Roland JC77 problems

[TarheelTechinTraining]

No I tested between secondary windings and the ground on the transformer.

[oc disorder]

1.Do you mean with the three wires disconnected from the terminals on the edge of the pcb the wires are measured to be shorted to the frame of the transformer which in turn is connected (bolted) to the chassis of the amp which is ground ?
2.Or do you mean the two ac wires that are connected to the outside terminals on the edge of the pcb are shorted to the wire that goes to the centre terminal on the edge of the pcb which is also ground.?
Please answer 1 or 2.
If "2" the secondary wires have a very low resistance between them it looks like a short circuit ..the resistance between the centre tap (one that goes to ground) and either of the outside connections is the same and measures half the resistance between the two outside ones.
You can check this by setting your meter to AC volts and measure the voltage at those wires.
It shouldn't matter if the amp is still plugged through the light bulb limiter.
Did you try this before ?
Unsolder the +'ve and -'ve pins on the pc board and measuring the DC voltage across them.The pins have red and brown wires wrapped to them.
Assuming the ac voltage is ok you will have to reconnect them to the terminals on the edge of the pcb making sure the centretapp
one goes to the um.. centre connection which is ground.
Now have we got Ac and Dc ??

[TarheelTechinTraining]

#1. When I measure AC , secondaries, its about 25v without the limiter. Thanks for everyones help !!

[TarheelTechinTraining]

When I test the secondaries ,plugged into the wall ,unhooked from pcb ,the voltages are 25 or less from center tap to Vac and about 50 Vac to Vac. All three wires are off the pcb

[oc disorder]

Ok thats good the JC120 was about 80v so it makes sense that the JC77 is 50vac.
Now working through ..if you can unsolder those pins (i thought it would be neater than unwinding the wire wrap on them)
you should be able to check the DC volts and make sure the bridge is ok.

[TarheelTechinTraining]

OK so I've got +25 on V+ and -25 on V-. but doesn't 50Vac seem alittle low shouldn't it be at least 60 Vac?

[oc disorder]

I think approx 50vAC is ok.... if you have the proper schematic what does that say ?
Anyway so far Transformer ok , bridg rect ok now with the bridge still disconnected can you measure across the back of the main capacitors with the ohmmeter hopefully they are not charged as the ohmeter may not like that... anyway measure one way you may get a small reading then it disappears.
Reverse the meter probes and do it again... slight reading then disappears ?
The voltage in the meter is enough to slightly charge the capacitor so when you reverse the leads the tiny voltage in the capacitor combines with the one in the meter and for an instant you get a reading 'till it charges again (this is a rough approximate description !).
Anyway we are looking for a serious short .
From your photo I can't see beyond where the caps are so I can't see if there are any links to disconnect the +'ve and -'ve PCB tracks from other parts of the circuit so you can test the supply with the caps connected and nothing else.
I do suspect that a transistor case is shorting to ground or something near the old switch has the supply on it that is accidently connected to ground.

[TarheelTechinTraining]

Yeah I do have the schematic for this amp , i did have the wrong one before. The caps c89 and c88 test good. With transistor how do I test?
I checked for grounding on all four transistors and zilch.

[oc disorder]

"With transistor how do I test?"
Just for a quick test which should show a short- low ohms setting on meter and test between emitter and collector.

Easiest way put probe on centre prong/connection and test either side with other meter probe. The collector is in the centre the other prong/connection is the base.
A short will show as very low ohms ... a transistor tester is needed for checking gain or open circuit ~ sometimes a small jig an be made
in conjunction with the meter for more complex testing.
(amp off of course !)

[TarheelTechinTraining]

All four read good. On the outside two legs the reading jumps but settles back to zero. From center to one side it jumps and the other side to center nothing.
All four are still attached to the pcbJC-77 Schematic.pdf

[oc disorder]

"From center to one side it jumps" I presume after it jumps it goes back to nothing (infinity or no reading).
Now Im gonna take a long shot... can you disconnect one end of 4 resistors in the power supply.
The idea is to disconnect the pre-amp supply and see if the amp still has a short and draws lotsa current.
R1, R2, R3 and R5.
R1 & R5 330ohms 2 watts (orange orange brown)
R2 & R3 5k6 half watt (green blue red )

With the light bulb limiter in does it light up ??? if not what are the voltage readings between the points the output transistors connect to?
Voltage between point 32 and 34
Voltage between point 17 and 15

[TarheelTechinTraining]

I disconnected the 4 resistors and the bulb gets bright for a second then dims to almost nothing.

[TarheelTechinTraining]

Also I lifted r82 r89 and c54 with the same results, limiter gets bright for a sec then dim.

[oc disorder]

Ahhh well it's looking to me that the "short" may no longer be there.
You can see what I was trying to do isolate parts of the circuit from the power supply to see what was shorting.
I didn't see the R82 & R89 connection well spotted !
The light bulb limiter getting bright for a second then "dying down" seems normal behavior for an amp with the initial inrush charging the main filter caps.
How about re assembling the amp without connecting the speakers and seeing if you think the short is still there on the +'ve.
If thats ok set your meter to DC volts and measure at the amp speaker outputs first one then the other.
Hopefully only a few millivolts not the whole supply !!!
Then if alls well reconnect the speakers one then the other this and the above still via the light bulb limiter.
We would then have the low level and perhaps the slightly lower ac supply to look at.

[TarheelTechinTraining]

Thanks OC I'm understanding Much better.