Peavey 400BH problem

[Drewn]

I bought an early 80's vintage Peavey 6 channel Powered PA recently. (Model XR 600B. (It says 400BH on the back.) It has been working great until yesterday. It was turned on and working when it "popped" and then cut out. The power light remained on. I tried turning it off and then back on. It would work for about 5 seconds and then "pop" and cut out again. When I got it home, I tried it again. Now it hums loudly. The inputs do work but they are softer then they were and sound distorted. Any idea where to start troubleshooting this?

Thanks!!

[clintronics]

Start here

Power supply caps go bad in these quite often. Check your voltage rails. See if they're there and look like they are being filtered.

[Drewn]

Thanks for the help!

Thanks,

Any good sources to order replacements?

[Enzo]

Before you replace caps, stick a volt meter on them. SHould be about 50VDC on each. Of one reads about half that, flip the meter to AC. SHould be almost no AC voltage reading. if it measuers 20VAC or something, then the cap is dead. These caps last a long time. The original 5000/55v caps are no longer available, but the common 4700/63v cap will work fine it their place. If you order replacement 5000/55v caps from Peavey, they will be sending you 4700/63 anyway. PV will sell you any part, but Mouser or Allied will be cheaper.

But try this first: Plug a signal into the power amp in jack on the lower right up front. Still hum? Since you are inside it, pull the four wire, five pin Molex connector off the pins at the corner of the 400BH board. Farthest corner from the caps. This is the cable from the front preamp panel. Without it, the power amp stands alone. ANy hum it makes is its own fault. If the hum went away, the power amp is not hummy.

We really need to make sure the problem is in the preamp or the 400BH.

[Drewn]

What Next?

You were right Enzo! I plugged an input into the amp in and it worked (not quite as loud but clear) and no hum. I tried unplugging the connector that I thought you were describing but it cut the power to the whole rig. When I did plug into the amp input it strangely picked up a radio station!? I tried plugging into the Aux input and it worked but there was a loud hum. None of the 6 regular inputs work anymore even with the hum. It seems as though it's the pre-amp board. I can't see any shorts or loose connections but I didn't remove the board from the metal front of the unit yet. Is this fixable (worth the time)? If so how do I proceed next?

Any help would be much appreciated.

Thanks,

Drew

[Enzo]

There is a white plastic connector with five holes (4 wires) in the corner of the power amp board on the rear panel. Follow it, it goes to the corner of the front panel. If you unplug this, the lights on the front may go out, but the power amp will still be alive. The four wires to the front are +15VDC, -15VDC, ground, input. The powr amp board supplied power to the front panel. Yes, pulling this cable will turn the front panel off. But all we awanted to do was see that the power amp worked. It does, you demonstrated that through the front jack.

Pull that conector off and make sure the p[owr amp is making the +15 and -15v to send to the front. -15 is the pin closest to the corner and next is ground, +15, and the one farthest from the corner is the input to the power amp. With power on and a speaker connected, if you touch that input pin with a finger, a hum should come out the speaker. That is my quick test for function. If it hums, at least it is functioning.

There are two more white Molex connectors over by the fuses. They are the wires from the power transformer, don't pull them. There is a second four wire Molex connector on th power amp, but it is not right at the corner. That is for the compressor LED up front.

If the power amp in jack worked, the power amp is OK< except the radio station thing makes me think there is a missing ground between the jack and the power amp. And if that is so, not only wil.l the front panel signals be screwed up, the power supply from the rear will not get to the front either.

Disconnect the whole amp from the wall power. Now measure resistance with a meter between the ground of the speaker output jacks and the ground of one of the channel input jacks up front. By ground I mean the metal barrel of the jack. They should be connected together, like very low resistance between them. If so, then the ground connection frront/back is OK, but if there is a large resistance, then the ground is open. That would have to be fixed before anything else will work right. The front to rear cable must be connected for this test.

Since the power amp seems to work, bolt it in and dismount the front panel, and lay it on its face. Power up and measure DC voltage from ground of the front panel to each of the four pins of that front to rear cable. Measure at the front panel end. This cable plugs onto pins right above that power amp in jack. The pins are from the end closest to the edge of the board, and countiung towards the center of the board: -15v, space, ground, +15v, preamp out. There is a wide pc trace around the outer edge of the board. Poke your black meter probe into that as ground. Make sure to poke a clear spot or poke all teh way through the green coating, or poke something like the leg of a jack that is soldered to that trace. THEN probe with the red probe at the pins of the connector. Do yuo get the two 15 volt supplies?

[Drewn]

OK this is what I found

Hi Enzo,

You are great at directions! I was able to follow them exactly. Here is what I found. I touched the input pin on the power amp board and it hummed as it was supposed to. Also the other pins gave me the correct voltage, -15, ground, +15. So that was fine. I did the ground to ground test from the front panel input to the speaker output and there was little resistance so that was fine too.

When I tested the 4 pin molex connector on the front I only got 1/2 volt from the -15volt pin. I did this with the connectors still on front and back and the unit powered up. I did get +15 where I was supposed to. I rechecked it reaching in to the power amp end of the molex connector to make sure it wasn't a faulty wire and got the same result. What does that mean? Is it a faulty power supply? Also, there was a 5th clear coated silver wire on that connector in addition to the space. The wires were green, space, silver, black, red , white going out to in. I had a good ground contact and retested it several times.

Please let me know the next step.

Thanks,

Drew

[Enzo]

The bare wire is just a shield and should connect to the ground wire at one end.

OK, so without the cable you get +/-15v at the power supply. But when the cable connects the front panel, then the -15 goes away, even at the powr amp end. Correct? In other words, the -15v supply is OK unloaded, but collapses under load.

Well it is a straight feed from the output of a 7915 three legged voltage regulator, so either that regulator cannot hold up under load, or the front panel has a failed part that is sucking WAY to much current. My furst three guesses would be a bad 7915.

But here is an easy fast test. It is certainly not foolproof, but it takes seconds and can't hurt. Unplug the power cable to the front. Now on the front at the now bare MOlex pins, measure resistance from the ground pin to the -15v pin. You should probably see a charging up type reading. If there is a low resistance to ground, something is shorted on the front panel board. Something up ther could be shorted anyway and not show on an ohm meter test, but like I said - takes seconds.

Even if the 7915 seems OK, I'd replace it anyway. If it is bad, then that dixes it. If the front panel loaded it down so hard it shut off, then it was stressed anyways, and for a dollar, replace it for peace of mind.

If you have a Molex pin extractor, pop the -15v wire out of the cable and insert a current meter and see what the front panel draws. The 7915 is good for an amp, but I expect draw less than half that in the circuit.

[clintronics]

heres link to schematics

http://www.clintronics.net/PV_400BH%20schematics.htm

[Enzo]

Thanks for the link. As an authorized Peavey repair center, I have all the documentation. The 400BH is a very common module, they used it a lot. COuple that with the XR600B being one of the all time mopst popular mixers, and we have seen quite a few of them over the years.

[Drewn]

Here's what I found now

Hi Enzo,

I tested the resistance between the -15v pin and the ground and it was about 1500 ohms. So that sounds like the preamp board is probably ok. I went and bought a new 7915 voltage regulator. Where does it go on the amp panel? I can't see where it is located. Let me know where it is and I'll swap it out. Are you thinking that this will fix the whole thing?! All for a buck and a half!!

Drew

[Drewn]

Found it!!

Hi Enzo,

I looked at those schematics that were sent and found the 7915. I think I'll change it out tomorrow as it's getting late. Any tips? Just change it out, put it back together and power up? Let me know if I need to do anything else, otherwise I'll let you know how it goes.

Thanks again,

Drew

[Enzo]

Change it and see what happens. It will either fix it, or you will at least know the 7915 is good.

In a commercial shop like mine, parts are cheaper than labor, and I stock all the common three leg regulators - 7805/12/15/18/24, and their 79xx counterparts. SO I don't have to make a trip to the store. SO maybe it is a little easier for me.

[Drewn]

Drumroll Please..........

Hi Enzo,

So I took apart the circuit board, unsoldered the 7915, took off the heat sink, put the new one in and resoldered it.

I put the whole rig back together. Then I did several prayers and incantations, I danced around to the PA gods and asked for their help and forgiveness for my bad singing.......

Then I flipped the switch..........IT WORKS!!!! Just like new!!!

It sounds great!! I can't believe it!!

Thank you so much for all of your help! Is there something I can get you to thank you? An Amazon gift card, a magazine subscription, Anything!!??

Let me know and it's as good as there!!!

Is there any reason that this thing blew? Does that just happen sometimes or does it mean that something else is defective and caused it?

Thanks again,

Drew

[Enzo]

Couple of hookers and a bottle of Old Grand Dad ought to about cover it...

Or just be kind to your fellow man, and give aid where you can.

Stuff happens, if the thing now works, we write it off as random component failure. After 30 years of bouncing around, getting hot and cold, vibrating, etc. some parts will eventually fail. The thing will work for another 30 years. If something else is weak, then it too will eventually fail, but who knows if or when? The 7915 has internal overload and thermal protection, so for the most part load problems won't hurt it.