Fender Roc Pro blowing fuses. Need troubleshooting help.

[oldsmobuick]

I bought an older Fender Roc Pro at Goodwill that was blowing fuses and I have it apart now for troubleshooting. The C->E resistance on all 4 output transistors is ~50 Ohms. Does this mean the transistors (TIP 142/TIP147) are not shorted/blown? The Zener diodes are toasted but I imagine I need to fix something upstream and not just replace those diodes. Any help appreciated- thanks!!

p.s. I should add that I have not removed or desoldered the transistors, I just checked them in place.

[R.G.]

Ideally, you'd have (or make) a light bulb limiter so you could hook up AC power and not blow fuses while you figure out what's wrong.

Failing that, pull out the power transistors. Fuses still blow? If no, you have bad outputs and possibly a bum part in the power amp. I always hated the bootstrapped opamp style power amps; they're fragile. If yes, you have power supply problems or a short on the other circuitry.

In that case, debug the power supply from the AC in. Remove the rectifiers, try powering just the power transformer. Fuses blow? If yes, you have a problem in the AC wiring before the transformer or inside the transformer. If not, you could have bad rectifiers, bad filter caps, or a short on the rest of the circuit boards somewhere. In this case, work your way forward from the transformer secondaries, testing parts and voltages as you go forwards.

[Jazz P Bass]

C-E at 50 ohms means you have one or more failed output transistors per side.

http://support.fender.com/schematics..._schematic.pdf

[52 Bill]

Agree with the others.

It would help us if you were a bit more clear in what you are trying to describe. First which Zeners are you talking about, the two in the low voltage power supply or the two in the power amp or any of the others? And by toasted, are you describing the diodes or are you describing the board area around them?

Does your meter have a diode test setting? Did you read across the C-E with the leads connected in both polarities? If you did, was there a difference in readings?

Do you have some sort of de-soldering tool or system to use?

[J M Fahey]

You are using the wrong multimeter scale.
"50 ohms" makes me think you are using a resistance scale, which is inaccurate measuring semiconductors, you need to use the "diode" scale .
Anyway, those 50 ohms strongly hint at they being shorted.

[oldsmobuick]

Thanks!

Thanks for the great suggestions, I have my work cut out for me it seems in troubleshooting this. I will post some pics of the zener diodes (looking burned on the board). I forgot to mention there is also a kind of a humming noise right before the fuse blows. If I do need to replace the transistors, do I need to buy "matched pairs" and if so where do I get those? I see the transistors on eBay for very cheap, but I don't want to put the wrong thing in. Thanks!

[J M Fahey]

No need for specially matched, but buy from a reputable supplier such as Mouser or Digikey, forget EBay , lots of fakes or factory rejects.

Parts are cheap, so order extra resistors, a couple extra diodes, ceramic 5W resistors, etc.
And donŽt forget proper micas and thermal grease or Silpads to remount your power transistors.
YouŽll pay the same minimum postage anyway.

Build a lamp bulb limiter (cheap and easy) to protect your amp while repairing.