Peavey Bass Mark III 400BH won't power on

[xr7390gt69]

OK. I bought a speaker cab and was able to get the guy to throw in a broken bass head with it for nothing. Cosmetically, it is in great shape. And, I was in the market for a head, so it'd be great if I can fix this one!

First of all, I must admit that I am a complete novice to electronics repair. I have a voltmeter, a soldering iron, and a layman's understanding of how to use them. I've successfully used both for car repair and home maintenance. But, I realize this is a whole different ball game.

So, if this makes me too ignorant to be helped on this forum ... I understand.

My amp can be plugged in ... and the switch turned on ... with NO fireworks. I have pulled the main fuse and it is OK.

Once turned on, I hear the power transformer humming slightly (if that tells us anything) ... but, the light doesn't come on at the front panel.

I pulled the back panel (which I believe is the power amp section) off for examination. I do not see any components that appear to be melted or leaking.

The only thing I noticed was on the main power switch. The flat copper strips that jump diagonally to the opposite corners show signs as if a small arc may have hopped across them and melted tiny notches out of each strip.

A few continuity tests seemed to verify that the switch is OK. And, as I said, the power transformer does hum when the switch is turned on (in either direction).

Maybe the arc burns are a clue ... maybe not.

Either way, I would appreciate any help. Thanks!

[xr7390gt69]

OK. The other 2 fuse holders were empty and looked so pristine that I thought they might not be used with this model. However, I found a schematic of some version of the 400bh. It said they should be 1amp.

I assume its OK for me to install a couple and see what happens?

[Enzo]

Can't think of any other way to see what's wrong.

Sometimes parts burn up, but MOST electronic components look no different when they fail.

[xr7390gt69]

Thanks Enzo!

Should I try popping in a couple fuses to see what happens? If so, any words of wisdom?

I checked the row of 8 "cement" resistors for open. They were all good. I don't know how to check the components directly under them (with the metal back and a nut on each ear). But, the "gasket" on one of them does look a little suspicious.

I checked some of (what look like) the "cement" resistors on the the power board. Some of those read open. But, I dunno what they are or whether they should or not.

PLEASE NOTE: I am not expecting anyone to walk someone as ignorant (on this subject) as me through a complete trace of this entire amp (unless they want to). BUT, if there are a few components that are common to fail on these amps ... I would like to try my hand at tracking those down and making the repair (with guidance from this forum).

If it goes well, I would consider taking some classes to have a fallback vocation as I am in my 40s and could be jobless one day or find myself unable to play anymore. At least in amp repair, I could keep my hand in the music.

Thanks again!

[Enzo]

Yeah well, I sure ain't getting rich fixing these things...

Those cement resistors all say on them what they are, most are 0.33 ohm - that is 1/3 of an ohm, yes - and a couple are 1 ohm. They are either OK or open, they won;t be "wrong." If they check open, they likely are, and that means one of those metal things with the screws is shorted. The metal things are your power transistors. They have two legs underneat, and that metal casing with the two screws is their third terminal. You can check with an ohm meter for continuity from the metal casing to either of the two pins. It should NOT be shorted to either. Your gasket is an insulator, typically mica or silicone, The transistor body is not to make contact with the metal frame it is screwed to. There may be some ugly white grease oozing out, but that is normal.

Clearly a lot more can be wrong than that short list.

[jrfrond]

If you can troubleshoot electronics at all, those power modules are relatively easy to service. However, from what you describe, it sounds like you may be in over your head. Just calling it as I see it.

You got it for nothing, so your best bet is to bring it to a real tech. Chances are it probably won't cost a whole lot to repair.

Did you actually try to pass audio through the unit? The reason I am asking is that Peavey amps have a nasty habit of the pilot LED in the front panel breaking it's solder joints loose from the PCB. Just a thought.....