6128 Plus video output distribution amplifier (splitting)

[Cwiiis]

2.4XV is fine Try the Audio

Working perfectly  Will update once I've designed and printed a suitable enclosure!

[Cwiiis]

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

[Cwiiis]

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.

[RetroCPC]

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).

[Cwiiis]

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.

[Cwiiis]

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:

   
• DIN8 socket for input - this would avoid the need to make a custom cable and also remove what is likely to be a point of failure, if this ever fails.

   
• Bigger and more spaced pads for the transistors. It was fine soldering them, but I'd put the difficulty level at relatively fine-pitch surface mount, which obviously defeats the point of using through-hole entirely.

   
• Better design for heat dissipation around the transistor driving resistors. These get hot, even with 1/2W resistors. I think giving them a little more space, but putting them nearer to the power input and widening the power tracks even further would help here. Possibly the resistors should stand up too - the current design can't really accommodate anything bigger than 1/2W.

   
• Power switch and power LED.

   
• What I intended to use for cable relief ended up just being locating holes - I'd relocate one to the other corner of the board for more accurate alignment.

   
• Bigger pads for all ground connections, and better thermal relief - these were a pain to solder and only the larger pads ended up with semi-decent joints. Maybe just a more shaped ground fill to make soldering a bit easier too.

   
• Headphone input?

   
• Replace all through-hole components with surface-mount and make it considerably smaller? I'm happy with the size, but it's bigger than it needs to be for sure.

[Bryce]

Great little project, but I still think there is a problem or mistake somewhere, those resistors definitely shouldn't be getting hot.

Bryce.

[Cwiiis]

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...

[Bryce]

The problem is, as long as you haven't found the issue, the device is using a lot more current than it should.

Bryce.

[Cwiiis]

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.

[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.

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.

[Cwiiis]

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

[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

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.

[Cwiiis]

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.

[Cwiiis]

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).