Peavey 400 Series w/ DubStep Wobble(problem)

[wildfirexxl]

I have recently gotten a Peavey 400 series bass head off ebay for a good deal. I love older/vintage equipment. I did not get to test it for a few weeks, as i did not yet own my own cabinet. The cab I got is a standard 2x15 cab that is the right ohms for my head.

The Problem
Head turns on, it's awesome, loud, clear, beautiful and then after about 15 seconds it cuts the volume by 3/4 and gets a resonant wobble that changes with whatever note I play on the fretboard. Then, if you turn it off for about 10 minutes to rest it works fine again. That is, for about 15 seconds.

It does it on both clean and dirt channel.
Sometimes it lets you play longer than 15secs/an hour.
I've taken it apart multiple times and cleaned the circuits thinking it's over heating from dust.
I've now tested it on two different cabinets: Acoustic 8x10 and Peavey 2x15 both rated at 4 ohms.

These are good amps. Lots of my friends have them and I see them in other bands all the time. So I'm not looking for any put-downs on my poor 400, just lookin' for a little wisdom on what might be wrong.

Thanks guys for the

[wildfirexxl]

anybody?

[sammyfallen]

sounds like tech time. I had a road 440 do something similar. It had a solder break when it fell in our van due to being rear ended by another driver.

[wildfirexxl]

What did you end up doing?

[TheSuzie]

Does the 400 use any socketed components like some other Peaveys (usually it is op-amps in the preamp section)?
If so, the first thing I would try is to CAREFULLY reseat all the socketed chips. Lift then slightly with a mini screwdriver without removing them from the socket and then push them back down to reseat them. While you are in there, you can inspect the board for bad solder joints and cracked circuit traces with a magnifier. Also look closely for any cooked components. Sometimes you won't notice them on first glance.

Hope this helps
S

[MuthaFunk]

I've seen this symptom before on vintage stereo receivers. What was happening on them then was a power transistor biasing resister had gone out of spec. and once it heated up it would drift in value to the point of shutting of the output transistor it fed.

You're going to need a tech to troubleshoot that problem as I think it's a failed component triggered by heat. What happens when you leaving it idle for 5 min and then play it? Do you get 15 sec of play time or is it already off by 5 min of idle with the volumes down?

[BassmanPaul]

I vote for tech time.

[wildfirexxl]

Took it to a tech, turns out two capacitors were bad. Easy and cheap fix.