Peavey XR400B not working

[Miss Anthrope]

This PA has barely audible intermittent output. There is no crackling when I turn the volume controls for monitor or master. Not even any noise when I take an input cable or mic cord out of one channel and into another. This is my first PA. It was free from someone's basement. I am not a techie and don't know much beyond basic electronics. Everything inside looks clean and dry, no corrosion, no blown fuses. Using 112H enclosure that worked fine on another head. All new cables. Any ideas?

[52 Bill]

I read the other post, where you said that you got a strong sound when touching the input to the power amp pin at the board.

Did you try the other things suggested in that post as well, like plugging in a patch cord from the power amp in to the graphic out?

[Miss Anthrope]

Thanks so much for responding. I haven't tried that. I am sorry to admit, but I don't quite understand what that means. I know what to do, but I don't understand what the result should be or what it indicates. Will I be leaving the speaker connected to the pa? Am I measuring the output with a fancy device or will I hear something? Can I use an instrument cable as a patch? I am not a techie. I am a quick learner and an intelligent troubleshooter, though. Thanks again.

[52 Bill]

What you need to do when troubleshooting any amp, is to try and isolate the problem. When an amp has no output, it could be the power supply, the power amp or the preamp that is at fault. If you approach the problem by isolating each section you can make what would seem like an impossible task into a simple repair.

When you got the hum from the power amp board you proved that the power amp and its' power supply was working. Therefore the next test would be the preamp section. So either the preamp has a problem or the signal from the preamp is not getting to the power amp.

A very common problem in all amps is the switching jacks used to connect the preamp output to the power amp input. These jacks can become dirty or oxidized causing the switch contact to basically open up and stop the signal from getting from the pre to the power amp. On the front panel of your amp are two jacks; one is the input to the power amp and the other is the output from the preamp. Plug a spare cord from one jack to the other, turn on the amp and see if the sound is restored.

If it does come back, you will need to clean the contacts of the return jack. If it does not come back, then you will need to look into the preamp circuit.

[Miss Anthrope]

OK. The front panel has six jacks plus one for each of four channels. The six on the master panel are FT SW, Aux in, Eff out, Monitor out, main out, graphic in. Given that, which two am I jumping? There are also two jacks on what I would call the back of the unit where the PA panel is. They are for the non powered speakers, right? Also when we took it apart (don't cringe) we found a metal box screwed to the top of the housing. There were four springs holding some kind of assembly suspended inside this box. One of the springs was dislocated and the assembly was resting on the bottom of the box. It appears that it shoud be attached. Could that cause some of my symptoms?

Edit-I looked back at the original thread I had read and was able to understand which two jacks to patch. Still don't get any sound other than an occasional pop when I tap on the microphone. Nothing from guitar at all.

[Jazz P Bass]

Also when we took it apart (don't cringe) we found a metal box screwed to the top of the housing. There were four springs holding some kind of assembly suspended inside this box. One of the springs was dislocated and the assembly was resting on the bottom of the box. It appears that it shoud be attached. Could that cause some of my symptoms?
That "metal box" is the reverb tank.
That spring hanging down on the circuit board may have caused any number of failures.
My honest advise.
I would advise taking the amp to a reputable tech.
This repair may be way beyond your ability to repair it.

[Miss Anthrope]

Well, as the amp was free and we are not professional musicians we were hoping to not invest any significant amount of money. This head would probably find its way onto freecycle long before my hubby agreed to sink steam into a professional repair. It is 30 years old BTW.

The reverb tank was not touching the circuit board, it had just collapsed unto itself. Perhaps this wouldn't affect the overall performance of the head just the reverb effect? I don't know...

Someone suggested replacing the ICs on the mixer panel. Understand what to do, but not where to buy new ones. If the IC for channel 1 is bad (it's missing a tooth) will the rest of the mixer still work or does it break the circuit to everything to have one go bad?

[52 Bill]

Well at this point, you can try and see if the signal from the FX out makes a sound going into another amp. But it seems that you have a problem in the preamp section of the amp.

The first thing that I would suggest would be to test the power supply for the preamp, but to do this you will need a voltmeter. Do you have one available?

[Miss Anthrope]

Voltmeter, yes. But will this test work with the opamp (right word?) from channel 1 pulled? It is missing a tooth.

[52 Bill]

Yes, by missing a tooth, do you mean one of the 8 legs of the chip is broken off? Most people liken ICs to bugs with multiple legs.

The preamp runs off of two 16v power supplies, one positive and the other negative in relation to the ground connection. Attach the black lead of your meter to the metal chassis and use the red lead to measure the dc volts.

Carefully measure the voltages on each of the socket connectors on the empty channel 1 socket. Hopefully you will find both the plus and the minus 16 volts on two of those points.

Remember that working on an amp that is plugged into the wall and turned on is dangerous and can cause you physical harm. If you don't feel that you are able to safely perform this test, then don't.

[Miss Anthrope]

Yes. A bug with many legs. We will try this as soon as we get the kiddos to bed. Those devices labeled Q8 Q9 Q10 and Q11 on the schematics... Do they generally have a white substance oozing out ever so slightly under them? Is it sealant to bind it to the board? It doesn't exactly resemble the consitantcy of any corrosion I've ever seen, but it could be. You have been such a help and I've learned quite a bit. Thanks again!!

[52 Bill]

I assume that you are referring to the output transistors here. They are metal cased, about 1.5 inches x 1 inch.

If you are, the white substance is heat sink grease. It is used to improve the thermal contact between the transistor case and the metal heat sink, so that the transistors run cooler.

[Miss Anthrope]

After a little 3AM research I realized that they are indeed output transistors. Another replacement part that peavey sells. If I did have a hum on the speakers way back during the first test I did on the power amp itself, does that mean that all of its components are working and the output transistors must be fine as well?

Our voltmeter doesn't measure volume of current, just whether or not it's present. I guess if replacing the ICs doesn't do the trick then there is not too much more I can do. Someone at a music shop told me today that this PA isn't worth too much more than the $18.00 I spent on the ICs even in working condition. We don't want to spend $75.00 at a shop when it's 30 years old and only 100W anyway.

This troubleshooting forum has been a blast and I have learned heaps more than I knew before. I'll let you know how I make out.