Commodore 1084S-D2: right sound channel is dead

[wurzel42]

Ok, so I have got this nice 1084S from Commodore (240V/50Hz region). And it works fine except for two things: the power switch was broken, and the right channel/speaker only gives my a quiet hum (50Hz?).

I repaired the switch myself, by replacing it with a fresh, new one. I tested the continuity of the speaker cables, of the sockets in the little breakout module, and even tested the speakers via a rigged cable with my iPad. Both speakers work fine on their own. So it must be something in between the input and the amplified(?) output before the breakout board.

I found a service manual for the P1 variant, but not the D2. Also, I am somewhat experienced in working on low voltage stuff, but I have never worked on CRTs. I watched a ton of videos, and now basic safety rules, but would consider asking a professional for help. The question is more: where to start, and how to analyze this problem?

And are there any people here who know who in Germany could help with such repairs? I could go to my local TV dealer, but I doubt they still do such repairs or if they will do a good job...

[Manul]

If you have quiet hum, it means that there is connection between speaker and amplifier. Also it should mean that amplifier is working. Hum on the output usually means that input of amplifier is not connected (floating) so it is picking up noise. Look for the most likely failure points - RCA socket and volume control pot, because they are likely to be damaged by mechanical stress. It may be a cracked solder joint.

[wurzel42]

That would be rather good news, if it's only that. The solder joints on the RCA jacks are pretty massive, but who knows. I can simply touch them up a bit, to make sure they are ok, and I can measure their resistance by following them a bit around the board... I have to check the volume pot. I would have assumed I get some crackling at the least.

[Manul]

This is schematic of the 1084S-D (older?), but there is good chance, that it will be similar. Is it similar? If yes, you can use multimeter to check the input path to amplifier (TDA1013) using this schematic for orientation.

[wurzel42]

Cool, thanks for the schematics. Yeah, I think it should be very similar. The RCA jacks seem to be the same, I guess the circuit won't differ much. I will check that later.

[wurzel42]

Ok, here are front and back of the amplifier section. It looks similar to the schematics at least. Haven't traced anything yet.

[wurzel42]

Ok, first trace: the right channel signal goes from the RCA to the header marked BN on the bottom of the board. From there a cable runs to the front of the PCB, which is obscured. I can't disassemble this without discharging the tube, so I won't do that for now -- don't have a screwdriver and jumper cable to do that properly...

But I could probe through the case grating and continuity on that cable is good. It arrives at the correct header. But from there I can't really probe further without disassembly.

I will try to work backwards from the amp.

However I reflowed the RCA joints and sucked away some solder. There were massive, round blobs of solder. Not very nice...

[wurzel42]

Going from I8011 (the right amp) to C8021 has good continuity. The cap also seems fine, is not shorted, not bulged. Looks good.

That goes further off to the breakout board. No problem there either. Continuity right to the socket.

So, next will be the path between the TDA1013 and the RCA input, assuming that the track from the pot is fine, since I can't reach that without disassembly.

[wurzel42]

So all the resistors that are around I8011 check out. A few deviations, but I assume this is down to the revision that I have. Nothing shorted, nothing open.

However when I measured R8011, it was indeed 47kOhm, while measuring from the RCA across R8011 I got almost 100kOhm! So I sprayed a bit of t6 spray into all RCA jacks, waited a few minutes and measured again. Now it's down to the 47kOhm. Maybe it was simply corroded RCA jacks all along...? I will put the case on and try it one more time, to see if now the right channel is working again. That would be too good!

[Manul]

Yes, from central pin of RCA you should measure 0 ohms to one side of R8011 and 47K to another side. It is not too good, contacts and mechanics fail often.

[wurzel42]

Well, I guess the RCA jacks were a bit corroded. But you know what else was corroded? MY BRAIN. Watch closely the attached image. See the colors?

Also, it all works fine now: https://youtu.be/ttP3x4LhbyE

[Haenk]

See the colors?

I think that happened more than once to me