Roland JC77 problems part2

[TarheelTechinTraining]

Ok I've gotten a 60w bulb for my limiter. When I turn the amp on it takes a while for the bulb to get bright.
One pair of the outputs is getting much hotter than the other pair and I've still got about .5 or .6 Vdc at the speaker out.
There is about 9 Vac at the BRD and 8.5 Vdc at the bridge.

[TarheelTechinTraining]

Well I removed the two output transistors that were hot and bulb still got bright but it took a little while.

[DrGonz78]

I am a lil bit confused reading that you are getting .5 or .6 Vdc on speaker out. Earlier you had said 5 and 6 Vdc so that part might need to be clearer. Also, how old are the filter caps on this amp and did you replace them?

[TarheelTechinTraining]

The filter caps tested ok. The voltages are acting alittle loopy. Should I remove the outputs to test ?
Seems to take the limiter a little while to get bright maybe caps charging?
I'm at a loss.

[DrGonz78]

Well my first thought on all filter caps is to ask how old is the amp? Now I don't really think that will solve the problem but it is a starting question for any old amp.

Seems to take the limiter a little while to get bright maybe caps charging?

That is why I was wondering about the filter caps. My thought is that when I turn on a working amp through the dim bulb tester it quickly lights up bright and then dims. So that charge seemingly happens very quickly and when you turn on this unit it is taking time to charge up the caps possibly. Well I am not an authority on this matter but just thought to share this little bit of feedback. Hopefully someone else might chime in as to why it might take the light bulb longer to get bright due to a bad component or short somewhere in the amp. I would guess that we are seeing those 15vDC rails out of the main power stage suggests that all of that portion of the amp seems to be working ok.

So maybe you need to look through the preamp/output sections. I just fixed up a late 70's JC60 and had found some bad resistors near the portion of the PCB that fed the output transistors power. It was really one of those shotgun testings/inspections prior to turning anything on to see if any of the resistors were out of tolerance or just open. I saw one that had a burn mark, but was actually still measuring. Got lucky there since I have no accurate schematic for my amp. Found another one that was a 100 ohm carbon comp resistor that was very lightly burnt and measured open. Almost all of the resistors could be quickly measured without lifting legs on my amp.

All those wires that hook up to the output transistors were questioned by Enzo earlier... Have those wires been confirmed as being hooked up correctly? Good luck I hope you get her working soon.

[TarheelTechinTraining]

Thanks gonz. I'm not sure about actual age of the caps. They have been changed before but can't say when.
About the dimmer I was thinking along the same lines but I'm not sure.
The output wires are on correctly as long as the transistors are B-C-E face up.
I'll nose around the resistors but so far they've all tested within tolarence.
Thanks again all help appriciated, Greg.

[TarheelTechinTraining]

Well I finally got it off the limiter !!!!
One of the outputs WAS bad, so I changed both of them.
Now after about 5 seconds a terrible static, horrible noise, starts coming out of one speaker.
Any ideas?? My dad, the owner of the amp, says to use componet cooler to test parts to find the one bad.
Thanks for all the input and patience, Greg

[TarheelTechinTraining]

Well she is all done. It ended up being a bad transistor ( Q7,8,23,24). Can anyone tell me what these transistors do/their job?
Also the chorus switch repair, where all this started, works great, put a on/off switch on the back of the amp.
Once again thanks for everyones input and on to the next project,disaster. LOL

[TarheelTechinTraining]

Well I finally fixed this puppy !! Ended up being a bad transistor ( Q7,8,23,24)
Can anyone tell me these transistor function or job in the circuit ?
Also the chorus switch repair , where this all started, works great. Put an on/off switch out the back of the amp.
Thanks again for all the help. Now on to the next project/disaster !! LOL
Greg

[Jazz P Bass]

Q7 & Q8 form a Long Tailed Pair input circuit (a differential amplifier)
The same is true of Q23 & 24.
Link: LTP transistor input circuit - Google Search