Need advice repairing SVT

[jeff]

 My uncle's buddy heard that I was into building amps so he's having me ckeck out his Ampeg SVT amp. He said that it blew a fuse so put a car fuse in it. DOH! I took it apart to have a look at it. As I was taking one of the power tubes out it sounded like there was somethin' in it rattlin' around. I figure this was his problem.

 As I inspected the circuit board I saw where a part of the trace was burned off and there's some dark smokey colored residue on the board. The trace is burnt out where the cathodes meet the - side of the bridge rect diodes. My best guess is that the tube shorted, all the current went straight through that trace, through the shorted tube, and burnt it out(sound right?)

 I think I can jump the missing trace with wire but is there any way to clean the circuit board totally?

 I've heard that when a socket arcs out it should be replaced because the carbon scoring(I think that's what its called) could leave a path for it to arc out again. Maybe that's because it's cheap and easier to replace a socket than cleaning out the nooks and crannies of the sockets holes, but can a board ever be cleaned and reused? What should I use to clean it? Can it ever be trusted?

      Thanks
       Jeff

[Willabe]

That amp is one powerful beast. High voltages and a lot of current.        Take your time, don't be in a rush.

Do you have a lamp limiter? Look in here if you don't. This is a must have/must use tool for amp building and repair.


http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


                       Brad         

[Jack1962]

There is a good chance that the tube went south and blew the first fuse and when they replaced it with a car fuse , it allowed to much current flow thru a bad tube or 2 and burnt the trace. Use the limiter as Wiiabe suggested and a new set of tubes and crank that beast back up.

[RicharD]

Once a trace is burnt, it's done for.  I'd remove it with an exacto knife and/or dremmel tool and replace it with a hard wire of sufficient ampacity.  You're gonna hafta be the judge on replacing the socket.  You can probably clean and re-tension it but w/o seeing it, it's hard to say.  There used to be a product made by Bishop Graphics (IIRC) which was a self adhesive trace replacement material.  I dunno if it's still made and furthermore I don't think it would be suitable for a high voltage, high current application.  Like the others have said, take your time and use a lamp limiter.

[PRR]

High-current burns (PCB traces) is different from high-voltage burns (socket arcs).

In the cathode return, just scrape any gross carbon and run a hard-wire.

Tell the owner "it's ruined beyond repair!!", buy the corpse cheap, and fix it for someone who will respect it (not use the 60A fuse from his boom-car).

[jeff]

Ok, I'll clean and jumper.

 What's the best advise for fixing an amp? I'm good at building amps with all new parts but don't have alot of experience fixing amps. I don't want to just assume that this is the only thing wrong with the amp. What else should I check/look for before just jumping the burnt trace and firing it up?

I aready went through and tested all resistors-tested OK.

How can I test caps?

Should I replace the diodes?

I want to make sure I have bias voltage first but that means juicein' up the PT.
What's the best way to test voltage on the transformer before I put the tubes in? If I fire it up with no tubes won't I blow the lower rated caps because there is no current draw through the dropping restsiors, and therefore no voltage drop to the preamp caps?

Anything else I should test before just jumpin and powerin' up?
       
           

      Appreciate the advice
               Jeff

[Willabe]

I want to make sure I have bias voltage first but that means juicein' up the PT.
What's the best way to test voltage on the transformer before I put the tubes in?

Use the lamp limiter.

I've never worked on an SVT, but there's a chapter in one of G. Webers books on setting the bias on them.

Is this amp you have an old one or a reissue? The old ones had built in test points and bias pots. He says to replace all the 10R 5w plate resistors and the 22R 1w screen resistors. Just incase they were strained by a bad tube or bad bias set-up.         

I'd bet physconoodler and Loose Change have worked on them.


              Brad      

[plexi50]

Ampegs are the most dangerous amps i have ever worked on. They get my total and undivided attention when recapping them
I feel like im working on a bomb when i get them in.

[PRR]

> What's the best advise for fixing an amp?

Use your brain.

Something's not right. Riddle: What could cause that?

Say it is blowing (proper size) mains fuses. Could that be a preamp tube?

> If I fire it up with no tubes won't I blow the lower rated caps

How long does it take to read a meter?

The caps over-volt on EVERY start-up, because it takes 11 seconds for the tubes to pull full current. Meter the DC between rectifier and main cap. If it comes up 550V 575V 600V 625V in 2 or 3 seconds, it is probably heading for a right voltage (for an unloaded SVT) and you can shut-down before the caps get much stress. (And OTOH, when I did a 400V conversion on a VT-40, and saw 500V 600V 650V..., I shut-down FAST because I knew I had made a 400V/800V mistake.)

And at some point, the most expedient way to find faults is to smoke them out. However this is safer/cheaper on a preamp than on a monster like an SVT.