Roland JC77 problems

[TarheelTechinTraining]

V+ and V- are shorted to chassis !!

[Enzo]

OK, so the transformer is OK. Not that it matters now, but when I asked about continuity between the transformer terminals, I meant the empty ones on the board. Of course the transformer would have continuity.

If the V rails are shorted to ground, then the first four suspects are those output transistors. Dismount them, leave them wired, and see if the short remains.

[TarheelTechinTraining]

Yep short is still there

[TarheelTechinTraining]

Also there is no continuity between the empty terminals for AC

[Enzo]

Well if you have a dead short to ground from both rails, it ought to be fairly straightforward to locate where.

[TarheelTechinTraining]

Well all 4 outputs test fine, out of circuit. But I'm getting continuity from V+ to 34,32,17,and 15 as well as V- to the same points. 34 & 15 are emitters and 32 & 17 are collectors. so the problem is in the power section?

[Enzo]

Let me get this straight. Both V+ and V- are shorted to chassis? And so V+ is also shorted to V-, even if just through the chassis? But did we not already test the main bridge rectifier for shorted condition? Since the V+ is wired to the bridge directly, as is the V-, even if the bridge is not causing it, its terminals should read as shorted together if the two Vs are shorted.

[TarheelTechinTraining]

ok so I removed the bridge and tested to the outputs, still shorted. removed the filter caps and tested to output , still shorted. Checked again for bad soldering and nothing. and V+ to V- terminals no continuity. Please be patient with me this is killing me. Also the pcb is out of the chassis

[TarheelTechinTraining]

I'm sorry V+ is shorted to 34 & 17 , collectors. V- is shorted to 32 & 15 , emitters. My bad enzo cranial overload,lol.

[Enzo]

Well, I expect V+ is shorted to 34,17, since it is wired to them.

But are both of them still shorted to ground?

[TarheelTechinTraining]

no now only the V+ is shorting.

[TarheelTechinTraining]

So I now have the correct schematic,thanks to this forum. Where should I start looking after the "usual" suspects , BRD filter caps and so on?

[Enzo]

All the advice you have been getting is good, just apply it to the correct schematic. There are only so many ways a V+ can get shorted to ground.

[TarheelTechinTraining]

Ok I'm pretty sure I've got a bad transformer. With the transformer unhooked from pcb both Vac are shorted to ground. Am I thinking correct here and if so where might I find a replacement?

[52 Bill]

Ok I'm pretty sure I've got a bad transformer. With the transformer unhooked from pcb both Vac are shorted to ground. Am I thinking correct here and if so where might I find a replacement?

So to understand what you have done here, you have disconnected the three wires of the transformer secondary winding from the pc board. Then you have read the resistance from one of the secondary windings wires to the chassis and you have gotten a low or zero ohm reading. Is this correct?