Fender Stage 160 Instantly Blowing Fuses

[Twintsupraz]

I've checked R81-R86 and the resistors that are connected to the output transistors already and they came up good but I'll double check to make sure. I'll take a look at Q10, Q11, D22, D23 and everything else as soon as I can. Thanks again for the help Enzo.

[Jam]

stage 160

Iam not a pro but I always had better luck to replace all the output transistors at the same time with the same brand

[Dave Curtis, dB AudioTech]

I believe he did R&R all of them. He said he was going to, anyway.

[Twintsupraz]

Sorry it's taking so long to write back. Work has been keeping me busy and not letting me have time for my toys.

Yes, all output transistors are replaced Jam.

Still have not found the solution to the problem yet. Here is the latest of what I've done and figured out.

The fuse only blows when the high voltage(AC) power is connected. When its just the low power(DC) the fuse does not blow. And when it does blow it shorts out 2 of the output transistors with it(Q15 and Q14 which are the first ones on each side). I was not getting the -+1.08V on test point 24 and 25, I can't remember at the top of my head what number I was getting for that but I know it was not 1.08V. Tested all of the resistors in the circuitry associated with the output transistors and they all showed good. Tested all of the other transistors associated with the output transistors are good, not shorted. Capacitors and diodes came out looking good also. I've pretty much tested everything in the block of circuitry connected to the output transistors and it's all came up good.

Any ideas guy?

Thanks!!

[Enzo]

I still think what is happening is that the bias circuit has failed, and both positive and negative transistors are being turned on fully at the same time. You should get that +1 and -1 voltage readings at the bases. If you get something like +15 and -15, it will blow up in an instant.

[Twintsupraz]

Sorry its taken so long to write back. Once again work has not let me have any free time to play.

How do you test the Bias circuit Enzo? And are you talking about the output tranisitors when you said to check the base for voltage?

Thanks!

[Enzo]

Yes, the output transistor bases.

[oc disorder]

"Sweet! I think I have some Arctic silver laying around."
Im not familiar with Arctic Silver but I gather some CPU thermal compounds
can be electrically conductive.

However from their web site:-

Not Electrically Conductive:
Arctic Silver 5 was formulated to conduct heat, not electricity.
(While much safer than electrically conductive silver and copper greases, Arctic Silver 5 should be kept away from electrical traces, pins, and leads. While it is not electrically conductive, the compound is very slightly capacitive and could potentially cause problems if it bridges two close-proximity electrical paths.)

Just thought it was worth a mention !

[Dave Curtis, dB AudioTech]

Good point. I'll add that any non-conductive compound would likely have the ability to become the dielectric (insulator) part of an unintended capacitor.