base pad - no need for the "twisted" track length to the second transistor Base's....

I'd also move the whole transistor block (with input termination Resistors) down the PCB - as close as possible to the output DIN sockets - this reduces the PCB track lengths (Video is High Frequency signals).

Also, rotate R1, R2, R3, R4, R13 - so that the Resistor end that connects to the transistor Base is as close as possible to the transistor Base - also the Ground end of the Resistor is now as near as possible to input Ground (Best for signal termination).

Last point, if you move the Via closer to the Opamp Power pin (Pin 8) - this will allow a wider ground path under both Opamps - at the moment, the Ground path flowing under the Opamp is unnecessary restricted by the Power Via on Opamp Pin 8 placed so far away from the pin / pad.

When I design PCB's, I always tray to move tracks to minimise Ground plane restriction if possible (Such as keeping the Ground plane as wide as possible under the Opamp).

I'm still freaked out that there is only one Input Ground connection (shared fro both Video and Audio)... This would trigger my OCD and prevent me from sleeping at night!!! I'm dont recall if you mentioned you already have done this, but just confirm if the DIN shell is not connected to Ground at the CPC side.. if it is connected, then no need to seperate them on the Buffer PCB.

Personnaly, I would connect them together and have full Ground fill... The more solid Low impedance Ground - the better...


You make a better PCB designer then I could ever be a Musician - I'm so envious!!  :P
You might surprise yourself with the music, much like PCB design, everyone should give it a try! :)

So I've rejigged all the transistor routing/positions - I think that was one of the first bits I worked on, so it definitely needed a revisit - that weird routing instead of just connecting the centres was a hangover from when they were all in a row I think, and some of the wonky tracks and 90 degree angles similarly were side-effects from moving components around. I'm going to look a bit more carefully at the other routes as well in case there are more obvious deficiencies - of course, every change is getting more and more awkward at this point, so I might call it a day soon!

Re the vias, I guess I'm having a hard time seeing where the ground planes get separated - I suppose given it's mostly through-hole parts, every resistor or capacitor ground connection is also acting as a connection between planes? Maybe there's a tool for this that I've not found that'll make it trivial...

So I just tested sheild/ground on the Plus and I'm going to have to eat my words - they are connected. So I guess I can just treat the shield as another ground input and tie it to the ground planes. I feel a little weird doing it as I feel the splitter should act like a cable in some respect, but if it's tied at the Amstrad anyway, may as well take advantage of it. I'm going to open an Amstrad up and triple-check this later...
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 13:25, 04 September 21
Quote from: pelrun on 07:43, 04 September 21
The 90 degree track thing is a complete myth; it has nothing to do with EMI and everything to do with manufacturing process issues from decades ago that no longer apply. If you're etching your own boards it might help, but if you're getting a proper fab to make them it's irrelevant.
https://resources.altium.com/p/pcb-routing-angle-myths-45-degree-angle-versus-90-degree-angle (https://resources.altium.com/p/pcb-routing-angle-myths-45-degree-angle-versus-90-degree-angle)
I'm still going to use them but only for aesthetic reasons :D

Not quite correct. Sharp points on the edge of tracks act as tiny antennas, however, this only happens when you get up to Gigahertz frequencies with high energy. This can be easily demonstrated with PCB trace spark gaps (great fun to experiment with). But for 99% of circuits it's totally irrelevant.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: pelrun on 13:35, 04 September 21
The exceptions are in the linked article, but nobody is making a 10GHz design without actually knowing what they're doing. Not if they want it to work, anyway.

Quote from: RetroCPC on 11:13, 04 September 21WRT the Altuim artical, I have to disagree as its very easy to see the reflections of 90Deg PCB traces with a TDR system - I'm afraid the Altuim article is too simplistic and therefore incorrect. Using a TDR system, any variation along the signal path can clearly been seen, with a sharp 90Deg PCB edge being really messy....
Your TDR is necessarily throwing exactly the sort of ultra high frequency pulses down the line that 90 degree bends dislike to detect issues, but that doesn't mean the circuit itself is doing it in normal operation. After all, the TDR is intended for those high-end applications as well, but it's overkill for a low frequency one.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 13:59, 04 September 21
pelrun,

People often confuse "operating frequency" with signal frequency content.

While the CPC operates at 4MHz (with a 16MHz Oscillator) - the typical edge speeds on the digital IC's are about 4nS - which equals 250MHz (and don't forget we are talking a squarewave - so signal content much MUCH higher then "4MHz" would suggest). With such edges speeds, you should really take care with characteristic track impedances etc... However, the CPC was design pre EMC days, when digital design was little understood in the consumer world - the introduction of EMC regulations forced designers to take more care with correct signal termination / Ground planes etc. - A cheap "fix" was to Series terminate digital lines with 33ohms to 100ohm resistors at the source...

With 4nS edge speeds, things get messy quickly when incorrectly terminated... and yes, YOU can see the "Ripple" caused by a 90Deg PCB bend even with a 500MHz scope (with correct probing), and things just get even uglier using a faster scope!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: pelrun on 14:12, 04 September 21
And we're back to "I can see it when I use high-frequency test equipment". It's still something that will *not* affect the operation of *this* device. We're also not talking about trying to meet commercial radiative EMI standards, which are extremely rigorous and also extremely expensive, hardly something that's going to be required here.
Again, if you're working on a device that *actually* requires proper certification or operates at frequencies where these things actually matter, then there's a hell of a lot more to worry about than just the angle of the trace corners.
(we can argue the point and who has more real-world RF EE experience but it's really not worth the bother. For this design and for almost anything we're likely to see that's CPC related, 45 degree corners are entirely an aesthetic choice.)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 15:08, 04 September 21
Now now, I think RetroCPC has been pretty clear - at least, as I understand it, while this may not affect this particular board, it is an issue that should be considered in more cases than you might be lead to believe and is good practice to consider, especially for someone like me that's learning. If it's a good habit, why not get into it now when the stakes are a bit lower and have fewer problems in the future?

Anyway, I think this is probably close to the final revision now as I'm getting a bit sick of looking at this  :D

I've moved the transistors down the board and flipped the terminating resistors - I've also re-routed most of those lines, so the video lines are shorter and have fewer kinks and no 90 degree turns. I also made the power line going to the audio section a bit shorter. I noticed that C2 and C4 were offset vertically, so moved those a little so that the traces are shorter and straighter and they're in line with C3 and C5 (I think this must've just been an oversight).

Shield is now tied to ground, so there are two grounds coming in at the top and 2-4 at the bottom, depending on the outputs connected.

I've removed the pointless vias - I'm pretty sure both copper pour layers are continuous (I can't see any disconnected sections) and given the number of through-hole ground pads, I don't understand where adding any vias would make sense. I'll definitely add some if someone can explain that to me though :)

I think at this point, this is probably good enough, but again I'll let it cool for a day in case anyone spots anything terrible, or anything easily implemented that's worth addressing now rather than in a later revision.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 20:24, 04 September 21
Quote from: pelrun on 14:12, 04 September 21
And we're back to "I can see it when I use high-frequency test equipment". It's still something that will *not* affect the operation of *this* device. We're also not talking about trying to meet commercial radiative EMI standards, which are extremely rigorous and also extremely expensive, hardly something that's going to be required here.
Again, if you're working on a device that *actually* requires proper certification or operates at frequencies where these things actually matter, then there's a hell of a lot more to worry about than just the angle of the trace corners.
(we can argue the point and who has more real-world RF EE experience but it's really not worth the bother. For this design and for almost anything we're likely to see that's CPC related, 45 degree corners are entirely an aesthetic choice.)

Of course. All this discussion is just theory and completely unnecessary for the circuit in question, but for someone who's learning, it can be an interesting discussion. Even a hobbiest can end up getting into technical issues/trouble when you consider the harmonics that a modern devboard like an RPi can produce on its clock signal. Yes, RetroCPC is gold-plating the design, but it is important "best practice" methods that one should learn.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 23:14, 10 September 21
Ok, update; I had the PCBs made and got them today - I had enough parts already to populate the video section, making it functionally the same as what I had on the breadboard. Good news: it works! Bad news: there's fringing and a red tint (visible even when the Amstrad is off but the monitor is on and plugged into the splitter) so I expect some of my soldering may have not been up to par or there's an issue with the design I missed. Neither issue were present on the breadboard. It isn't really visible in the photo, but it's very visible in real life.


The only circuit difference between this PCB and my breadboard prototype at this point are the added decoupling capacitors (I only tested with a smoothing cap on the breadboard). I also powered the breadboard via USB rather than barrel jack, but I don't see why that'd make a difference... I reckon either a ground short somewhere or a poor ground connection, based on how I felt about the soldering, but if you guys have any specific ideas of what to look for, I'm all ears :)

Re the design, if I was going to spin another board, there are two things I'd change:
- the input may as well be a DIN8 port - This would make the soldering a bit easier and you can buy DIN8-DIN8 cables, so it'd save making a custom one up. I don't know why I didn't think of this before  :picard: As it is, the cable will be integrated.
- The transistor footprints should have the pins spread out more... They're way too close together and I wouldn't be surprised if I had a short or two somewhere there... I should've checked the footprint more carefully, maybe I'd have noticed before this point. Lesson learned!

Otherwise, it went surprisingly well! Here's hoping I can fix that image degradation and then I'll design an enclosure for it.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 02:30, 11 September 21
Found the mistake - R1 which is meant to be a 100 ohm resistor on the red line got mixed up at some point and is a 10K resistor - so that'll be that! Will fix tomorrow 🙂
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 12:58, 11 September 21
Wow that was Quick - and great to see professional quality PCBs :) Very nice work :)

Lets us know if the resistor swap resolves the issue :) Then you can be brave and try the Audio section :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 13:46, 11 September 21
Quote from: RetroCPC on 12:58, 11 September 21Wow that was Quick - and great to see professional quality PCBs :) Very nice work :) Lets us know if the resistor swap resolves the issue :) Then you can be brave and try the Audio section :)
So yes, I'd mixed up R1 (100 ohm) with R13 (10k). A not-so-quick (jeez, that ground plane really sinks heat...) swap and the video portion of the board performs perfectly! Even with the temporary cable I've put on that I used with the breadboard, the picture quality is bascially as good as plugging straight in - glad I followed all your advice :)

Still waiting for the opamps to be delivered, hopefully they'll come in the next week or two, then I'll put together that portion of the board and report back - fingers crossed! I dare say this is the first Amstrad Plus video amplifier in the world  :P I'll have to do a celebratory live-stream once it's finished!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 14:09, 11 September 21
Yeh - pretty sure its the first (and only) CPC video splitter :)  I admire your determination and the end results  :)


Just a little worried about the Audio opamps input Bias current, you might need to change a couple of resistor values in the "Virtual Ground" circuit to compensate if it proves an issue... nothing complicated.

When you have the Audio section built, then measure the DC voltage on the opamps outputs and confirm its about 2.5V (before the output capacitors). It should be half the input power voltage so 5V/2.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 14:21, 11 September 21
I've a CPC video splitter, but it's only video, no audio and it was made by (ugly) modding a VGA splitter, not an original design.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 21:23, 18 September 21
Quote from: RetroCPC on 14:09, 11 September 21
Yeh - pretty sure its the first (and only) CPC video splitter :)  I admire your determination and the end results  :)


Just a little worried about the Audio opamps input Bias current, you might need to change a couple of resistor values in the "Virtual Ground" circuit to compensate if it proves an issue... nothing complicated.

When you have the Audio section built, then measure the DC voltage on the opamps outputs and confirm its about 2.5V (before the output capacitors). It should be half the input power voltage so 5V/2.
Just completed the board (got the chips in earlier this week and then discovered I was 1 capacitor and 1 resistor short, bloody typical! Just got those in this evening :)) - measuring the opamp outputs, it is indeed 2.5V (well, I measure 2.46, but this is on a 5V power supply under load, so I assume that's a normal reading). Is this what's expected? After your comments, I'm a bit cautious to just plug the monitor straight in, so I'll wait for your confirmation first in case!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 23:15, 18 September 21
2.4XV is fine :) Try the Audio :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 00:40, 19 September 21
Quote from: RetroCPC on 23:15, 18 September 21
2.4XV is fine :) Try the Audio :)
Working perfectly  :D Will update once I've designed and printed a suitable enclosure!
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 01:30, 19 September 21
Feeling around on the board, I notice the transistors get a little warm (I wouldn't say hot, so I guess that's fine), but resistors R5-R12 get very hot to touch, possibly more so on the right side unless I'm imagining it, but all are what I'd call 'hot'. I can hold my fingers on them, but it's uncomfortable (my fingers are extremely calloused, so it's a bit hard to tell exactly how hot - possibly it'd be more uncomfortable for someone with more normal hands!)

Is this ok? I don't really understand why those resistors are what they are, given that I haven't changed their values vs. the VGA version of the circuit... Perhaps 1/4W resistors aren't enough? Or should I have raised their values? I don't really understand their relationship to the monitor's resistance, but if when the monitor was 75 ohms they were 100, should I also add some headroom and switch them out for 125 ohms or similar? Given that they're pull-up resistors, I'd be concerned about them failing, turning into shorts and putting 5V straight through, I expect that wouldn't result in anything good happening :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 02:09, 19 September 21
So measuring the voltage and current across one of those resistors, it's about 4V and 500mA (rounding down a bit), which I suppose is 2W... Which is way, way higher than 1/4W... Should I be looking for a problem elsewhere, or do I just need to use resistors with a higher rating?

For now, I'm trusting the fact that it works and what my multimeter is telling me and I'm going to switch them out with 100 ohm 5W resistors on Monday. If I think that the max draw will be 600mA at 5V, that would be 3W, so 5W should be plenty of headroom.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: RetroCPC on 04:17, 19 September 21
Chris,

You (hopefully should) have 5V PSU rail so even worst case 5V / 100 (ohms) = 50mA

50mA *5V = 250mW (This is worst case as you will not see the full 5V swing on the transistor Emitters).

Just confirm you have 5V PSU input NOT higher (such as 12V).
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 15:08, 19 September 21
Quote from: RetroCPC on 04:17, 19 September 21
Chris,

You (hopfully should) have 5V PSU rail so even worst case 5V / 100 (ohms) = 50mA

50mA *5V = 250mW (This is worst case as you will not see the full 5V swing on the transistor Emitters).

Just confirm you have 5V PSU input NOT higher (such as 12V).
I'm pretty perplexed now... I double-checked everything. Input voltage is 5.11V, the first time I checked the resistors, I was getting the same results as before - about 4V and about 470mA... Now I checked again and I still get that 4V, but consistently reading 0.62mA! They still get hot, but I suppose those figures are well within operating parameters. Maybe time to check the battery on the multimeter...


I may replace them with higher-rated resistors anyway, I don't like the idea of anything being that hot for an extended period of time unnecessarily. A shame I didn't think to put both a power switch+light (though this is easily hacked on) and some kind of cut-off if the output isn't connected (less easy?) But I tend to switch this setup off at the plug anyway, so not too big a deal.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 15:27, 25 September 21
I think we can call this done now :) I replaced the 1/4W resistors with 1/2W ones - no values change and they still get hot, but not as hot as before, so hopefully that'll be ok. Thanks a lot everyone, especially RetroCPC for helping with this - I'm extremely pleased with the outcome!

If I was going to do a revision, these are the things I'd change:
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 16:22, 25 September 21
Great little project, but I still think there is a problem or mistake somewhere, those resistors definitely shouldn't be getting hot.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 16:55, 25 September 21
Quote from: Bryce on 16:22, 25 September 21
Great little project, but I still think there is a problem or mistake somewhere, those resistors definitely shouldn't be getting hot.

Bryce.
mm, quite possible/likely even, but I'll be darned if I can see where/what it is! Hoping the 1/2W resistors will do the job well enough...
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 19:21, 25 September 21
The problem is, as long as you haven't found the issue, the device is using a lot more current than it should.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 19:32, 25 September 21
Quote from: Bryce on 19:21, 25 September 21
The problem is, as long as you haven't found the issue, the device is using a lot more current than it should.

Bryce.
Indeed. My best guess is that R5-R12 are actually too low in value. Looking at the datasheet for the transistors, their suggested circuits have both lower voltage and considerably higher resistance, so presumably much lower current. I don't know what the relationship with the current at this point in the circuit is with what is necessary for the output though, so I'm loathe to just swap them out without better knowledge of the effects.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 20:11, 25 September 21
Quote from: Cwiiis on 19:32, 25 September 21
Indeed. My best guess is that R5-R12 are actually too low in value. Looking at the datasheet for the transistors, their suggested circuits have both lower voltage and considerably higher resistance, so presumably much lower current. I don't know what the relationship with the current at this point in the circuit is with what is necessary for the output though, so I'm loathe to just swap them out without better knowledge of the effects.

Actually, that's one of the most fundamental calculations in electronics - Ohms Law: V=IxR, so the voltage drop across the component divided by the resistance determines the current that will flow. The watts dissipated is calculated with P=IxV. So if it's getting too hot (too many watts) you need to lower the voltage (not possible in this case) or increase the resistance which will lower the current.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 01:43, 26 September 21
Quote from: Bryce on 20:11, 25 September 21
Actually, that's one of the most fundamental calculations in electronics - Ohms Law: V=IxR, so the voltage drop across the component divided by the resistance determines the current that will flow. The watts dissipated is calculated with P=IxV. So if it's getting too hot (too many watts) you need to lower the voltage (not possible in this case) or increase the resistance which will lower the current.

Bryce.
I understand that, but I wonder why the original VGA splitter circuit, which seems to be replicated all over the web, chooses 100 ohms at this point... Does the current just need to be that high, or could I substitute them all with 250ohm resistors (or more) and expect everything to work ok? I would just try it out, but I'm a little wary to modify something that works and has things attached that I really don't want to break :)
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Bryce on 10:11, 28 September 21
Quote from: Cwiiis on 01:43, 26 September 21
I understand that, but I wonder why the original VGA splitter circuit, which seems to be replicated all over the web, chooses 100 ohms at this point... Does the current just need to be that high, or could I substitute them all with 250ohm resistors (or more) and expect everything to work ok? I would just try it out, but I'm a little wary to modify something that works and has things attached that I really don't want to break :)

Because it was designed for the inputs of a VGA Monitor, not a CTM. Increasing the value to 220R or 270R (250 is not a standard resistor value) will not damage anything because it's lowering the current, not increasing it. I would definitely increase the value to 270 if they are getting warm.

Bryce.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 12:41, 28 September 21
Quote from: Bryce on 10:11, 28 September 21
Because it was designed for the inputs of a VGA Monitor, not a CTM. Increasing the value to 220R or 270R (250 is not a standard resistor value) will not damage anything because it's lowering the current, not increasing it. I would definitely increase the value to 270 if they are getting warm.

Bryce.
Ah, so I now see the part on the Arnold V Specs wiki that says "The new monitor must present an input impedance of 100 ohms to 0V, and accept an analogue input current of 0-10mA for each gun" so indeed 100 ohms is way too low... If we plan for 5V and 10mA, that would mean 500 ohms - So I suppose something in the range of 300-400 ohms (taking into account voltage dropping) ought to be fine. I'll see what I have and change them out soon.
Title: Re: 6128 Plus video output distribution amplifier (splitting)
Post by: Cwiiis on 15:44, 28 September 21
Replaced them with 330 ohm resistors (the next up I had was 470, which may or may not be skirting the line...) and that did the trick :) The transistors remain cool to the touch now and the resistors seem to warm up a tiny bit if there's no monitor connected, but were cool to the touch after running through a few tests with everything connected.
I've attached a zip of everything needed to build one of these, along with all the source (Schematic/PCB/Case all included). I can't imagine there are too many people in the market for one of these, but at least this makes it easier :) I also have 3 spare PCBs if anyone wants them for the price of postage or want to swap for something - bear in mind that R5-R12 have the incorrect resistance marked on the board (this is updated in the source).
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