Peavey 2000 draws huge amounts of current and im stumped.

[Andrew58]

I got a peavey 2000 a few days ago, i tend to work on these amps alot and so far have been able to fix each one but this ones causeing me headaches it came in with the complaint of it smoking. so i got it, replaced the sacrificial triac. and a speachel series ic i looked for any obvious troubles like shorted drivers, outputs, and diodes and the like. all seamed to be fine. i tunred it on slowly with just channel b in it with a variac it draws 5 amps at 100 volts with or with out a signal through it. when i put a wave through it, it put out a good sine wave. but ive fixed a few of these and never noticed it draw so much current. Then i was like well ok ill try channel a. so i did the same to it And it draws Huge amounts of current at about 20-35 volts it pulls 6-10 amps, but it puts out a nice sine wave . i know if i were to plug it in it would def blow. im really lost in this and have called peaveys tech support several times. they seamed to be stumped to. so please if any one on here knows any solution or has come across this problem before then give me some pointers or lead me to the right place to get some answers. Thanks

[Enzo]

To be fair to Peavey, they are not stumped on how to fix it, they are not able to tell you how. Unlike here where we can take three weeks of back and forth, and explain anything that needs it, they have the time to point out areas to explore and that is about it. They don't have time or manpower to teach. I don't mean that unkindly.

First, the amp has serious issues, so DO NOT use a load. If you load an amp and there is DC offset on the output, it will indeed draw tons of current - most of which goes into the load. Once the amp makes full output signal and does not require 20 amps to do so, THEN we add a load and see what happens.

Whether it passes signal or not at reduced mains voltage is really pretty irrelevant. To rebuild an amp, first we get it to sit there, stable, with all its DC voltages correct or within limits, no DC offset on the output, and current draw at reasonable levels. Only after that do we care about signal. If DC ain't right, signal won't be right either.

Once the DC is OK, then we apply a signal, but NO LOAD. We make sure the signal will amplify to almost the entire power supply rail voltage, and that it will either limit or clip there. ANd any limiting or clipping must be symmetrical. The amp when not loaded should not draw any more current when amplifying a signal than when it is not. The signal should not have any visible distortion.

Once THAT is accomplished, THEN we apply a load and slowly bring the signal up frome to work with. If it still draws 5A, then something is turning on the outputs together. I would start by tacking a shorting wire across CR108 or CR110, or both. They are over near the input IC. On the drawing they are in the vertical column of diodes. That will reduce the on bias and hopefully cool things off for testing. Leaving one shorted may be a permanent thing, though. If that brings current down somewhat, we need to explore the DC voltages. The pos and neg sides shuold be more or less the same, except polarity. The output bases should sot at about .4-.5v. The driver bases should sit about a volt. (Q106,107) The bases of the pre-drivers should sit about 1-1.5v (Q104,105) And note that the predriver bases are kept apart voltage wise by that column of diodes. Those are two dual dioses and two single diodes, for a total of about six junction drops, or roughly 2.5-3v, which s exactly what we wanted to find between the predriver bases anayway, right?

On the drawing, everything to the right of those diodes will be involved in getting the DC voltage and currents right. Once it is OK, then the IC U100B will yank it up and down with the signal. But even when it is doing so, that 3v separation will be constant. WHen I short a diode ro two, it leaves the base voltages a little lower, so the transistors are not turned on as much, so less current flows.

Then the channel with the big time problem, is still blown somewhere. Either the outputs are shoprted or something is turning them all on at once, or there is a serious DC offset fighting the triac there. Lift the triac off the board to clear up the testing. And work without a load until the current is tamed.