KR-6600: No sound in one channel after amp board repair

[benjaminlee]

I had some issues with transistor noise on one channel, which I fixed, but some similar issues appeared on the other channel. I decided to go ahead and replace everything on the amp board with a repair kit off of eBay; essentially everything but the resistors, in hopes of future proofing a bit. I also replaced the outputs in the faulty channel to be safe.

I took around 7 hours to do this repair, making sure I followed the included instructions carefully, replacing both channels of a given component before moving on to the next, and keeping the old components sorted, which I still have. I powered it on after finishing, and it left protection mode, but only the right channel seems to be working.

There is very faint noise at higher volumes but I assume that is some sort of bleed. I was fairly tired at this point so I just ran a few tests, it seems like the voltage across the transistors on the working channel was ~6v not long after powering off, while the non working channel’s shows close to 0v. Tried different speaker groups/different speakers, same issue.

I reflowed the joints on the working channel, and checked thoroughly that the orientation of components was correct. I will send pictures when I get a chance. If you have any advice or suggestions let me know! My current thought is just putting the old components back on that channel one by one until it comes back to life.

 

[Dave1027]

My current thought is just putting the old components back on that channel one by one until it comes back to life.

That sounds like an unnecessary pain. Do some circuit checks to narrow down the odds. Replace those parts more likely to be the cause first. Which transistor has zero volts?

 

[benjaminlee]

That sounds like an unnecessary pain. Do some circuit checks to narrow down the odds. Replace those parts more likely to be the cause first. Which transistor has zero volts?

I agree, probably unnecessary. It's been over two years since I picked this amp up and it would be nice to finally have it functional.

I tested again, and it seems that whatever voltage discrepancy there was has vanished from the transistors. I noticed that the working channel is distorted, though I assume that has to do with the bias not being set as I installed new trimmers and haven't set them yet.

What circuit checks do you suggest I try? I have become significantly more familiar with this stuff since getting this amp with no experience, but I still struggle with understanding the schematics in a meaningful/applicable way.

I will share some before/after pictures of the board.

 

[benjaminlee]

I went to check bias and it is showing 0mv on both channels, both trimmers seem to be connected to the circuit via continuity test on dmm. DC offset is 25mv on the working channel, while the non working channel fluctuates between 0 and 12 inconsistently. Hopefully that gives some clue? I may try replacing Q7/8 to see if that fixes the bias issues, though that is just a blind guess.

Quick update: replaced those and the previously working channel now is extremely quiet, put the new ones back and still quiet. I suppose I'll leave it alone until I get some guidance.

 

[Dave1027]

Could mean flakey differential transistors or cap. Get a cheap transistor tester and check them. Those usually test caps too. Be sure to check the power supply voltages.

 

[gort69]

Tap the relay with a toothbrush handle or chopstick or something nonconductive.

The sound inject switch/pot is also a frequent culprit for losing channels especially if it’s pulled out

 

[benjaminlee]

Could mean flakey differential transistors or cap. Get a cheap transistor tester and check them. Those usually test caps too. Be sure to check the power supply voltages.

Gotcha, I guess I will order that. Anything I can do in the mean time? Components you suggest I replace based on the symptoms?

 

[benjaminlee]

Tap the relay with a toothbrush handle or chopstick or something nonconductive.

The sound inject switch/pot is also a frequent culprit for losing channels especially if it’s pulled out

Nothing from relay, before this I was diagnosing an issue with the relay and that turned out to be one of the transistors on one of the two power supply boards. Ended up replacing most if not all of the caps on those boards in the process so I can probably safely rule out anything other than the amp board. Both channels were functional before I attempted the full repair of the board, it was just that one had some loud pops not long after powering on. In hindsight I probably would have solved that issue by replacing the input pair, but here we are. :/

 

[Dave1027]

Try comparing the bad amp board with the good one. Write down a list of the transistors and list their voltages, collector, base and emitter.

Not having an oscilloscope, you could input a noise signal from something like a phone app and check for AC along the signal path.

 

[benjaminlee]

I did end up getting that transistor checker, but while I was waiting on it I decided to start replacing components to see where I had gone wrong. It ended up being Q17-20, the guide instructed me to put them in the opposite direction as the old components, and I probably should have verified the correct orientation with a datasheet before blindly following the instructions. I ended up just replacing them with the originals in the original orientation, and that seemed to bring back both channels.

I then had an issue with one channel only working when I moved the amp, so I took it back apart and reflowed every joint on that channel, which ended up working. There still might be some slight issues with that channel, gets a bit fuzzy sometimes, but it is inconsistent. For now it is going to have to do, I'm getting tired of thinking it is completely fixed and putting it all back together just to find another issue and removing all those screws again.

Thank you for the guidance though, hopefully someone else has a similar issue and is able to gain some benefit from my troubles.

The kit I ordered was: https://www.ebay.com/itm/114517406585?chn=ps&google_free_listing_action=view_item