Advice needed for vintage amp repair: one channel distortion

[Pixel]

Document your progress and let us know how it goes with the TC-50. I'll just say that if you use rubber rejuvenator liquid you should be prepared to let the thing air out for a few days outside because that stuff is awful.

[Leizhg]

Thanks Pixel ! My next (under going actually) project is to restore a Sony TC-50. Don't have too high hope but let's see!

[Pixel]

That is awesome. You did a great job. You should go out and find some matching gear and save that too!

[Leizhg]

Reporting back after a long delay. Finally received the ceramic cap yesterday, replaced the existing one and still no success. So I resorted to replacing IC3. Woohoo, it worked!

Thanks everyone for the advice along the way! Very happy about the result and a satisfying journey.

Here is the video if anyone is interested:

https://photos.app.goo.gl/d4YwnTdkob7uRYwU6

Thanks again!

[Pixel]

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!

I just wanted to make sure that we were working on the correct channel.

Do you still have the oscilloscope? If so you could see if there is good signal coming into the IC. Its great you have the good channel to compare to, but be careful not to touch more than one pin at a time on the ic with the oscilloscope probe.

I'm not convinced that the IC is bad yet. If you did something to make it go bad we should try to find out what that was or the replacement will just go bad too. You didn't try to get the IC out already did you? That might be enough to damage it, but just soldering caps around it probably wouldn't unless you had the soldering iron on the board a very long time. I always dread removing ICs, but at least you don't have lead-free solder to deal with on these older boards.

[Marshtech]

I would replace IC3 at this point.

[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!

[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?

[vintagekiki]

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

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

[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!

[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.

[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.

[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]

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]

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