Peavey XR 600 400 R

[Wes]

I can't find a schematic for this amp. I'll have to wait until tomorrow to contact Peavey.

My problem is related to the +15 supply.

Without plugging in the preamp board- the supplies sit right at ~ +15 -15.

Plugging in the preamp board with the TL074s and 4558s removed- the supplies are still fine.

If I add the chips to the board, the -15 rail is fine, but the +15 rail will begin to sag lower and
lower. It will fluctuate 8+ to 12+ and then sink to 4+ and then even go negative to -8 or so.

I inserted two new TL074s, alone, and that was enough to cause the issue.

I have replaced the caps on the low voltage supplies, replaced a couple
bad looking resistors, and replaced the 2.2 electros on the preamp board.
Again- unloaded it's fine, as soon as you add a couple chips....

What else could I look for?

[Enzo]

perhaps the ICs are in their sockets backwards?

Pull the ICs. Power up. Pick a 4558 socket and measure voltages at pins 4 and 8 right at the empty socket. Pin 8 should have the +15. make sure the IC is in the socket so its pin 8 is in that corner. if you install only the 4558s and no TL074s do the rails hold up?

On the 14 pin socket, pin 4 has the +15 and pin 11 the -15. Make sure those are in correctly.

I don't have the plain old XR600 drawing here, so I am looking at the XR600B just for fun. Does the 600 have the two preamp boards? One for input channels, and the other for master section? If so, there is a ribbon cable connecting the two, so you can pull that ribbon and leave only the master section powered. That will tell you which board is loading. Hey, make sure the ribbon is on the right pins as it sits.

On the XR600B anyway, the TL074s on the mixer section do all face the same way, but on the master board some have pin 1 up and some have pin 1 down. if you installed them all facing the same, that may be the issue. CHips in backwards may have been damaged by the experience.

Though I have dwelled on the one thing, of course other possibilities exist as well.

[g1]

Not sure if this schematic is what you need. Aside from checking what Enzo mentioned, if it has regulators, have you replaced the +15V reg? Have you checked the rectifier diodes that feed it?

[Enzo]

That is the XR600C, which is even later than the B.

But that is a good clue, the +15 regulator may be bad.

[TomCarlos]

This copy of the schematic also has pc board layouts and you can see the position of pin 1 for the Op Amps. You might need to increase the view size to 400% or larger.

[Enzo]

That is also the XR600C

I now have my paper copy of th XR600. In the master section all the ICs have pin 1 towards the bottom.

[Wes]

Thanks for the quick responses, guys!

I was careful to keep my chip orientations correct. They are, as Enzo has pointed out, all pointing down.

It is the model where there are separate preamp boards- one for the channels and one for the master controls and eq business.
I have only been testing with the master control board and the back, main board that has the power amp and power supplies. I
excluded the channels board early on, but oh yeah- having that plugged up made it worse. With that plugged up there was about
-8 to -10 VDC on the signal wire to the power amp and the +15 rail.

From the get go the voltages on the signal/power plug looked like this:

+15 rail -8/-10
-15 rail -15
signal out -8/-10
ground 0

Plugging into any channel allowed no signal through, but you could hear the DC scratchies when you turned the level pots.
Plugging into the POWER AMP IN resulted in a nice pop, but signal did get into the power amp.

So as far as the regulator- is it possible that it could not sustain the load? The diodes feeding these two regulators are the main HT
regulators- it does not seem that they have their own.

TIP 31 would be an appropriate replacement it seems.

The main filter capacitors are original- is it possible that they could have an effect? When you hook up to speakers you can hear the
put-put and pops that sound like bad caps...

[Enzo]

Bad caps make it hummy. If you think the basic supply is drawing down, monitor it with a meter.

the power supply is on the power amp, and your +15 is collapsing under load. So look at the regulator, under load does the collector voltage fall or does it stay up? Schemo says 40v in and 16v out. is that 250 ohm resistor intact or way high? And at the other side, is the 4.7 ohm resistor close to value or way high? Might as well check the 4.7k at the base. The schemo has voltages at all three terminals of the pass transistor, how close are yours?

Don't just guess and change parts, FIND where the problem is, then replace only the parts that need it.

[Wes]

It turned out to be a burnt up 4.7k at the base. The big 250R was a little high and looking a little rough, so I replaced that as well.

All ICs were good to go!

[TomCarlos]

Good job Wes...

So is the amp working now or any other issues?

[Wes]

It works fine now. I did replace the main filters as well. There was a little popping and hum. They were only 38 years old........