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Deutsches CPC-Forum => Off-topic => Topic started by: tjohnson on 15:11, 06 April 20

Title: Good forums to help fix electronics?
Post by: tjohnson on 15:11, 06 April 20
Massively off topic for Amstrad but thought someone might know some good forums where I could seek help on fixing a double cassette deck.

The original issue I started to diagnose was the recording deck played fine but the recording would intermiitently distort on one channel.  I have access to a scope and observed the bias signal voltage on the affected channel changing very erratically.  However in my effort to diagnose I seem to have introduced a playback issue on that deck which plays back at massive amp and hugely distorted however it still records.  I think it may be in a multiplexer IC which takes the inputs for deck A and B then outputs them.  Deck A seems to play back more or less ok.  Same IC and outputs but seperate inputs.

I have the service manual and it has a very good circuit and block diagrams but with my little experience struggling to pinpoint this new fault but being a hobbist and complete amateur I'm very much trying to learn as I go along.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 16:45, 06 April 20
Send me the schematics and I'll see if I can help.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 23:56, 06 April 20
Hi Bryce, thanks for the offer of some help.

I've attached the main schematic and a block diagram.  The original issue appeared to be a bias issue, i hooked up the scope to capacitors C130 and C230 and could see on record the bias on the right channel C230 bouncing up and down alot in voltage.  Anyway I was trying to determine the cause of this when I then noticed the playback on deck B had gone completly distorted and pushing out massive amplification where previously it had worked fine.  I've attached a sound clip with playback on deck B and then playback on deck A which still seems to works.  I was suspecting an issue in the multiplexer IC301 on in that region of the circuit as playback A still works, however I'm really clutching at straws here and could really do with some guidance on where might be suitable places to check and understanding the circuit digram, it would be really appreciated.
Trevor
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 10:11, 07 April 20
Ok. Obviously the first culprit would be a bad electrolytic capacitor, but I assume you've already given the board a visual inspection. The next step would be to disconnect the heads and connect a signal generator to each channel at the head connector and compare the signals after the bias traps (L104 and L204) with a scope.

Bryce.
 
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 14:54, 07 April 20
I should say that playback on deck B was fine until I started probing around, I thought I was very careful not to short anything out but I'm not exactly sure when it went wrong but I think it was after I connected the scope up to the legs of C129 and C229.
I had previously checked playback by playing a sinewave 1000hz into the inputs, recording and then playing back.  I hooked the scope up to the legs of C135 and C235 to see the tone and it was fine on both deck A and B, now B is fubar.

On a further visual inspection of the board I noticed that the large 4700uf cap on the powerboard C909 has a small bulge in it.  I had previously dropped the board out and resoldered a few points that looked a little iffy, nothing else looks bad from what I can see.  Annoyingly I don't have a spare but could order one in, fingers crossed it arrives in all this madness.  Unfortunately I don't have a function generator and getting one at the moment could be a challenge!
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 17:38, 07 April 20
An MP3 player with a 10kHz sinewave recording can be used instead of a signal generator.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 17:51, 07 April 20
Thanks, I had that thought after I wrote my response.  Hopefully I'll be able update tomorrow on the results.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 01:49, 08 April 20
Sorry about the bullet point, the forum put everything into one big Paragraph.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 23:43, 08 April 20
 I have spent ages trying to work out what is happening and have now have an interesting update.
 
I then fed in the signal from deck A on one channel into the B connector CN041 on the board again and played from deck A.  I heard nothing but upon connecting deck B back up it now plays perfectly fine without any distortion but recording is now not working where it was before. 
 
 So the issue is now reversed, Deck B is now playing without distortion where it wasn't before but now doesn't records silence where it recorded before.
 
 The board has alot of ICs and a main processor, I can see in the block diagram something called record mute, I'm wondering if this is some sort of record muting issue.  I'm not quite sure how that's suppose to work but maybe it should mute on playback but not on record and wasn't muting before on playback so putting a high voltage signal in during the playback hence the massive distortion but is now stuck in mute.....atleast now I have more of a chance to tracking the issue down I think as I can monitor for the record signal....the mystery continues.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 10:58, 09 April 20
If this is an expensive piece of kit and you seriously want it back working (and both of those seem to be the case), then the first thing I'd do is replace every electrolytic capacitor. When the more complicated parts such as multiplexers, Op-AMP's, CPU's etc have issues, you usually loose an entire channel or the entire device is dead. Distortion means it's working, but something is outside the specs. On really old valve radios, this could also be a resistor drift issue, but not here, the only thing that goes bad are the electrolytics.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 13:25, 09 April 20
Its now a bit of a mission to fix it!  I think I'll start with the power supply board which has most of the big caps on and the large 4700uf which is on the 12v line shows some signs of bulging.
I did some more testing yesterday and after the rather unusual change in the issue where playback now works on deck B but record doesn't , i.e. the issue is reversed I can pickup the audio signal on the record path just before the pre-equalisation amp input (IC306) but not on the outputs.  On record I see the signal rise a bit approx 100mv when recording but there is no audio it stays flat.
I feel this is a bit like looking for a needle in a haystack with only half an eye open!
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 15:40, 09 April 20
How much ripple/noise is on the power rails? Observe them during the different functions.


Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 22:22, 09 April 20
I'll give that a try, just gotta work out how, looks like IC306 is driven off the +8v and -8v lines.  I'm wondering about TR317, that appears to be involved in muting record.  from the circuit diagram it show the base hooked up to GND, the emitter is on the rec mute from the  main processor and then the collector is hooked up to the base of 2 transistors in the rec output, so I'm assuming that this TR317 transistor is acting as a switch to mute the recording output via the two other transistors but I don't really know enough to fully understand how this circuit works.
Trevor
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 17:03, 16 April 20
not much of an update but I recapped the power board, no change, this thing still doesn't record.
My next area of investigation is here.  I could pickup the recording signal in the area before the pre-equalisation amp marked in green but not on the outputs.  There is a transistor TR317 involved in record muting and a multiplexer that sets equalisation depending on tape type so will look at these.
This is probably a question for bryce, on the transistor it is a DTA114TS (PNP) which appears to have a built in 10k resistor on the base, if measuring with DDM positive on the base then meeter OL when neg to Emitter and Collector but forward volatge in reverse?  I couldn't quite work out the datasheet if there is a resistor from base to emmitter too and whether this would affect the reading?
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 17:52, 16 April 20
Those are relatively difficult to measure. You may be able to test it with the diode tester on the multimeter, at least between Base and Collector (with black probe to base),  but the Base - Emitter diode won't be testable due to the series resistor. Anyway, those three transistors are the mute circuitry, so if you remove TR108 and TR208 the mute is removed and you should get a recording signal (if that was the issue).

Btw: I hope you've seen that C125 and C225 are bi-polar electrolytics! So if you've swapped them, you better have used BP Electrolytics too, otherwise you'll have built a smoke machine.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 17:56, 16 April 20
cheers I haven't changed those caps, I only did the power board.  I can't believe those caps have both failed all of a sudden together, good idea on trying to remove those transistors, I'll take them out and see if that makes a difference to recording.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 16:24, 17 April 20
I pulled transistors TR108 and TR208 and that has made no difference still recording silence.  Need to get the scope back out now and have another look at what is going on again.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 17:32, 17 April 20
Then the problem is most likely IC305 or one of the control signals going to that chip.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 20:35, 17 April 20
That is my concern  :o , I'm really hoping the issue is going to be IC307, that chip provides the parameters to IC305 so if anything is bust hopefully it's that chip as it seems to be an available part.  On IC307 the only control connected is the ground on pin 5.  All the other control pins appear to be NC on the circuit.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 20:42, 17 April 20
CXA = Sony part. so a pain in the arse to get in many circumstances. If that's fried it will be difficult to find, but I would be suprised if it was, it's not a high current or high load part, so why would it fail?

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 21:24, 17 April 20
Ok was checking IC307 and have found D308 has 8.2v on one side and 3.5v on the other which doesn't sound right at all I don't thnk it should be dropping over 4v.  On the negative side D307 I see -8v and -7.2 so the expected drop.   That lower measurements is on the cathode side, it feeds both IC306 and IC307.  Neither IC seems hot.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 02:47, 18 April 20
Oh man my head hurts.  Diode D308 had failed.  D307 was ok.  So I've replace them both anyway. now the awful noise on playback on deck B has returned, recording partially works but on left channel only, so I'm back to before but with the loss of one channel while recording.  Deck A is still playing.  As I say my head hurts time for bed!
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 10:18, 18 April 20
If D308 has failed, the IC307 most likely has an internal short circuit. Disconnect one end of D308 and measure the current. It's a low power multiplexer, it shouldn't pulling more than 300mA which is what the 1N4148 can handle.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 10:53, 18 April 20
Sleeping on it has helped I think.  I reckon D308 is a bit of a red herring.  Before I connected my phone to the header for deck B I could record both channels but play back was completely dissorted.  After doing so playback was fine but no recording.  D308 is on the left channel which is now recording again so it seems connecting my phone somehow blew the diode but now I've lost recording on right channel.  This probably explains why I'm now almost back to the original issue.  I definitely remenber checking the voltages across that diode before and they were fine.  I'm now back to thinking that muting transistor tr317 is the fault in that it's not muting when in playback or the control ic that sends the signal to mute.  No idea why right channel has gone, I wouldn't think a phone could put out the voltage or current to damage something but maybe it has since the head itself is just a coil isn't it?  So I'm going to try to replace the muting tr317, could I use a normal pnp with a 10k resistor soldered to the base, that is what the service manual suggests it is. I don't have this type and it seems no longer produced.
 
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 10:55, 18 April 20
But you already said you removed both muting transistors and that didn't change anything?

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 11:27, 18 April 20
I'm assuming that the transistors only mute when on, driven from tr317 so removing them allows recording but isn't muting the record circuit on playback distorting the audio so the record and playback signals are mixing together. I'm not sure what else it could be really.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 13:18, 18 April 20
I can see this particular transistor type is used elsewhere on the board so I think ill swap one over and see what happens.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 14:04, 18 April 20
D308 is only feeding that IC with power so the only thing that could damage the diode would be too much current = Short in the IC or reverse current which wouldn't do the IC any favours either, so you can be pretty sure, no matter what you swap, that the IC needs to be replaced aswell.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 03:24, 19 April 20
This is getting perculiar now.  So tonight yes at about 1am in the morning I swapped TR317 on the record mute for TR203 in the playback equalisation section.  On testing recording is now back on both channels and it plays fine on deck A still.  Testing Deck B I'm getting horrible distortion however I've also now found that on deck B if I play a silent section of the tape then it gets to a section of music it will play and i can hear it without distortion however if I start playback on a section of the music then the meters go off the scale and I get horrible distortion so perhaps I've been looking in the wrong place.  I checked both the transistors with a meter and they measured practically the same so I think the transistors I swapped around are both fine.  This is hurting my head although I'm kind of enjoying the detective work it would be nice to find the issue!  Time for bed.
Scratch that playing back from B seems intermittent and doesn't seem to relate to the silent section, tried it 5 times and it started ok on the fifth time.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 10:56, 19 April 20
Two of the things you said just there would point at a dry joint!

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 12:41, 19 April 20
Yeah I thought that too I've had a good look on the main board and had already soldered anything that looked suss to me but I haven't really checked the control board that the decks both plug into, where the buttons are and the where the main processor sits.  It works one minute then not the next.  Why is it now affecting playback on deck B only and intermittently.  More investigation required.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 12:44, 19 April 20
The fact that it "jumps" from one channel to the other, means that it's at a point that effects both channels. As buzzing is involved, I'd be looking at the ground points first.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 13:27, 19 April 20
New observation, I did my latest test with chrome tapes where I has been using normal before.  I seems to ocassionally work with chrome tapes but not at all with normal tapes.  For example I tried it 5 times with a chrome tape and it played 3 out 5 times ok, but not once with a normal tape.  When that diode had blown playback was working ok on normal tapes.  The diode supplies both IC306 (the Sony chip) and IC307 a 4051 multiplexer which is involved in setting parameters for IC306 based on tape type.  The fact that when both ICs were probably out of action due to the diode failing could point to either one of these chips but why is it affecting playback when this part of the circuit is involved in recording.  Or again maybe this is some red herring.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 13:45, 19 April 20
They may be involved with a completely different function, but they are all somehow connected together, so a dodgy chip could be pulling a signal low or injecting noise or interferance onto the signal.

Bryce.
 
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 14:17, 19 April 20
The other place were tape selection is involved in the HX Pro chip, see attached which is also involved in recording and this is actually where my original investigation was.  The original issue was on recording the right channel would distort and drop out then this issue around playback occured.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 03:01, 20 April 20
Another late one, so I dropped out the mainboard and the control board checked and reflowed alot of joints, nothing jumped out as dry and there is no change.  I have tried turning down the Recording Gain pots RT103 and RT203 just before that sony IC and when turned all the way down there is a buzz and very high pitch noise so that is obviously what the IC is amplifying, using my phone the high pitch nouse 14khz, I'll get the scope out tomorrow to have a better look.
Re. your early point about looking at ground points what do you mean?
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 10:28, 20 April 20
These type of systems usually have multiple PCBs and it important that all their grounds are at the same potential, so there will be multiple ground paths such the PCBs all having a screw bonding them to the chassis, plus a ground wire between PCBs etc. A problem with any of these could cause buzzing.

However, 14kHz is pretty specific. You need to find out what part of the circuit uses a 14kHz clock.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 11:12, 20 April 20
These type of systems usually have multiple PCBs and it important that all their grounds are at the same potential, so there will be multiple ground paths such the PCBs all having a screw bonding them to the chassis, plus a ground wire between PCBs etc. A problem with any of these could cause buzzing.

However, 14kHz is pretty specific. You need to find out what part of the circuit uses a 14kHz clock.

Bryce.
There are three PCBs, the control board with the display and processor on, the mainboard and the power board.  The powerboard and mainboard have a bonding with a screw to a metal plate which holds the face plate on.  The mainboard has 2 earth bonds to the metal casing although during testing disconnected because I've got the lid off.  There is no bonding wire for the display, this connects to the mainboard via some ribbon cables.  How would I check their potential are the same?
I'll get the scope out to try to check the frequency of the noise and try and locate it.  Is fixing electronic devices always this difficult?
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 11:25, 20 April 20
It's only difficult because: You don't have the experience yet, you don't have the required setup and because of that you are trying things that are normally unnecessary.

Normal process:
1 - Download the schematics and datasheets.
2 - Check the power rails and repair if necessary.
2 - Inject a known signal.
3 - Trace signal up to the point it disappears or gets noisy.
4 - Replace the faulty part.

The entire repair would normally take around 2 hours. Then you leave it running for a few hours to make sure the issue doesn't return.


Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 23:50, 20 April 20
I've had another quick look tonight, I setup my jerry-rigged mobile function generator cable again but I don't get the same distortion when using it.  I only tuned up the volume to the first notch and was to pickup the signal in the playback section.  Looking at the playback sections I've noticed there is A/B selection after the equalisation too and 70/120 in equalisation which I hadn't really seen before.  i feel part of my issue is being able to visualise how the circuit is working under different conditions.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 03:35, 24 April 20
I've had another looks tonight, I can only get the noise when the head is connected to the board and the noise appears everywhere in the playback circuit when it's happening.  What I have found and maybe you can advise @Bryce (https://www.cpcwiki.eu/forum/index.php?action=profile;u=225) is that the rec mute line isn't going fully 5v high when playing.   I measure just about 1.18v when playing and 0.1v when recording so all I can think of is that this is allowing feedback from the recording circuit to reach the head during playback of Deck B and be amplified but it can't reach Deck A and I'm assuming when the phone is connected there is no head to feed the noise back in.  Looking at the circuit this record mute appears to come straight off the main processor, I checked the voltage it at the pin of the connector and on resistor R663 and R315.  So if it's a fault in the main processor then I could well be stuffed unless there damage in the wiring loom which is causing the voltage drop.  I'm wondering how I can test this theory.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 09:52, 24 April 20
That's going to be quite difficult to test. Does the Processor have any sort of header for JTAG/SPI/RS232 ?

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 10:09, 24 April 20
No unfortunately it's soldered to a seperste daughter board on the face plate, without any kind of diagnostic capability.


Could I remove the pin from the plug then try and pickup 5v from somewhere else to inject into the main pcb? Would that be too  risky do you think?
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 10:45, 24 April 20
If you are sure that there's meant to be 5V there, then it shouldn't be an issue.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 10:49, 24 April 20
I'm not 100% sure it should be 5v but every other signal line is 5v from the processor and the service manual indicates high although I think it has an error in the manual as it says output High High, where I think it should say low high.  This is where having access to another unit to check would be very useful. Tbh I've got to the stage where I have nothing to lose as the deck is basically scrap if it doesn't work.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 12:01, 24 April 20
@Bryce (https://www.cpcwiki.eu/forum/index.php?action=profile;u=225) pulled the record mute signal line from the connector and this restores playback, recording now doesn't work.  the signal line is only measuring 1.18v high.  I'm now getting 3v on resistor R663 on the +8v supply  and and the same on R315 where I was getting the 1.18v.
So it does appear that this is the cause that the record mute is not working and feeding back into deck B.  I'm now trying to work out if it's a processor fault, and if there is a way I could generate a substitute signal.  The pins on the processor are tiny so trying to check at the pin would be virtually impossible maybe there is a pad on the daughterboard somewhere.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 12:50, 24 April 20
Hmmm there is a little defect on the pin where record mute is supplied, I think the next course of action is to hit that pin with the soldering iron.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 13:03, 24 April 20
Give it a go, but it doesn't look like a short and at the low currents involved, creepage and gap aren't going to be an issue either.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 15:08, 24 April 20
As you rightly guessed it didn't make a difference, it must be close to the edge as playing chrome tapes occasionally works.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 01:26, 03 May 20
HI @Bryce (https://www.cpcwiki.eu/forum/index.php?action=profile;u=225) this thing is driving me round the bend!! I'm not so sure it is this record mute section now.  It is now acting intermittently on play back with normal tapes where before it wouldn't play them at all.  I have noticed if I wobble the ceramic caps C104 / C204 that it can cause this noise to start.  I've replaced both caps and that hasn't made a difference.  Trying to probe any component in this section also.  Any ideas what this could mean?
Sorry just noticed this is in the German section!!!!
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 17:40, 03 May 20
Hast du die beiden Dioden schon getestet? (because we're in the German section :D )

Have you tested those two diodes yet?

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 21:59, 03 May 20
Yeah, I must have created it in the german area somehow.  The diodes test ok with my multimeter, have taken the fets out too and tested and they seem ok with a basic MM test.  The slightest touch on a ground point with the scope or meter can trigger the distortion.  I'm back to my theory that it's feedback on the muting circuit, that the circuit is not muting because the processor isn't putting out high enough.  I was thinking of ways to recreate the signal.  The record out signal is inverted, so it's low when playing and high when recording but the muting circuit is reversed.  Do you think I could use some sort of small logic gate IC to pickup this signal and split it, inverting it for the record mute and passing through for record out?  It must be able to pull it low somehow.  The thing I notice is that at the connector I pickup +5.1v high and -2v low, looking at the circuit diagram it seems to connect to the negative 8v rail via a 10k resistor.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 10:02, 04 May 20
A single NPN transistor and a resistor is all you need to invert a logic bit.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 17:13, 04 May 20
Hi Bryce, I found your old post (here (https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/getting-ready-signal-from-a-teac-fd-235hf-b391-floppy-drive/msg140032/#msg140032)) where you show how to create an inverter, my main concern is that if I pull the pin from the processor then measure the base of the transistors I get about 3v.  From the circuit diagram it looks like this is from the +8v line via R663 (4.7k).  however when I plug in the pin from the rpcessor and measure I get the following at the base 1.2v (high) or about 0.1v (low) so somehow overrides this.  Do you think this would work?
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 20:40, 04 May 20
Hi Bryce, I found your old post (here (https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/getting-ready-signal-from-a-teac-fd-235hf-b391-floppy-drive/msg140032/#msg140032)) where you show how to create an inverter, my main concern is that if I pull the pin from the processor then measure the base of the transistors I get about 3v.  From the circuit diagram it looks like this is from the +8v line via R663 (4.7k).  however when I plug in the pin from the rpcessor and measure I get the following at the base 1.2v (high) or about 0.1v (low) so somehow overrides this.  Do you think this would work?

Wait. Are you measuring the voltage on a pin that's not connected to anything? That's a floating voltage and could be anything. You only measure voltages that are in a circuit, because without a current, there's no knowing where it could be. This is why you also only measure a PSU output when it's connected to a load.

As far as whether the inverter will work: It's a Bipolar transistor not a MOSFET, it switches on current, not on voltage. If the current across Base and Emitter is 30mA or so, the transistor will be fully biased and will output a logic LOW. With anything less than that the transistor switches off and the resistor from the 5V to the collector will pull the line high.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 21:24, 04 May 20
As I say this is hurting my head!  I'm pretty sure I measuring in circuit which when the rec mute is plugged in I pickup circa 1.2v when in high state and effectively ground when low.  If I pull the pin out I'm still get voltage and drop across resistors and the mute function seems to work so I assume when the processor goes high it mutes and when it goes low it acts as ground and there is low current flowing to the muting transistor base to switch them on.

It seems to work intermittently so maybe this damn processor has taken a hit on this pin but not completly knocked it out.   I'm nervous of messing around with other parts of the circuit but trying to work out a bodge to get this thing working again.  I was thinking could I use a a NPN with the record mute output connected low side.  If the processor is low the transistor is off and it pulls the circuit to ground but if it's high (whatever is left of high) that would supply the current to switch the transistors on.  Another crazy idea I was having was whether to use an arduino to check the voltage at the pin and then switch a pin high or low if it detects. the voltage rise therefore replacing the processor output.

The other one was that a 15k resistor is used on the main muting line so maybe dropping this resistor to slightly would improve it.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 22:37, 04 May 20
I still think that the issue is either a PSU or a grounding issue. Especially as you said: "The slightest touch on a ground point with the scope or meter can trigger the distortion." ie: The scope (if it's a real scope) is grounding the otherwise floating ground. This definitely shouldn't happen, so you should be focused on finding out why it does happen.


Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 15:09, 05 May 20
I agree it does appear to could be something else and maybe I'm getting fixated on this muting circuit being the issue. 

Do you have any hints on finding grounding issues?  The only thing I've found on the PSU is that the 12v line is very high measuring approx 16v and 14.5v under load when dubbing but I don't think this is likely to be causing an issue as it's used on the motors and mechs which all seem to be working very well and it's not used on the main PCB board.

TBH I'll probably have to throw in the towel on this one, it would be nice to fix it but I'm spending way to much time on it going round in circles ,it's just nagging me now as I want to know what the issue is!
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 17:33, 05 May 20
Connect the crocodile clip of the scope to the best ground point you can find on the secondary side of the PSU. Then probe every other ground point ypou can find around the audio circuits. You are looking for any points that are at a different potential or display any sort of noise.

You should also measure all the voltage rails with your multimeter set to ACV and see if you get any high (above 50mV) readings. This will give you a good indication of the health of the capacitors.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 12:00, 09 May 20
Update on this Bryce, I kind of fixed this.  I couldn't find anything else obviously wrong using the method you suggested so as a last ditch attempt I purchased a £2.50 ATtiny85 micro controller board off eBay, set it up to read the voltage from the main processor and set a pin high when it read over 1v and otherwise low.  Wired it into to the 5v on the main pcb for power, removed the mute line from the socket and connected to to the Tiny85 and then fed the new high low signal from the Tiny85 onto the main pcb and it now works perfectly recording and playing.  So it does appear to have been that muting circuit.  Strange I don't know why the mute line is failing to go fully high and put out 5v there must be a fault inside the processor.  I also managed to fix the original issue I was investigating by replacing a couple of variable resistors.  So it looks likes my tape deck is now useable again, how long it lasts is anyones guess.  Thanks for your help !
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 12:55, 09 May 20
It sounds like the CPU has open-collector outputs and the pull-up resistor associated with that signal has gone open circuit. Is there a resistor anywhere between that line you now control with the ATTiny and to the 5V rail? Is the resistor ok and if so, is there really 5V on the rail at the point the resistor connects?

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 13:26, 09 May 20
I've attached a picture of this part of the circuit.  There is no connection to the 5v rail in the main circuit.  The record mute signal controls 2 sets of transitors.  2 NPN on the input to the record amp and a PNP which then controls 2 NPN on the output of the record amp.
At the top right of the picture this is connected to the +8v rail via a 4.7k resistor.  There is also 15k resistor prior to the 2 input muting transistors which after the 4.7k resisitor in the circuit.  Both these resistor are ok.

At the moment the tiny85 is replacing the high/low signal that would have come from the processor, I've connected the output to the pin on the connector.  When the processor line was connected in the circuit wasn't muting the majority of the time.  When I removed this pin from the connector the circuit was permanently muted so wouldn't record but played ok.  It was working very intermittently with say 9 times out of 10 not muting.
Title: Re: Good forums to help fix electronics?
Post by: Bryce on 20:16, 09 May 20
Unless there are dry joints on the 4K7 resistor or a crack in the PCB trace, it really could be internal damage in the CPU.

Bryce.
Title: Re: Good forums to help fix electronics?
Post by: tjohnson on 22:54, 09 May 20
Unless there are dry joints on the 4K7 resistor or a crack in the PCB trace, it really could be internal damage in the CPU.

Bryce.
I think it's internal damage on the CPU, I've pretty much reflowed the whole board including that resistor.  I didn't see any cracks but I know they can be tiny so I may have missed them.  I have soaked tested my fix and it seems to work and is stable so I think i'll have to stop there.  I've spent inordinate amount of time on this already!  The deck sounds surprisingly good really, tape was never a great format and considering most people scorn these twin decks it makes good recordings.  I think the control board is a little cheap, it has hard soldered connections with hot glue on them, but the  rest seems well made and being single sided is very easy work work on.