ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

previous: Erik Miller I've got a tough one on the bench r... -- 1131217264

Re: Ampeg SVT 3 Pro output has DC

11/7/2005 5:40 PM
Enzo	Re: Ampeg SVT 3 Pro output has DC The driver transistors test OK. Famous last words. Your tester down't put the full power rail voltage on them, nor does it ask them to conduct any current. If the amp is spitting out DC, why is there a load on it at all? RUn it naked until you get rid of the DC. That will prevent burning out your load, and it will put a hell of a lot less strain on the electronics. The schematic is like 12 pages 11x17" and includes parts layout drawings and parts lists. Isn't that worth a few bucks? There is a speaker relay, so monitor the output of the thing on the amp side of the relay, that way you can see what it is doing before the relay clicks in. The .47 ohm ballast resistors are on the output bus, monitor things at one of them. Otherwise defeat the relay. it runs off the 65v rail, so you can't just ground the low side of the coil when variac-ing, But the NC contact on the relay does not appear to go to ground - it doesn't short the speaker terminals - so you can tack a short across the contacts and then the output bus will be on the speaker terminals no matter what. If this were my repair, I would run just one output transistor on each polarity. Why blow up 8 transistors when you can only blow up two? Once the problem is solved, stuff the remaining power xstrs. For that matter, there is probably good work you can do with ALL of them out. This is not a big circuit. If the outputs are shorting, make sure the ballast resistors are all good. Make sure the gate resistors are all good. I would replace the 3440 and the 5415 automatically, Q3,4. There is a 3.3v zener associated with each, check for shorts. In fact, just grab your meter and go down the board in diode test and check each and every diode on the board. it will take five whole minutes. A bunch of them are zeners. Q2 is the little MPSA06 in a TO92 leaning on the outside of the heat sink near the bias pot. IS it OK? it is the bias xstr. The small xstrs work at low currents, so set your meter on diode test and check teh junctions of all teh TO92s. Get the schematic and check the resistors for opens.

11/7/2005 5:40 PM

Enzo

Re: Ampeg SVT 3 Pro output has DC

The driver transistors test OK. Famous last words. Your tester down't put the full power rail voltage on them, nor does it ask them to conduct any current.

If the amp is spitting out DC, why is there a load on it at all? RUn it naked until you get rid of the DC. That will prevent burning out your load, and it will put a hell of a lot less strain on the electronics.

The schematic is like 12 pages 11x17" and includes parts layout drawings and parts lists. Isn't that worth a few bucks?

There is a speaker relay, so monitor the output of the thing on the amp side of the relay, that way you can see what it is doing before the relay clicks in. The .47 ohm ballast resistors are on the output bus, monitor things at one of them.

Otherwise defeat the relay. it runs off the 65v rail, so you can't just ground the low side of the coil when variac-ing, But the NC contact on the relay does not appear to go to ground - it doesn't short the speaker terminals - so you can tack a short across the contacts and then the output bus will be on the speaker terminals no matter what.

If this were my repair, I would run just one output transistor on each polarity. Why blow up 8 transistors when you can only blow up two? Once the problem is solved, stuff the remaining power xstrs. For that matter, there is probably good work you can do with ALL of them out.

This is not a big circuit. If the outputs are shorting, make sure the ballast resistors are all good. Make sure the gate resistors are all good. I would replace the 3440 and the 5415 automatically, Q3,4. There is a 3.3v zener associated with each, check for shorts. In fact, just grab your meter and go down the board in diode test and check each and every diode on the board. it will take five whole minutes. A bunch of them are zeners.

Q2 is the little MPSA06 in a TO92 leaning on the outside of the heat sink near the bias pot. IS it OK? it is the bias xstr.

The small xstrs work at low currents, so set your meter on diode test and check teh junctions of all teh TO92s.

Get the schematic and check the resistors for opens.

Replies:

Erik Miller "If the amp is spitting out DC, why... -- 11/8/2005 12:35 AM