Ampeg SVT-4 PRO repair amp output

[tinchom23]

I have a problem with 2 SVT-4 PRO, both had burnt both channels, all the mosfet, the resistances of .47 ohms, also those of 47 ohms, the preset of the bias, the bias transistors, the exciters, and almost all the MJExx.
The problem is that of the 2 amps, I could repair 1 channel, in the other 3 channels I have 2,6 negative vdc in the output, 14vdc negatives in the paw 1 of the 5532 (schematic TP4 + / - 1vdc),
I attach an image of what I measure in the exit with the oscilloscope with 1khz sine wave.
I don't have a lot of experience with mosfet amps, but I would be interested to learn.
I would thank any help.

Pardon for my english

[MarkusBass]

I have SVT4Pro with similar symptoms (even slightly worse - 40V on output). I cannot find TP4 on the schematic. Is it the pin #1 connected to R111? I would check all following transistors: Q102 - Q107. Q103 has base connected to +15V.

Mark

[tinchom23]

Hi Mark,
Yes, TP4 is the pin #1 of IC1, what you mean with Q102 - Q107. Q103 has base connected to +15V, to me that is not right, check the trimpot maybe is burnt.

Q103 BASE +5VDC
Q104 BASE -5VDC

Tincho

[Enzo]

First off, the base of Q103 is not connected to +15v. Look again. There is a resistor from there to +15, yes, but that is not the same thing. More important is that there is a whole circuit there, so look at the overall picture. There are three 4.7k resistors in series from +15 to -15. That means about 10v across each will drop. And that means we should expect about +5 and -5 at TP6,7, just like the chart predicts. Those parts serve to bias Q103,104. If I had to call that circuit something, I guess I'd call it a level shifter.

The pin 1 of the 5532 drives the output stages. If you have already replaced it, then it is probably OK. Since the input to the IC is fed by the feedback from the amplifier output, any offset there will also show up at the 5532. The 5532 will try as hard as it can to "correct" that offset.

Go along the power tranasistor board and check every diode on it. Check every transistor on it. Measure every resistor on it. Note that R138/139 will be in parallel with the other four sets of resistors in that row, and similar things happen in the negative side. SO in circuit it might red 300 ohms instead of 1.5k. But if any are open, that changed that. You can always lift one end to measure each.

And on the driver circuit page, I would also check every single part to the right of TP4 (pin 1) on that schematic - Q102-107, plus the diodes and resistors.

It is not as large a job as it sounds, and really it is a lot simpler and faster than trying to figure out just which parts are and are not faulty.

[MarkusBass]

I ment just checking transistors Q102, Q103, Q104, ....., Q107. They may cause the negarive voltage on the output. Have you got signal on the output of IC1? The IC is also suspected.

Mark

[MarkusBass]

Of course I didn't mean that Q103 has base connected directly to +15V but rather through a resistor. The same applies to Q104. What are other voltages on Q103 and Q104 (emiter and collector)? +5V on the base seems to be correct.

Mark

[Enzo]

Sorry, i can only take it at face value. I saw two guys discussing this transistor base connected to +15 and not thinking that was right. It is actually connected to +5, I am seeing, so i pointed it out. By stating it is connected to +15 rather implies one ought to find 15 volts there, which is not the case. No big deal, we are all on the same page I hope.

That is why I suggested shotgun troubleshooting as being more efficient - no one has to fathom the circuit.