Advice needed for vintage amp repair: one channel distortion

[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)

[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.

[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!

[vintagekiki]

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

[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.

[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!