New Ampeg SVT Classic Problem

[bnwitt]

I have brand new Ampeg SVT Classic in for service. The amp works fine for about 20 minutes and then it will loose output power and become crackly when you hit a note. When the note decays it will come back to full power.

I am about to check the preamp out and power amp in jacks by putting in a patch cord. There is also a slave out on this amp but i don't know it's function so I'm not sure if it would break the path to the pwr amp.

It would be nice to have a schematic for the power section, but I don't have one. I've pulled the preamp chassis and checked for bad solder points, jacks etc but everything looks good in that part of the circuit. Any advice is appreciated.

Edit: The problem doesn't appear to be in the loop jacks and now that it has really warmed up the output is lost completely. You can do the Enzo test (bang on the top of the cabinet) and hear some crackly output so it is obviously a loose connection of some type. I'm pulling the main chassis now.

Edit #2: I feel like such an idiot. I tore this amp completely apart. those of you that have torn an Ampeg SVT Classic apart know how much fun that was. I checked every cable, every jack, every connection, every solder joint. I checked everything with a meter and magnifying headset. Nothing wrong anywhere. So I was stumped. For jollies I swapped out the two preamp tubes in the preamp chassis box. Problem solved. The very first thing one should do and I saved it for last. I guess it's time to fix my Hickock 750 that went down on me a couple of months ago and go back to my routine of testing every vacuum tube from and amp before going any further. Man I feel like a total dork.

Edit 3: Hey I'm not an idiot. It wasn't the tube after all. When I put the preamp chassis back in and tightened every thing up, bingo problem came back. Turns out it was the noval socket losing contact at the preamp board when the chassis flexed at tightening.

[bnwitt]

Final Post

Ok with my double magnifying wonder hat (its tough getting old) I finally found the real problem. One terminal of C3 right next to the V1 socket had a cracked solder joint. Boy that was hard to find. The amp is good to go. I hope this thread helps someone out there with a similar problem. At least I now know how to get this amp apart easily.

[Enzo]

Glad we could help. I can't believe you didn;t spot that the instant you got back from SUnday School.

[Amp Kat]

That's the easiest amp I've ever helped someone fix.

I've seen on that exact type amp the preamp board heater traces blown in half right after those little blue and white wires that are going thru that clear piece of plastic. Didn't find it of course until I took the whole board off and looked underneath. it

[bnwitt]

Glad we could help. I can't believe you didn;t spot that the instant you got back from SUnday School.

Yeah. I have been working 7 days a week lately. I need to stop that.

[bnwitt]

I've seen on that exact type amp the preamp board heater traces blown in half right after those little blue and white wires that are going thru that clear piece of plastic.

There are two things I don't think should be put on a Printed Circuit board.
1. Jacks of any kind
2. Filament circuits

Of course the jacks get stressed from jacking in and out and cause pin solder cracks. The Crate Palomino I am working on right now has a separate little piggy back PCB on the jack body with wires going to the main PCB. That's how it should be done to avoid flex stress.

I have seen many traces go up in smoke on filament circuits and believe they should always be wires not traces.

[Sir Cuitous]

After all that I'm surprised you didn't add tube sockets to that list. Don't they get pushed and pulled as well?

[bnwitt]

After all that I'm surprised you didn't add tube sockets to that list. Don't they get pushed and pulled as well?

Point taken. Added to my list for sure.