Good forums to help fix electronics?

[tjohnson]

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.

[tjohnson]

I can see this particular transistor type is used elsewhere on the board so I think ill swap one over and see what happens.

[Bryce]

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.

[tjohnson]

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.

[Bryce]

Two of the things you said just there would point at a dry joint!

Bryce.

[tjohnson]

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.

[Bryce]

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.

[tjohnson]

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.

[Bryce]

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.

[tjohnson]

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.

[tjohnson]

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?

[Bryce]

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.

[tjohnson]

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?

[Bryce]

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.

[tjohnson]

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.

[tjohnson]

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 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.

[Bryce]

That's going to be quite difficult to test. Does the Processor have any sort of header for JTAG/SPI/RS232 ?

Bryce.

[tjohnson]

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?

[Bryce]

If you are sure that there's meant to be 5V there, then it shouldn't be an issue.

Bryce.

[tjohnson]

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.

[tjohnson]

@Bryce 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.

[tjohnson]

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.

[Bryce]

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.

[tjohnson]

As you rightly guessed it didn't make a difference, it must be close to the edge as playing chrome tapes occasionally works.

[tjohnson]

HI @Bryce 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!!!!