Rotel RB991 with -61V on output terminals

[Jaimo]

I have a Rotel RB990BX power amp with both channel outputs sitting at -61V. The voltage across the main reservoir caps is +- 66V.

I have in circuit tested all output devices, the driver tranny and the 1N4003 diodes that sit between the output and voltage rails and all these components check out okay. The Emitter resistors and Rail fuses also check out fine.

Any ideas what could be blown, resulting in the negative Vout?

 

[abraxalito]

Is there only one pair of fuses serving both channels? If so, I'd re-check these when present in their fuseholders (not out of circuit as the holders themselves may have fatigued) as we need a single point fault mechanism to explain why both channels give -61V.

 

[Jaimo]

RB991 Fusing

Each Channel has it's own pair of fuses - and I re-tested both channels' fuses in circuit and verified that the positive and negative rails are connected to the collectors of both sets of output transistors.

Thanks for the input.

 

[abraxalito]

OK. So the next thing I'd do is check the voltages at the collectors of all the output transistors. If these do indeed show up as +/- 66V then the problem is probably earlier in the signal chain. Are the input and driver stages fed via the same fuses as the output devices?

 

[jaycee]

Keep checking backwards. Drivers, predrivers. Power it on through a bulb and measure voltages on the bases of transistors.

 

[east electronics]

broken idle trimmer can also do that ....krack on the pcb can also do that ...open resistor can aslo do that...... but both chanels ???that is odd ...