Peavey Combo 300 Bass Amp makes loud noise when turned on.

[PfeifferElectronics]

As soon as the amp is turned on, a loud, distorted hum comes out of the speaker. Front control knobs do nothing, which makes me think it is in the power out section. This amp uses 400BH drive board and output board.

Combo300.pdf

I'm having trouble taking live readings because the noise is so loud. Is it ok to disconnect the speaker on an amp like this? I've always made sure there was a speaker connected as to not over load the output transformer but that is on a tube amp.

With the output board removed, I took readings on the power transistors pins (pins going through the board on 6357s) and they are all around 1.6 ohm except the drivers which are 7.5M to 8M.

I also took readings on the P600D diodes and found them to be very different. Forward resistance (black probe on cathode), both read .5M. However, reverse resistance (black probe on anode), one reads 35M, the other only 15.5M. Not sure if this is the problem or not.

I've got a lot to learn but I sure love this stuff. Thanks for any help.

PS: also, someone modified this amp with a switch on the speaker out jack as to bypass the internal speaker and redirect to an external speaker only.

[Jazz P Bass]

Peavey 400BH Schematic

Yes, disconnect the speaker, please.

Get on the output jack & measure volts dc with your meter.
I think you will find a high voltage which means the output stage is blown.

The readings that you took on the output transistors indicates one or all are shorted.
At this point it is best to remove them one at a time until the short clears.
Make sure you check the 0.33 ohm resistors for opens.

Do you have a 'diode test' feature on your meter?

[PfeifferElectronics]

I disconnected the speaker and connected my voltmeter to the speaker output. I'm reading .5V DC and 20V AC

[Enzo]

That is actually a good sign. Instead of a blown output stage putting large DC on the speaker, it looks more like you have a bad filter cap, or one with cracked solder.

You have two large filter cap cans at one end of the power amp board, try resoldering their connections, worth a shot. If that doesn't work, probably one is bad. You should have two 50v DC supplies, one each polarity. See if one is very low and full of ripple. If so, it needs a new cap.


Those caps were 5000uf at 55v, totally non-standard values, and they are also no longer available. The current standard value cap to use would be 4700uf at 63v. Mouser or someplace will have them. If you order the original cap from Peavey, they will send you the 4700uf ones instead.

ON the new capss, the spacing for the connections will be different, so we usually drill a new hole in the board, being careful where we do it.

[PfeifferElectronics]

You were right on Enzo! I pulled the drive board and the power caps had already been replaced with 4700uF caps but whoever replaced them, did a really poor job. I actually soldered in a piece of copper solder wick into the connection because the trace on the board was coming up. I removed all the solder and resoldered. It works great now. Thanks so much!