Peavy Session 400 Problems

[BlackOrchid]

My dad gave me his old Peavy session 400 amp about a year ago and I haven't touched it much. I got it out recently and it either plays fine for a while, then makes a loud cracking sound and the volume goes out. Sometimes it turns on but just doesn't make any sound. I must admit that I know practically nothing about electronics in general so I have no clue what is wrong. I did the basics, made sure it was plugged in, checked all of the connections on the inside and tried different cords or plugging my instrument into the other jack. I do not have a spare amp or speaker to try to isolate the problem.

Another thing I forgot to mention is that my father put in a Peavy Black Widow speaker, either the 15" or the 18". It used to work fine for him like this, and I had gotten a chance to use it a little before I started to have these problems.

Any suggestions or help any of you have would be much appreciated. Thank you!

[Enzo]

OK, turn the reverb down first.  Now when it does this, ball up your fist and whack the top hard.   Does that make the sound come back?  Seriously.   You could use a rubber mallet, but fists work fine.  If that works, it means there is a loose connection inside.  COuld be wires, could be cracked solder, whatever.

I don't think your amp has any extra jacks, like FX loop or power amp jack. Right?  But inside there are a number of white plastic "Molex" connectors, where wires plug onto the circuit board.   Make sure all those are snug, maybe pull each half way off then push back down.  Especially one on a cable from the preamp board to the power amp board.

With something playing through the amp - I use a feed from my stereo receiver, but a CD player or most anything will do - use a wooden chopstick or other insulated thing, and go down the board pushing on and slightly moving each part, looking for something that reacts.  Again most likely bad solder.

After that we are into electronics, and I'd be looking for DC voltages that are wrong, or electrolytic capacitors that are not working right.

[BlackOrchid]

The sound cut out and I whacked the top pretty hard and the sound cut in and out when I did. I'd have to open it up to look for loose wires and cracked solder then right? If so i can do that easily. Thank you!

[Enzo]

Good, that means we don't likely have a voltage getting out of line from a leaky cap or something.

Spray cleaner into every jack, and push a plug in and out a few times to spread it around.  Check or just resolder all solder joints on all jacks.  I make a policy of checking the solder on all controls as well, as long as I am in there already.

[QReuCk]

Well, my top on list suspect in that case would have been input jack, but if it does it with both input jacks, that reduces the likelyness of it.
Next suspect would be the connexion between power amp and speaker, especially as the speaker has been replaced: the speaker wires are exposed in an open style combo and the connexion could have been mechanically stressed easyly (as for the input jacks).
So really thoroughly check these connexions (input jacks to preamp board and power amp to speakers).

If preamp and power amp are on different boards, I would also check meticulously the connexion between them.

As it happens after some playing, next suspects on the list would be any solder or connexion near some place where there is heat (probably near transformers, power amp chips, etc...). So check for places on the circuitry that are a bit warm after some playing. The fault could be located not to far from here.

So basically follow Enzo's advice but maybe start with these connexions rather than checking every trace in the circuit. At least that's what I would do.

Pots sometimes crackle and function intermitantly, but dirty pots are usually pretty obvious to spot without even openning the amp, and usually they do their thing either at powering up or just when you turn them up or down.

[BlackOrchid]

After I opened it up I saw a broken capacitor. I'm planning on replacing this but could it be the cause of my problem?

[DrGonz78]

If everything else is looking great then it probably is the problem. I mean it looks like a little critter came up to it and took a good bite out of it. While you are soldering a new part back in it would be wise to take a good look at other solder joints to re-flow. However, you might just want to change out that part first, then test it. That way you know exactly the outcome of the bad part in relation to the amp repair. There still could be other problems to address so take it slow and methodical.

[BlackOrchid]

Thank you! As an added note, does anyone know where I could find that capacitor or an equivalent? I've looked for a while, but honestly, I have no idea of what the numbers on it means.

[Enzo]

It is a plain old vanilla 0.05uf 100v disc ceramic cap.

Not a critical value, and any higher voltage part will also work fine, if you have them.    In modern times they will be more likely printed 0.047 rather than 0.05, but the difference is irrelevant.


I bet there is another one nearby.  The white connector with three red wires next to it goes off the board to one of the large filter caps clamped to the cabinet, right?   Is there another cap by the connector for the wires to the other cap?

That could be making noise, I doubt it would cause a complete loss of sound output unless it blew a fuse.   You can just snip it off the board during testing, and if the amp now works flawlessly, install a new one.

[Roly]

I'm going to stick my neck out and opine that this broken cap isn't the cause of the problem.

[DrGonz78]

I am still dying to know the outcome here... Don't give up is my best suggestion. Replace that one part to get it out of the way and as Roly say's "isn't the cause of the problem" Come back and let's get this thing figured out.

[Roly]

I have a mental file with lots of jobs that were left hanging in the air for all sorts of reasons.

One in particular was the sequel to this Gibson repair.

Some time after these events she approached me and told me that the amp had "died".  Since this was only two or three months later I pressed her to take advantage of my "free return" conditions (and while I say "three months" I will accept any reasonable return out of concern for my reputation).

Well she didn't bring it back to the workshop, but every time I was at the local store and ran into her she would make a big song and dance about it in front of everybody, and I would once again tell her to bring it back because I stood by my work.

Eventually after about six months of this nonsense I got a bit edgy and told her to bring it back for a free repair or shut up, to which she responded that she had taken it to somebody who had charged her a lot of money to "fix it properly".  Given the time I had spent on what turned out to be a love job, no profit at all, I was very unimpressed.  :trouble   I guess I shouldn't be surprised when I was brought a "valve amp" that turned out to be solid state.   :duh

But that's sometimes how it goes.

[DrGonz78]

Hey Roly! I had read this on your page a while back and it was a great read. My favorite line you wrote... "I have developed an idea over years of faultfinding (ranging from mine safety to Intensive Care) that the real problem is you, not the equipment. The fault simply is what it is, just lying there waiting for you to recognise it. It isn't hiding, or being difficult, it's just that you haven't looked at it yet in the right way to see it."

That is a really well put statement!  :dbtu:

[Roly]

Meh, that's nuthin' only took 40-odd years of fixing stuff, blaming the equipment for my inability to see what was wrong, to come to that insight.  I could have saved myself a lot of frustration had I realised that sooner - the reason you don't understand what is wrong has nothing whatsoever to do with the faulty equipment and everything to do with your perception of the problem.  (Joseph Weizenbaum touched on this in his Computer Power and Human Reason, talking about "the law of the fault", it just took a while for the penny to drop for me).

It's a long time since I wrote that, but I can tell you what the associated bit is without looking; it's the assumption that whoever had done the previous repair had used a transistor in the VAS that had sufficient voltage rating, and it was only when I made that assumption specific and checked it against the datasheet that I suddenly saw what was happening from a different perspective - that assumption was wrong.

{in a similar way, there is no such thing as a "buggy" computer programme, both buggy and un-buggy programmes run in some way (which makes no difference to the computer if we cheer or scream with frustration and pound the lid), it's just that the "buggy" one doesn't produce the desired result - so a faulty amplifier is running in some manner, just not the way we want.}