Peavey 400b bass amp series III - fuzzy/distorted tone and sound dips in and out

[Kingforaday]

Hi guys,

I bought this amp a few months ago and with lockdown etc there wasn’t really an opportunity to try it... but it was cheap enough and came with the cab (which was the bit I actually wanted anyway really)

I took it home and tested it and the pots were crackly and there was a constant distorted tone and the sound dips in and out.

I took it apart and got some switch cleaner into the pots and switches and the crackles have gone, but still the distorted dipping in and out. Unfortunately lack of time and feeling like I’d been sold a dud, meant I put it to one side and haven’t looked at it since. I’m now finally keen to get it sorted.

So, what would be the next thing to try?I’m handy with a soldering iron but lack the knowledge of the inner workings of something like this. I’d really love to understand how to fault find on things like this, as I know these old amps are considered easier to work on.

I’ve modded amps before but it’s always been via very detailed instructions, so I’m sorry I say I don’t feel like I actually learned from those experiences!


thanks

[Jon Snell]

I would say either crossover distortion; check there is 1.3volts across CR13 and someone has mistakenly replaced it with a single diode as it is a double diode or you have a damaged loudspeaker with a rubbing speech coil; gently move the cone by hand and listen for scratchy noises.

[Enzo]

Isolate the problem. First, the power amp IN jack. The whole signal runs through it. If the cutout contacts there gert dirty, this can result. MEASURE the resistance of those contacts. If they are more than about half an ohm they need cleaning. If they measure say 2 ohms, it is tempting to assume they are OK, what is 2 ohms after all? But that 2 ohms instead of zero means the contacts are not clean. But beyond that...

MAke sure to use real speaker cords and not guitar cords for the speaker. Try the amp though a different cab. Still distorted? Plug your signal into the power amp IN jack. You now lose the preamp, but is it still distorting?

Also,, plug into the regular input, and run a cord from the preamp out jack over to some other amp. Does it sound OK over there? Or does it distort over there too? (Unplug the speaker from THIS amp for that.)

SInce it comes and goes, apply a signal - I use a CD player - to the amp, and poke and prod the boards with a wooden chopstick, or other insulated tool. ANy reaction to poking?

Verify your power amp board is 400B/G

REmove the small IC from the power amp, any change? NOTE WHICH WAY IT GOES IN FIRST.

[Kingforaday]

Wow! Thanks for such a quick response guys! I posted on a bass forum and waited a couple of days... zero replies. I’d also messaged the previous owner and got a “it’s an old amp, you have to expect these things” kinda response.... so to get some knowledgable and potentially helpful replies already is great!

[J M Fahey]

Bass Forums are typically *player* Forums.
Nothing wrong with that, but that´s their focus.

Here you got into the Musical Tech Forum and got answered by one of the best available, lucky guy.
Follow his advice and any other coming forward here.

[Chuck H]

I’d also messaged the previous owner and got a “it’s an old amp, you have to expect these things” kinda response....

What a charmer.

Like JM said. Two solid tech's responding already. If you do any requested tests and report you'll get sorted out for sure.

[Kingforaday]

Ok, I’ve had a little time to open this back up and have a look... I can’t seem to upload pics of my findings?! Every hosting site link I try says ‘invalid link’ and when I try to upload (heavily reduced) pics it says my ‘user group upload quota’ has been maxed.

Anyway, going through Enzo’s fault finding tips:

no it’s not the cab. I’d already tried a different cab and run a different amp into the original cab.

yes, I always use speaker leads (pretty high quality ones I may add! Ha!)

tested the power amp in jack - 0.6 across the contacts?

I plugged into the power amp and it’s the same, distortion and sound dipping in and out... I also noticed that if I play really gently no cutting out (still distorts though) the harder I play the more it cuts out.

tried the preamp into another amp, other than being a little crackly, it sounds good! No distortion, no dipping in and out.

which points to the power amp, yes the board is 400 b/g.

Whilst looking in to confirm that, I did notice something that looks very wrong. As I said I’m not really up on the inner workings of amps, but it just doesn’t look well! As soon as I figure out how to upload a pic I will

[Enzo]

Oh just describe it for now.

AS a new member, you may not have enough posts to allow certain uploads.

[Kingforaday]

well I seem to be able to upload YouTube videos, so here’s a 15 second video, five seconds on each pic, getting closer in on the suspect component... what is this and is it meant to look like that?!

[Enzo]

That is an inductor. Look at your schematic, over to the right at the output, it is L2, the one with a 22 ohm resistor in parallel. They all look like that.

Give it a gentle tug to see if one end of the wire has snapped. Likely not, but takes 2 seconds to check.

[Kingforaday]

Ok, yeah seems connected (it just looks odd but like I said I’d never seen one before)

here’s a vid of the normal input, then power amp input - showing it’s the same on both

[Kingforaday]

and here’s a video of the preamp out into another amp, it’s working but now has a strange constant noise

[Enzo]

I don't have sound on my computer, sorry.

[Kingforaday]

Checked the last couple of things suggested Enzo;
No reaction to poking around with a chopstick as far as I can tell and I tried removing the small IC from the power amp section and it made no difference.

anything else you can think to try, that a mere mortal like me can do? Or do I need to give in and pay a professional?!

[Jon Snell]

The diode CR13 is a double diode. I wonder if it has been changed.
With care, measure the voltage across it. I suspect there will be less than 1.2volts and that will cause crossover distortion.
Have fun and keep safe.
Now you have youtube video with sound, it could be just a dry joint but to guess is folly.