Ampeg SVT 3 pro- raise from the dead project

[spacerust]

Where I live there isn't a competent tube repair person. I have to drive almost 50 miles just to have someone look at any of my tube amps and I'm not really satisfied with his work. Anyways I decided to try to fix my Fender Deville that was crackling, ended up with a leaky filter capacitor and I also changed out some resistors suggested by Justin Holton site. Now it works like a charm.

Anyways I've had my Ampeg SVT pro 3 sitting for at least a year and now I want to try to get this one going. It went dead during a gig and I noticed the fuse was blown. I opened it up and I saw that one of the ceramic covered resistors is cracked and another one shows signs of burns. I cant really find a troubleshooting guide for this amp or schematics. I was going to replace the fuse and turn it on to see what it does unless anyone has any suggestions. Originally it just lost complete sound the last time I used it.

 

[DF96]

An expert with the amp in front of him may be able to repair it without a schematic, provided the design does not include anything unusual. Everyone else needs a schematic.

 

[spacerust]

Im no expert, but im pretty good at figuring things out and learning...thanks for the advice though. I like the challenge

 

[Tarzan]

Hi,
Can you give us more details on wich resistors?
Are you confident in reading a schematic?
Do you have some testgear, like a multimeter?
Let us know...

 

[kevinkr]

This thread actually belongs in Instruments & Amps forum, I'll move it. Lots of guys there to help you out.

 

[Andrew Eckhardt]

Sounds like you have at least two capacitors to replace before you're ready to replace a fuse and do a "smoke test". I wouldn't just replace the fuse unless you like snaps and flashes. Get yourself some safety glasses and put together a "dim-bulb" tester at a minimum before powering up with the chassis out of the cabinet. If you go poking around with a meter with the unit live or with unknown charge on capacitors, observe the one hand rule...

 

[gingertube]

Ampeg SVT- pro 3
Thats the one with tube preamp and MOSFET Output Stage right?
Fixed on of those for a local kid (18 yrs old) - he lent it to the organizers of a rock festival, one of the bands bass player pulled it off the top of the speaker stack and it expired when it hit the deck. 4 days out of warranty and AMPEG were unmoveable.

He had no money (spent it all on the amp)so it came to me to fix. The entire output section was blown, all of the MOSFETs and all of the white "rats coffin" resistors in the MOSFET sources.

I put $200 of parts into it and charged him $240 which his dad came good with.

Here are the setup / specifications documents I got for it.

Will go chasing the schematics and post them if I find them.

Cheers,
Ian

 

[gingertube]

Manual including scheamtics can be downloaded free from here:
AMPEG SVT-3-PRO Service Manual free download, schematics, eeprom, repair info for electronics

Cheers,
Ian

 

[spacerust]

Thanks for the info. Yeah, I'm going to replace those first, place a new fuse, test the tubes, and then if all if good, plug it in and see what it does. From there, I will be able to determine what is going on.

 

[gingertube]

When replacing the MOSFETs then replace the insulating washers too.
I actually spent a bit extra cash on high quality SIL PAD insulators on that repair. That reduced the thermal resistance a bit and allowed the MOSFETs to run just a little cooler.

The 5W Resistors i teh MOSFET Sources (current balancing resistors) looked fine but were all open circuit. This caught me out, I replaced all teh MOSFETs and still nada - checked those resistors to find all of them open circuit.

The extra info is what was sent to me in response to my query to teh Ampeg service agents of "what do I set the idle current in the power amp to". Note it says set this when amp is cold. The idle current changes as it warms up.

Cheers,
Ian