Peavey Vegas 400

[catstrat]

I have a Peavey Vegas 400 that has low voltage at pin 8 of all the op amps. Pin 4 is the right voltage of 15 volts negative. I replaced both zeners in the low voltage supply and still no difference. R 78 burn after just a second or so. If I remember those resistors come off of pin 3 of U 4 and go to ground. What is the reason for both positive and negative voltage going into pin 3 of some of the op amps ? Any help is appreciated. Thanks

[J M Fahey]

Catstrat, you are an old timer and know the drill
Post the schematic, otherwise R78, U4, etc. mean nothing to us.

As of

both positive and negative voltage going into pin 3 of some of the op amps ?

there's a little contradiction there, a voltage can't be positive and negative at the same time

[g1]

There doesn't seem to be a schematic available online. However, Teemuk was kind enough to draw one up elsewhere.

[J M Fahey]

1) thanks Teemu
2) how can R78 burn while R79/80/81 in parallel live a happy and productive life?

[catstrat]

Vargas 400

Hello everyone, I have a schematic Peavey emailed me. However I'm forced to use my wife's iPad because the main computer is down and i have a bad enough time using that one but this iPad really confuses me. I'm not computer savvy at all. But I'll try to post the schematic. There are things with this circuit that just don't make sense to me. I was hoping some one else already had the schematic. The amp has a hum that originates in the pre amp probably due the the voltage problem. I've seen this problem before in another Peavey and it was a Zener. Give me time to try to post the schematic. At least I did fix the Fender stage 160 a few months ago. Thanks.

[52 Bill]

My first guess would be that if R78 burns, IC U4 may have a problem. Are the chips socketed on this one? If they are try pulling U4 and see if the voltages at pin 8 comes back up.

The diodes that connect the + and - supplies to some of the inputs (pin 3) are there to try and keep the input balanced at zero volts between the two supplies.

[Enzo]

I am doubtful about the four parallel resistors, The similar Nashville 400 has two in "parallel" there, but each actually goes to a separate ground circuit, they are not strictly in parallel.

Ask your wife how to upload a file from her pad. The schematic is after all just a file, no different from posting a photo.

The pairs of diodes near input and output jacks are clamps. If a signal or other voltage comes into the input and exceeds either 15v supply, the diodes go into conduction and clamp it there. This prevents inputs to the IC above the rail voltage, which as far as the IC is concerned would be reverse voltages and thus destructive.

You do have zener regulated supplies, but in the case of the low one, did you check the voltage coming into the zener? The supply starts out as something like 25v, then drops through a 150 ohm 5w resistor to the zener. If that resistor is funny or the 25v low, your supply will be funny too. If the +25 filter cap is tired, there will be excess ripple, which would likely leave a lot of ripple on the +15. That would cause it to read low and also cause hum. Worth checking.

I agree with Bill that if something associated with U4 is burning, remove U4 and see if it makes the difference.