Peavey Session 500 Mark IV - Loud buzz when turned on

[Amp Kat]

Well it sounds like you have a either a lose ground or a bad input jack as the buzz is obviously at the front and the back it just basically gets louder depending on where you take the signal from. Could be a loose cap or bad filter cap and that amp is pretty old right ? I would start at the front though because if you can't get it clean from the input jack to send out it won't be good past that.

[Enzo]

OK, several thoughts.

I agree with Jazz, it looks like your power amp is a 400BH. You posted a photo pf the parts side of your power board. Look on the solder side of the board - the underside - and see if the letters 400BH appear.

But I don't think the power board is the issue.

There are two sets of 15v power supplies, two of each polarity. The TL074 on the power board runs on one set - and that set comes from the two zener diodes, NOT the 7815 and 7915. The 7815 and 7915 are ONLY for the preamp. In the uppe4r left of your power board is the cable connector for the preamp, and you have the 15v pins marked. With the preamp connected, you want to measure those two 15v supplies to that ground pin, and also to chassis. See that they are up to voltage on the DC scale, AND they have about zero volts on the AC voltage scale.


But here is the one I bet will pay off: No guitar or anything, just plug a cord into the effects send jack. Now on the free end of that cord, measure for DC voltage between the plug tip and plug sleeve. Do you get about 15vDC?


So with no guitar, it hums, did you say that the GAIN controlk does not turn thge hum down? Ninth control from the left. Also, you have tone controls HIGH, LOW, PRESENCE, and PYRAMID. Did you say those do not change the tone of the hum in ANY way?

Plug something into the PEDAL RETURN jack. Does that stop the hum?

[YYZ]

FIXED

Hello, everybody! The problem is fixed!

Since the possibility of ripple voltage has been the initial main concern for everybody, I took a closer look at the filter capacitors, C37 & C38, on the regulated +/-15v rails. Lo and behold I noticed some discharge from a hole at one end of a C38! Not much, but enough to tell that is was fried.

I replaced both with some caps from Radio Shack (2200uF, 35v), and after turning the amp back on there is absolutely no hum/buzz.

Many thanks to all who guided me through this. If I only had a scope, we could have fixed this ALOT earlier.

Regards,
Brad

[Jazz P Bass]

Glad you have it sorted out.
I thought that you proved the issue back on 09/13/12
Quote:"I got a tiny speaker and wired some leads from it to use as a probe. One lead I included a resistor.

Using this probe with one end connected to ground, I probed the +/- 15v rails.

On the +15v rail I couldn't hear anything since it was pure DC voltage.

But, on -15v rail I could faintly hear the hum/buzz."
That was the clue.
You can measure Vac ripple with a simple volt meter, by the way.
No need for a scope.