Peavey Bandit 112 TransTube

[ChancesR]

This amp has loud hum (not affected by volume adjusting), no sound output otherwise. I pulled the power board (PY049A), have checked all solder connections; pulled the filter caps - test fine (Sencore LC103); checked all diodes for shorts, all good; checked output transistors - all good; fuse never blew; the smaller 8-pin op amps all check great (BA4560), there are 3 of these on the board. The TL074 has no DC on the outputs, but the neg VCC is a little low (-14.64). Switching to AC, both go to '0'. So that's good. No DC on the speaker leads. Speaker tests good. Used a second speaker, also same symptom as above. Also have checked resistors for opens, and the bridge rectifier *appears* ok - didn't remove from circuit.

What am I missing here....preamp bad? I've gone through this power amp board multiple times - still no go.

The preamp board is a PY048/PY049B.

Thanks in advance,
Regards,
C

[pecorporation]

Does the hum appear on the preamp out jack as well?

[ChancesR]

Good advise - will check and re-post. Thank you!

C

[Enzo]

DOn't get locked into thinking all problems are bad parts. SOlder cracks cause many many problems.

You have a loud hum and no signal. Isolate the problem. Is there DC on the output to the speaker? That indicates trouble in the power amp. Your outputs may not be shorted, but the drivers could be faulty, or a resistor could have opened. You report no DC>

Or perhaps a loss of filtration. You pulled filter caps and tested them, but are they doing their job in the circuit? The power amp runs on what? +/-25vDC maybe? Are both rails up to voltage or is one low. ANd check them for AC, is one rail full of AC? You checked the +/-15v, but even if the 25v filter fails, the 15v has its own which would cover for it. And a cracked solder joint somewhere between the filter and the rest of the circuit is the same as no filter.

The transistor front end has its own power supply. Did you check it? That +30v needs to be clean.

Plug into the power amp in jack. Still hum? Then it is in the power amp stage. And send the preamp out signal to some other amp. OK or hum?

Various points in the circuit, mostly near jacks, are protected by clamping diodes to the -15 and +15 rails. One of those shorts and the signal path is clamped to a 15v rail. That will kill signal and make hum.

[ParthaD]

One of the internal cable connectors loose.. maybe?

It's amazing how many amps, especially SS amps, that I've worked on in the last few months with the exact same symptoms (loud hum unaffected by any control), where the problem was that one of the internal cable assemblies or some grounding cable had got loose. Just pressing down on all of them might be worth a shot!

Good luck!

[Enzo]

Certainly, and don't overlook loose nuts on jacks and controls, and loose hardware on transformers too.

[ChancesR]

FIX!!!!!!!!

Just wanted to say 'thank you' to all who replied - and give the final fix on this.

I continue to amaze myself how I can get SO fixated on the symptom=THIS. When I saw that I had the 15V - I quit there. What I didn't have was the 25 and 35V rails......and then discovered 3 of the 6 22ohm flameproof resistors had gone open. VOILA! I substituted 22ohm metal oxide long enough to validate that nothing else was amiss - and am waiting on the replacement flameproofs now.

Again - thank you so much for getting me back on track!!

Regards,
C