Peavey 400BH problem

[misterc57]

Similar Problem, need advice...

Having similar problem and I have been following this thread. Upon opening the amp I discovered that the main filter caps had already been replaced with 4700/63V.

The front panel is made up of two sections; an 8 channel input (the larger left side) and the main control (smaller right side).

Using the previous instructions…there is low resistance (1.4 ohms) to ground on the front panel between ground and the -15V pins.

I have isolated the hum to the left side of the front panel. If I disconnect this side the amp works fine (example; a signal going into AUX input). I also read high resistance with the left side disconnected between ground and the -15V pins.

So what is causing the hum on the left side? Per a friends suggestion I measured resistance across every capacitor looking for low resistance, they all read high resistance. I also removed all the op amps but the problem persists.

A picture of my board in question can be found at http://www.noalibirocks.com/000/pv680c.jpg

I am open to suggestions! Thank you!

[Enzo]

Welcome to the forum.

My first suggestion is to start a new thread for your amp, instead of tacking onto the end of a thread that died in 2007. You will get a lot better response. Also, this thread is about an XR600, while you apparently have an XR680C?

But for now...

Take the mixer board alone. Right above the row of jacks is a long row of small parts. Scattered along that row are a bunch of small glass diodes. Go down the row with your meter and check them ALL for shorted. I prefer to use the diode test function instead of the ohm meter, it's faster, but whatever works. Replace any bad ones with 1N4148.

Op amps rarely short anything to ground, they more likely short +15 to -15. SO you already eliminated them anyway.


Upper right corner is the 8-pin connector to the master board. I think the top pin is the ground, but make sure. Determine which pin is -15v. If the top pin is pin 1, then I think -15 is pin 6, with pin 7 being +15. Make sure by comparing to the master. Now measure that resistance between pins 1 and 6 (or whatever pins you find are correct) Measure very carefully with solid probe contact. Let us assume you find the 1.4 ohms.

Now go to the lower left corner of the board by the Ch1 jacks. Again carefully measure -15 to ground - for example pin 4 of the op amp there to the ground of the input jack. Is it still 1.4 or is it higher or lower? Wherever the short is, the resistance will be lowest. The farther from it, the higher the resistance. Even if it is only a half ohm difference, it helps us isolate the problem.

And here is a tip - might pay off, might not - using the same resistance approach. Within each channel grouping, just above the topmost IC, and just to the right of the third control down, is a group of three small parts - a resistor and two small caps that probably look like green resistors. Those two caps are from the +15 and -15 to ground. Go down the row at each channel and see if any reads lower than the others. If in doubt, unsolder one end and lift it and remeasure it

[52 Bill]

Mister C
Good job of isolating the problem!

I wish everybody here would take this same logical approach.

[Techknowman]

Another 400BH

I'm hopeing for a bit of help here.
I have a 400BH amp section that was damaged that I received to repair.
It was passing a small amount of DC to the speaker.
I have a schematic.
I pulled the amp section and found Q7 and Q12 bad as well as R26 bad.
I replaced them and no more DC on the speaker. but unit goes into compression almost instantly and sound is distorted and very weak.
I put a 200hz signal into the Aux input.
At the 5 pin connector I have a clean ac signal all the way into R36.
At U14 (pin14) the signal is very distorted. I ordered a new U1. Used a TL084C in place of the TL074C. No change.
So I did some research and found that there may be issues with U2 (peavey 87478) which is now obsolete.
Contacted peavey and they said it was replaced with a OTA module. I orderd and installed. the problem is worse. it goes into compression instantly with any signal at input.
I know this is wordy but just want to get the facts out to see if someone can help.
Thanks in advance.

[Enzo]

Hi, welcome to the forum

Do us all and yourself a favor and start a new thread for your amp. This thread has been dead well over a year. You will get better response that way.


But to answer: A tiny DC is not a problem, and I mean a tenth of a volt or something. If it measures in volts, then yes, fix it. Until we know for sure the amp is not making DC, get rid of the speaker and any speaker loads. Apparently that has passed.

I have to say, I have few problems with that compressor circuit. Once in a while it fails, but not nearly as often as other things.

If it makes things more convenient, you don;t need the preamp panel. I usually lay the amp panel down flat and clip a test signal to the Molex pins where the preamp cable plugs on, eliminating the preamp cable.

U2 is the compressor chip, that 87478 IC. Pull it, the amp works without it, of course without compression now. We will put it back when the amp is fixed. Your amp cannot put out a clean signal, and U1d distorts the signal the opposite way trying in vain to compensate. That is your distortion at U1d. Pin 14. The compressor is coming on because of all this, just doing its job. Not that it matters with U2 pulled, but in servicing, either pull the four pin molex to the compressor on/off switch or at least turn it to OFF.

You found Q12 and R26 burned. And Q7. In that case, there is a REAL good possibility Q2 is bad, and probably someone over in Q13-16 land. Check Q2 aqnd R54,55,56. Those resistors like to open when transistors fail. Q7 fails when currents try to flow through it after other parts open. Check CR14 for shorts or low junction drop.

Q1, Q7 are limiters and not neded for basic amplification. If they might be involved, unsolder one end of CR14 and CR10 and lift them from the circuit. That disables those two circuits. We'll restore that later, and recheck all those parts too.

There are two long 0.1 ohm 10w resistors between the two three-pin Molexs to the transistor board. Make sure they are not open. (probably OK) Unplug the two thre wire cables there, and on the transistor board, check EVERY resistor for open. Then ther are two groups of four transistors. Note the end one of each group is wired as a driver to the other three. Check them all.

Just to humor me, make sure the two resistors top center on the page R19,R20 are OK.

Once any open resistors and bad semiconductors are found and fixed, fire up the amp. No load yet. The schematic has DC voltage references all over it. Check them all and see that they are all more or less close to the drawing. Note: most are with respect to ground, but a couple are across a resistor, NOT to ground.

Once that is all OK< apply a test signal. 200Hz is fine, I use 100Hz myself. Hell of a lot less ear fatigue than 1kHz. Scope the output bus and run it up. DO you get a full and clean waveform? If you do, then apply a load or speaker and try again. Does it now work? Or does it fail under load? A clean waveform that collapses under load is a different problem from a waveform that is never clean.

I should have entered this near the top. Ther are two sets of +/-15v. The regulated set is for the preamp. The zener pair is for the power amp itself. Make sure both sets are OK, don;t measure the +/-15 at the preamp and assume they are all OK. A missing or low 15v on the power amp ICs will cause trouble.

[clintronics]

New link

yamaha em150 schematic p2050 peavey at clintronics.net

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