Advice needed for vintage amp repair: one channel distortion

[Leizhg]

Newbie seeking advice here - solute to the experts.

I recently got into vintage amps and my first one was a 1972 Panasonic RE-7412, a little Am/FM receiver/amp - just love its mid-century look!

The amp worked well in general, with some hum and static noises. To address those issues, I started recap the baby. It went well in the beginning with noticable improvement of sound quality with the fist few new caps that I put in, but unfortunately turned south at some point after more caps were replaced - the sound of the left channel became completely distorted and weak.

I tried to troubleshoot by viewing output of 1000Hz sine wave passing through the amp with oscilloscope. The output that I measured directly from the Audio Power Amplifier IC's (NEC upc20c) pin shows distortion from the bad channel comparing to perfect sine wave from the good one. See images attached.

At this point, can I be certain its the IC that turned bad and should be replaced (I've purchased replacement chip)? Or could it be some other cause?

The schematic of the amp is also attached.


Thanks for your input in advance!

[Pixel]

I would look at the caps you replaced in the bad channel to see if any are backwards, poorly soldered, or have solder whiskers making unintended connections. If that seems ok then use your oscilloscope to trace the signal along its path from input to output to see where it went wrong. Depending on where you got the caps from its also possible one of the new ones could have been bad to start with too.

[vintagekiki]

Before any intervention, measure and compare DC voltages on IC3 and IC4 and on TR6 and TR7.

[Leizhg]

Appreciate the quick response!

Pixel, I removed all caps around the bad channel that I put in, measured their capacitance before putting them back, the problem persists. It could be my terrible soldering skill - can't visually tell though. BTW, I didn't touch any of the ceramic caps, could these gone bad too?

vintagekiki, thanks for the advice! Following are measures:
IC3 (bad channel):
1: 0.2v 2: 1.16v 3: 1.075v 4: 3.79v 5: started at 1.71v, gradually reduced to 0.44v in 2 minutes 6: 1.16v 7: 8.22v
8: 9.46v 9: 15.7v 10: 15.2v 11: 4.5v 12: 5v 13: 1.82v 14: 0.84v
IC4:
1: 0.2v 2: 1.14v 3: 1.05v 4: 3.64v 5: 1.59v 6: 0.93v 7: 7.3v
8: 8.47v 9: 15.7v 10: 15.05v 11: 4.33v 12: 4.83v 13: 1.8v 14: 0.83v

TR6: B: 1.25v C: 4.84v E: 0.75v
TR7: B: 1.23v C: 4.89v E: 0.73v

Measures from TR6 & TR7 are pretty consistent so I assume that's good news.

There are quite some different readings between IC3 and IC4. Though I'm not sure what range of the difference is considered abnormal, it's immediately obvious that pin 5 of IC3 (from the bad channel) behaves weirdly. The reading started at 1.71v when the mutimeter probe touched this pin, and gradually reduced to around 0.44v. Does this suggest a bad IC3 or maybe other components (caps?) around this pin?

Thanks again!

[Enzo]

Hi
Pixel asked if any caps cold be backwards, you said you removed them and measured them. But that doesn't say whether or not any were backwards. And pin 5 does have an elecrolytic connected to it.

APply signal to the amp, now run lines from REC OUT to some other amp for a listen. Is the distortion there or it it OK?

Meanwhile, isolate the problem. Run a signal into the TAPE IN jacks and select as input. Still one bad channel?

Before this project, both channels worked. You may have had some issues, but it basically worked, right? And now after your work one channel has a new problem. In repair work, 99% of the time, when this happens it is because of something you did.

[vintagekiki]

IC3 (bad channel): 1: 0.2v 2: 1.16v 3: 1.075v 4: 3.79v 5: started at 1.71v, gradually reduced to 0.44v in 2 minutes 6: 1.16v 7: 8.22v
8: 9.46v 9: 15.7v 10: 15.2v 11: 4.5v 12: 5v 13: 1.82v 14: 0.84v
IC4:
1: 0.2v 2: 1.14v 3: 1.05v 4: 3.64v 5: 1.59v 6: 0.93v 7: 7.3v
8: 8.47v 9: 15.7v 10: 15.05v 11: 4.33v 12: 4.83v 13: 1.8v 14: 0.83v

TR6: B: 1.25v C: 4.84v E: 0.75v
TR7: B: 1.23v C: 4.89v E: 0.73v

Check (replace) the cap (pay attention to polarity) that goes from pin5 to R69 (1k).
Eventually check (replace) cap C113 (.0022u)

[Leizhg]

Wow! Thank you all for the advice! I'll give it a try tomorrow and report back.

[Pixel]

I removed all caps around the bad channel that I put in, measured their capacitance before putting them back, the problem persists. It could be my terrible soldering skill - can't visually tell though. BTW, I didn't touch any of the ceramic caps, could these gone bad too?

If they don't look cracked or burnt then ceramic caps are usually ok. Ceramics don't have electrolyte in them to dry up like the can caps do. I was wondering about where you got your caps from because electrolytics have a shelf life and they can go bad just sitting on the shelf. There are fake caps on places like eBay too.

Send us a picture of the board, especially the potential problem area around IC3. A picture IS worth a thousand words.

Edit: Pictures of both sides of the board.

[Leizhg]

First of all thanks again for the advice from everyone - the help I've received from the forum has been way beyond my expectations.

It's been a bit of a long day, so I was only be able to follow vintagekiki 's advice to check the cap connecting pin 5 to R69. The polarity seems correct to me. But again, this is my first amp repair and first soldering since 25+ years ago, so mapping schematics with what's on the board on both sides got confusing sometimes.

And per Pixel 's suggestion, a few thousand words are at the following link (somehow can't upload photos because the size exceeds the forum's limit):

https://photos.app.goo.gl/kPa1kZ1o8Nm5DPru9

Thanks again!

[Pixel]

Hi

APply signal to the amp, now run lines from REC OUT to some other amp for a listen. Is the distortion there or it it OK?

Meanwhile, isolate the problem. Run a signal into the TAPE IN jacks and select as input. Still one bad channel?

Definitely do this before you pull the soldering iron out again. Since you have a oscilloscope you could use that here as well. Circuit boards can only take so much soldering before the traces start to come off the board.

[g1]

It's been a bit of a long day, so I was only be able to follow vintagekiki 's advice to check the cap connecting pin 5 to R69. The polarity seems correct to me.

You should replace that cap.

[Leizhg]

Got some time to implement all advices:

@enzo:
- Rec Out to another amp: no distortion
- Tape in from external source: distortion with one channel

@g1:
- I replaced the cap on Pin5 IC3, still the same thing

@all:
100% this was something I did when recapping, though I've been very diligent replacing each cap one by one to make sure capacitance/voltage/polarity were same as the old ones.

I just fear IC3 was damaged during my work (for instance by heat from soldering?). Now the cap connecting to Pin3 was replaced again and proven okay, is bad IC3 a more possible explanation for the weird reading from Pin3?

Thanks again!

[vintagekiki]

- I replaced the cap on Pin5 IC3, still the same thing

Check (replace) cap C113 (.0022u) ? #6

[Pixel]

Got some time to implement all advices:

@enzo:
- Rec Out to another amp: no distortion
- Tape in from external source: distortion with one channel


Thanks again!

When you did the Tape In, was distortion on left or right channel?

[Leizhg]

vintagekiki I'll source a ceramic cap to replace C113.

Pixel it's the same channel (left) that distorts when I feed tape in. BTW, before the latest replacement of C79, there was some very low sound on the left, now it is almost completely muted.

Thanks again!