fixing solid state amp

jimistone

jimistone

long standing member
I found a pair of eminence patriot speakers in the junk pile at the local music store. The owner said he would take $40 for the pair if the amp was a junker. They were in a peavey FX 212 amp cab. He rummaged around and found the power section. He put it together just to make sure it wasn't a good amp, and also that the speakers were good. The amp, at first, seemed to work flawlessly. But, after a minute or 2 it started cutting in and out....almost like a tremelo effect ecept sporatically. It got worse at higher volumes and better with lower volumes. He said it probable doesn't need much and he would sell the whole amp for $75. I gave him $50 for it. Its,a really nice amp and I would like to fix it.

So far:
I plugged in to the "power amp in" input and it doesn't cut out at all.
I reflowed the solder on the 2 guitar inputs

I have noticed 2 things...
The indicator light for the clean channel dims each time it cuts out and the gain channel indicator light flickers dimly each time it cuts in and out.

Also, there is a bulged cap that's fractured at the top and the stuffing is exposed. It's one of the 4 larger black and white colored cylinder shaped caps.
I have a good cap just like it in another circuit board, but it's gonna be a bitch to pull that cicuit board so I haven't replaced it yet.


Could a bad cap cause that kind of cutting in and out problem?
 
jimistone said:
could a bad cap cause that cutting in and out problen?
Click to expand...

Definitely.
Full electrolytic recap before you power it on again. ;)

Not sure what experience you have but be careful...The larger caps in the power section will retain a hell of a kick and quite happily share it with you.
Disconnecting mains isn't enough - drain the caps too.
 
Thank steen. Yeah, I always drain the caps when messing with tube amps, but I didn't know solid state amps would hold a charge.
 
Greg_L said:
Lol. Just fix that cap. I don't know if it's your problem, but it is definitely a problem.
Click to expand...
Yeah I'm going to. All the effects work flawlessly on this amp... and the sound very good too. If I can get this one problem fixed it will be a good amp for taking out to jam or gig with. Then I can leave my other Amos in my studio.
 
The blown capacitor will likely be one of a pair. You need to find out if the amp circuit is the now almost universal +V 0 -V style and with a DC coupled speaker.
Otherwise it will likely have a single 60-80V +ve supply and a coupling cap, 2,200 to 4,700mfds to the speaker. IF that naggered cap IS the speaker coupler DO NOT use the amp until you change it! You could easily burn out speaker and amp.

If it is a split rail you must change them as a pair otherwise the amp will hum like cnut and the other cap surely fail shortly (no pun intended!) .

After the work connect a 100Ohm or so 5W resistor in place of the speaker and check the DC voltage across it. Ideally zero but you can live with 10 millivolts or so (that's 10/1000 VDC! i.e. not a lot) More than that and the amp could suffer thermal runaway. N.B. This last only applies to split rail DC coupled amps. In fact even with those, for geetar purposes it is often better to AC couple the speaker through 1000-2000mfd anyway.

Very often with these old transistor designs it is easier and better to just rip out the old PA and fit a modern module.

Lastly. Yes, the caps could hold a charge but only a few tens of volts, you would barely feel it unless you have very girly hands! Watch out for sparks tho! Can bugger transistors.

Dave.
 
The Peavey parts department is usually pretty reasonable on pricing repair parts. You might want to call them to see how much they want for a new power supply board. They are located in Meridian.

GLen
 
jimistone said:
Thank steen. Yeah, I always drain the caps when messing with tube amps, but I didn't know solid state amps would hold a charge.
Click to expand...
All caps hold charge. That is pretty exactly what they're for. Not much different than a little rechargeable battery. Bigger caps will store more charge, so take longer to charge up and drain off, and in most power supply situations there isn't a particularly good path for it to discharge through, so it will hold charge for quite a while. Those bigger caps storing more charge means they also have more current available to ZAPP you with. They say "it's not the volts that gets you, it's the amps", but I think it's actually the power which is volts times amps. The big fat cap across a power supply plugged into the wall can have a whole lot of both, so...

I say pull it out of there and see what happens. If it's a power filter, it'll hum, but should work. If it's a speaker blocker, it won't make any sound, but I can't imagine why it would hurt anything more than starting up the amp without a speaker attached. That's not good for tube amps, but SS usually don't mind a bit. Course, if it's a speaker blocker, it can't really be the problem you're trying to fix.
 
the cap is in a group of 3 caps. They have clear stuff holding the together. It looks like they've been squirted with clear silicon. Anyway, I pulled them already...the amp is completely disassembled. I bought 3 caps from the TV repair guy today. The caps are 1000uf 35V.
I think a new loaded circuit board is a little over $200. (according to a discussion on another forum.
I don't want, or need, the amp bad enough to sink over $200 in it.
 
jimistone said:
the cap is in a group of 3 caps. They have clear stuff holding the together. It looks like they've been squirted with clear silicon. Anyway, I pulled them already...the amp is completely disassembled. I bought 3 caps from the TV repair guy today. The caps are 1000uf 35V.
I think a new loaded circuit board is a little over $200. (according to a discussion on another forum.
I don't want, or need, the amp bad enough to sink over $200 in it.
Click to expand...

You are lucky it is clear silicone (aka snot) Hitachi used to use some black shit that resembled roofing pitch. Made getting bits out a bloody nightmare.

Have you found a schematic? I cannot from the model number given. The caps are probably part of a split rail, a 25 -25 supply would easily give 50W into 4 Ohms.

Yes, LV trannie caps store a lot of charge but the circuit drains them somewhat (a cold valve don't) The charge stored on a 50mfd at 450V is about the same as that on a 1000mfd at 25V but skin resistance does not allow much current to flow at 25, even 50 volts. But! Don't bet on it! Wet skin can have a low enough R for very low voltages to be dangerous. (I am presently messing about with solar power and car batteries and although I KNOW 12 volts can't hurt me my training over 50yrs still won't let me touch a bare terminal!)

Dave.
 
Thee is no schematic available on the net as far as I can find. Peavey sells the schematic for $2.50.
 
Well, I soldered the 3 capacitors in today and tested it out. I turned all the volume pots down and plugged a guitar in. When I turned the power on it was just humming loudly. I pushed the channel select button and it started making a loud feedback type noise and the same capacitor that was blown out before was bulged and very hot to the touch. The other 2 were not hot and didn't bulge. Also, it blew 2 fuses when it make the loud feedback noise.
Oh well, ya win some and you lose some. I only paid $10 more to get the amp than I would have paid for just the speakers. I got a nice 2/12 cab with wheels on it for the extra $10 so I'm happy.
:)
 
Cool.
Sounds like it's not big loss but if you do decide to tinker or investigate any further, I'd strongly recommend testing with a speaker you don't care about.

Without a schematic is tricky but hey..post up a few decent pictures of the board if you're bored. :)
What's the exact model number? I'm finding nothing for FX 212.
 
The only thing I can think of to make an electro cap fail like that would be reversed polarity. You did make sure to orient it correctly? I mean, I'd imagine the factory part was in correctly, so...

Could there be something fucked in the rectifier section that's letting either the wrong side of the rail or just the full AC wave hit that cap?

A schematic would be soooo nice. Don't spose you feel up to tracing out the circuit around that cap?

Edit - or I guess just way over voltage could do it. That still points to something upstream like the rectifier/regulator section of the power supply.
 
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