Kenwood KR-8050 Triggering Protect

[CaseyWKC]

Hey all, I’ve had a KR-8050 that I had rebuilt the left channel in and got back going a while back.


Replaced the outputs that were blown while somebody was trying to adjust bias themselves and shorted one.

It was working fine for two - three weeks and now increased volume on the left channel keeps triggering protect. Not a lot either.

At one point when probing the protect circuit it went away and performed normally. I forget what transistor I was monitoring but I figured maybe it was a bad joint or TR so I replaced it and then could not get it to work intermittently again. Back to the same deal.

Any increase in volume on left channel just triggers protection real quick.

Happens regardless of a load connected as well.

Any insight or good places to start would be great!

I figure at this point it has to be on the left channel amp board.

Thanks!

 

[CaseyWKC]

Bias is 240mv with trim set to min in left channel now, narrowing things down.

 

[rschweitzer]

Drivers, pre drivers, capacitors, bias resistors, emitter resistors are all suspect and most likely bad if the outputs went. The outputs are no longer available and the cheap offerings on the bay are no good. If that is what you have, this could also be an issue.

You need NOS outputs if you can find them. The alternative is insert TO-3P style. I tried this with the 8050 I have. It worked well, but was able to find the original T-03 transistors and reverted back. Unless the market has changed over the past few years, equivalent transistors are all available for this receiver. I replaced most of them in mine.

Check the offset at the speaker terminal. Looking for as close to 0mv as possible.

Use a dim bulb tester with 100 watt incandescent bulb. Takes a few more seconds, but the relay will click.

 

[CaseyWKC]

Drivers, pre drivers, capacitors, bias resistors, emitter resistors are all suspect and most likely bad if the outputs went. The outputs are no longer available and the cheap offerings on the bay are no good. If that is what you have, this could also be an issue.

You need NOS outputs if you can find them. The alternative is insert TO-3P style. I tried this with the 8050 I have. It worked well, but was able to find the original T-03 transistors and reverted back. Unless the market has changed over the past few years, equivalent transistors are all available for this receiver. I replaced most of them in mine.

Check the offset at the speaker terminal. Looking for as close to 0mv as possible.

Use a dim bulb tester with 100 watt incandescent bulb. Takes a few more seconds, but the relay will click.

None of these are the issue and performed as expected for weeks after the repair.

I have gone over the driver board and protection board at least 5x now, I've repaired what looked to have been cold solder joints, I replaced the 2sc2259 dual trans on the driver board that was sending the unit into protect when probing. All voltages match the schem, I've even swapped the outputs to confirm it's not an issue I'd need a curve tracer to truly resolve or see.

I'm at a loss at this point, I've had this unit for a while now.

Bias is still pegged at around 237mv in the left channel, trigger protect anytime volume is increased.



Maybe this helps some, one output emitter on the left channel (the channel I'm having trouble with) is reading 237mv, that high bias, the other transistor reads fine, around 15mv, as I powered it off before fully warming up.

 

[CaseyWKC]

Drivers, pre drivers, capacitors, bias resistors, emitter resistors are all suspect and most likely bad if the outputs went. The outputs are no longer available and the cheap offerings on the bay are no good. If that is what you have, this could also be an issue.

You need NOS outputs if you can find them. The alternative is insert TO-3P style. I tried this with the 8050 I have. It worked well, but was able to find the original T-03 transistors and reverted back. Unless the market has changed over the past few years, equivalent transistors are all available for this receiver. I replaced most of them in mine.

Check the offset at the speaker terminal. Looking for as close to 0mv as possible.

Use a dim bulb tester with 100 watt incandescent bulb. Takes a few more seconds, but the relay will click.

and no, replaced with a genuine Mj21194g from mouser

 

[CaseyWKC]

Still fiddling with this guy, I intermittently have 260-600mv of bias on the collector of Q23 2SC2607 , protect triggers with any volume increase as well.

Transistor isn't getting hot either. And it's only the collector of Q23 on the left channel, the other output seems to behave normally

So -6.59v on the collector of Q23 and -7.07v on the collector of Q25.

I have not lost hope.