Peavey Bandit - living up to its name

[jbmdisco]

I have taken a peavey Bandit 65 in with a buzz and hum. The slight hum I believe is factory fitted but the buzz is curious. No matter where you touch the amp, be it the chassis, the casing, handle, anywhere at all, you get a buzz. Have given the pots and jacks the customary spray with servisol to eliminate anything there and also removed the (factory fitted) spiders and webbing. All components and soldering in good condition. The buzz happens at a slight touch and if tapped is more so. Have checked grounding is OK and I have 'chop sticked' all components to eliminate dry joints. I predict this is a job that will sit in a corner until a eureka moment.

[TomCarlos]

Just curious.... Are you doing your "touch" test with anything plugged into the input jack? Not sure if that would make a "huuuuuuge" difference but it is something to eliminate from the equation.

You say there is both a "buzz" and a "hum" and the "buzz" only occurs when you touch the amp?

Maybe step 1 would be to find the source of the "hum" and work on that. Typically, the "hum" would come from the Power Supply caps. I would check or replace the caps on the high and low supplies.

I'm am thinking the "buzz" from touching is a bad solder or mechanical connection. I am sure the other MEF members can weigh in.

Bandit 65 Schematic

[nickb]

A couple of things to check - the continuity from the earth pin of the power plug to the chassis and for 47 ohms between chassis and input jack ground (R98).

[TomCarlos]

I saw this before but learned something today.... yeah!!

Nick mentions R98. That is the Flame Proof (FP) resistor. It is located on the outer edge of the PC board, above the Pre/Pull Bright pot and below the PC Board ID number. I found an old post on the Peavey forum site where Enzo explains that "The resistor is ther to keep the sections connected together in case a ground connection goes open."

On the schematic, that resistor can be found just below the reverb circuit (look for U4B and U4A).

[nickb]

I saw this before but learned something today.... yeah!!

Nick mentions R98. That is the Flame Proof (FP) resistor. It is located on the outer edge of the PC board, above the Pre/Pull Bright pot and below the PC Board ID number. I found an old post on the Peavey forum site where Enzo explains that "The resistor is ther to keep the sections connected together in case a ground connection goes open."

On the schematic, that resistor can be found just below the reverb circuit (look for U4B and U4A).

In this particular amp, there are two grounds. One for the power amp and the other for the preamp. I don't think I missed any more. The bridge between them is done by U3B. It is configured as a differential amplifier that is referenced to the preamp ground but also has an input from the power amp ground. Thus it bridges the 'gap' in the grounds Look at it's +ve input - it is connected to both grounds. I believe, but not certain, that intention is to prevent hum loops should you connect something to the one of the jacks and is also connected to the safety earth.

I've seen these hum if any of the chassis connection points are poor.

[Mick Bailey]

The first thing I always check is the mains plug is fitted correctly. Loose or bad connections can give rise to all kinds of problems. At least eliminate the possibility.