Roland JC77 problems

[TarheelTechinTraining]

With the amp plugged in the limiter and powered up I'm getting fluctuating mV from - to + 60 mV at the speakers
plus I still have resistance from V+ to ground and continuity from V- to ground

[oc disorder]

That 60mv is fine .. insignificant compared with 18vac at full power with 8 ohms so there is no "dangerous "DC at the output which will burn out a speaker .
As you can now see if a output transistor failed the full power supply could go to the speaker.
Common problem -guy plays amp too loud transistors overheat breakdown just before the fuse goes speaker burns out.
"Ohh amp stopped working" ..replaces fuse with foil turns on amp full dc down the speaker wires all other speakers burn out etc etc

Regarding this remaining reading from V+ to ground can you tell me how many ohms and can you reverse the meter leads and read how many ohms again.
I'm hoping the readings are different.
I better print out the schematic. I'm using a borrowed Mac laptop and normally use a PC so am a bit clumsy at the moment....

[TarheelTechinTraining]

on V+ to ground the resistance is climbing one way , reverse leads no resistance
amp off

[oc disorder]

Well my thoughts are the short is no longer there ..the light bulb would have stayed illuminated if it was.... (had a look at that point you discovered before that connects to the supply R89 & R82 , it connects to fets Q5 and Q20 ..this appears to me as a de thump circuit for when you turn the amp off it mutes the input to both power amps.)

I think the resistance you measured is a capacitor charging up ..not sure why the negative supply doesn't show the same characteristics .

I would now connect the speakers with the light bulb limiter in place and turn it on . . . .the L.B. Limiter may illuminate a little more now a load is connected but shouldn't be bright after it settles down.
Feed a signal guitar etc and try it... the light bulb will glow with the audio

EDIT.. I missed this "continuity from V- to ground" can you measure like you did with the positive reversing leads etc ?

[TarheelTechinTraining]

Light bulb very bright

[52 Bill]

on V+ to ground the resistance is climbing one way , reverse leads no resistance
amp off

The climbing reading is the filter cap being charged by your meter. When you say "no resistance" do you mean zero ohms or no reading on the meter/infinity reading?

[TarheelTechinTraining]

It was zero

[oc disorder]

forgot to ask what wattage is your light bulb ?

Hmm not good "light bulb very bright" only connect one speaker at a time.. with only one speaker connected
does one amp (or side) when its speaker is connected make the light bulb glow more than the other ?

I'm trying to see if a fault is only in one of the two amplifiers in the JC77.

[TarheelTechinTraining]

Still bright with either speaker. limiter is 25 watts

[oc disorder]

Just thought should rewind a little..to this "set your meter to DC volts and measure at the amp speaker outputs first one then the other"
If its definatly DC and no more than + or - 100mv double check please

[TarheelTechinTraining]

one speaker is getting + or - 100 mV DC and nothing on the other

[oc disorder]

Can you try a bigger light bulb say 60 or 70 watts? It should initially glow brightly then dim down.
Also maybe need to clarify a few terms zero ohms means a short circuit ie if you hold the two leads together you should measure 0 (zero) ohms but because of the resistance of the leads and possibly not the best connection (holding them together) .. the reading would be more like 0.01.

However when you have the meter on the ohms scale and you hold the leads apart and the dial doesn't move that is open circuit millions and zillions of ohm which is infinity or no reading.
The air we breath would have very very high resistance dependent on distance but to me its an insulator unless it's raining !!

[TarheelTechinTraining]

with one speaker connected the other reads infinity and reversed. Still have continuity from V+ to ground only its coming and going
I'm sorry I'm writing this wrong I've got 5+ Vdc at one speaker and 6+ Vdc at the other. Still have continuity at V+ intermidently

[oc disorder]

Ahh well it would seem we have both power amps showing dc at the output so I suppose it's time to start looking at blown drivers or a diode.
I guess the 60mv measurement was either AC or decimal point misplaced.
You have already checked the output transistors for a short circuit. They should be either 2SD716 or 2SD718.
We have been pursuing a possible short circuit between the positive supply and ground.
It now seems that perhaps something is open circuit upsetting the balance and producing a positive voltage at the output.

have a look at this as it's copyrighted I better not paste it here Transistor Testing

Ok the output transistors are 100v 6a NPN type. This utube vid shows how to test
http://www.youtube.com/watch?v=8VK_4gZU__I
He's actually testing an NPN type and has a diode test function on his multimeter.
It best to test for this out of circuit.. ( i just remove the base and emitter leads to test).
With the collector in the middle the base of the transistor is to the left and the emitter is on the right.
After the 4 outputs are done move back one per the schematic and test the drivers 12,13 29 & 30.
13 and 30 are PNP types.

Before this perhaps you should double check where you removed the switch in a good light with a magnifier.
Make sure there no small bits of solder shorting anything out and that there are no broken tracks anywhere.
It's unusual for both amps to go faulty .. do you think the collectors (back of o/p transistors) could have accidently
shorted to the chassis when power was applied ?

[TarheelTechinTraining]

ok here is what I've got
tested Q 6,7,8,9,11,12,13,15,16,22,23,24,25,28,29,30,32,33
all tested good
also tested D12,22,19 and all good.
So if I'm testing correctly both amps are good so far .?
next in schematic are a couple of FETs my question is how to test them with a DMM?
Thanks again for your patience all,Greg