Peavey xr600b distorted output

[Alan]

Hello,
I tried searching for an answer, but, no go,,,,so I guess I have to bother you folks. I have a Peavey XR600B with the 400BH power setup. Pretty common stuff I gather so I hope this will be a simple fix. The amp was an ebay purchase several years ago and hasn't seen much use. The pots were all pretty tight from years of club use cigarette smoke and such i guess. I got some cleaner/lube and went to work tearing it down. I got the pots loose and they don't crakle anymore, but the output is distorted(it was that way before the cleaning) and the compression light is not lit at all. I can use a meter but don't have the schematic...couple of questions....where do I start and if I'm going to need a schem., where do I get one?

Thanks,
Alan

[Enzo]

You can get the complete schematic set from Peavey customer service.

What is wrong? Isolate the problem.

Have you tried a different speaker and cord?

Plug a signal into the POWER AMP IN jack on the front. Does that come out the speaker clear or distorted? If distorted, your 400BH has a problem. If clean the power amp is OK and the mixer portion has the problem.

Likewise, plug a cord into the MAIN OUT on the front and send it to the input of some other amp and speaker for a listen. CLear or distorted?

How about the monitor out, does it sound clear through another amp?

Do ALL the channel inputs ahve the same distortion, or only one?

[Alan]

Good evening Enzo,
Thank you for the help. Yes Did try another cord. Learned that one a long time ago the hard way haha!

The problem exists in all 6 channels. I followed your diagnosing map above and got:

Guitar through Power Amp In jack = Distirtion
Guitar through channel 1 with main out to another amp = clean!!
Guitar through channel 1 with monitor out to another amp = clean!!!

so....I gather the problem is in the 400BH then?

alan

[Enzo]

Pretty sure 400BH schemo has been posted here, use the search.

It doesn;t blow fuses, it is just distorted, so your output transistors are not shorted at least. Well probably not anyway.

Start by checking for the presence of the high and low voltages, both polarities each. The highs will be something like 45-50VDC and of course +/-15VDC for the op amps. There are two sets of +/-15VDC. One set runs from the two voltage regulators 7815, 7915. That set is for the mixer only - it runs off board through the molex connector in the corner. There is a second set for hte power amp ICs, and it runs off a pair of zener diodes. Obviously your regulated set is OK, since your mixer is working OK. SO check the zener set.

You could get lucky and just have a bad TL074.

[Alan]

Thanks Enzo, I found the Schematic. checked all of the 15, 16, and 52 vdc points on the board and they all came in real close. I swapped out a TL074 from the mixer board and that killed it! Now its dead. no led on the front and now I'm tired and shouldn't have my hands testing around for that kind of voltage. There is 124VAC across the fuse, but thats as far as I can go today.When the cancer tireds hit ya, ya quit and try again another day. Its Back to the drawing board. I'll check in after I get power back...thanks

[Enzo]

It matters which way that TL074 faces, if you stuck it in backwards, it will probably be damaged and no good will come of it.

If there is 120VAC from one end of the fuse to the other, then it is blown.

[Alan]

The 074 was put in notch toward the bottom of the amp just like the limiter is oriented,,,, and you lost me on the fuse....I pulled it and checked for continuity....it was fine. If I read it with power on and in the fuse holder, end to end it should show 120V because its not blown right? if its blown then it shouldn't show any voltage right because there would be no internal connection? Can't hurt to replace it anyway and have some around so I'll hit the Radio Shack. I just get the feeling its going to end up being the switch for this power issue(or at least I hope)
Like I said I'll check back in shortly when follow through on your instructions again Thanks Enzo

[Enzo]

Your reasoning is backwards on the fuse. Think about it, what is a fuse? it is a piece of wire inside a glass tube. A wire. so putting a volt meter at each end of a fuse is the same as connecting them together with a piece of wire. When a fuse blows, then you get 120VAC across it because the two sides of the AC mains are connected to the two ends of the fuse - one side through the transformer and switch of course.

We are talking with your probes put one on each end of the fuse now, not measuring to each end of the fuse from ground.

Right, TL074 and limiter IC both have pin 1 at same end.

[Alan]

My fault good sir! I think theres a bad pun in that opening...I did remove the fuse and check it for continuity, and it was fine. However I do understand what you are trying to teach me. (Thank you very much for your patients, you make a great teacher)

[Alan]

Ok Houston, we have power. I poked around the fuse holder and found it to be intermittent...replaced fuse holder and got some new fuses for just in case and I have power again.
i started all over again.
Main out is clean.
Power amp in is distorted and Monitor out to another amp is distorted.
All 6 channels have the same problem so the issue is still in the 400bh correct?
All the 52 , 15, and 16vdc checked out and I have gone back over them again and got the same readings.....triple checked the zeners and got +14.85 and -15.08

I will definately be learning from this problem......where to next?

[Enzo]

Interesting that not only is the power amp distorted, but also the monitor out.

This sounds weird, but do it. Take a cord and plug into each of the patch jacks along the lower right. Main out, power in, etc. The cord is just to make readings easier to take. At the free end of the cord, take DC voltage readings. ANy jacks have DC on them? Probably 10-15v if they do. If you find that, you probably have a shorted protection diode for the jack. Look on your schemtic, most jacks have a pair of diodes one to each rail.

If a clean signal to the power amp in comes out distorted in the speaker - and the speaker is OK - then start by verifying all the DC voltages on the schematic for that 400BH.

[Alan]

Main out 0vdc
monitor out 0vdc
effects out 0vdc

Aux in 27.9vdc
power amp in 0vdc

So this tells me that there are faulty diodes associated with the Aux in jack? I have been working off of the 400bh schem. I'll guess those would be on the front xr600 board? I'm waiting for that schem from peavey, otherwise, all of the voltages on the 400bh check out as per the schematic I got from this site.
Am I chasing more than one problem here? I'm not sure I follow how this particular set of diodes would affect (distort)the output from the entire amp.
I really thought it would end up being a bad transistor. I know you have plenty of experience with these amps and I'm a newb so lead on Macduff
Alan

[Enzo]

Yeah, but I only expect 15v, not 29. I can;t imagine where 29v even comes from there.

The AUX jack is next to the power amp jack. Look right above the power amp jack on the master board - see the group of four little diodes? SHould be easy to find the shorted one there. They are just plain old 1N4148 types.

No, I have no idea why that might affect the power amp. Probably doesn;t. Probably coincidence.

If a clean signal into the power amp in jack comes out distorted then the 400BH is the bad guy.

One thing you can do is remove the little 8 leg IC from the board - the compressor. The amp will work fine without it - minus the comopression feature of course. And if it is involved, that will tell us.

Right near the pins where the speaker wires plug into the board is an inductor - a coil of wire. Probably has brown paper wrapping. Tug up on that to see if either end has broken its lead wire.

ALL the voltages are OK? For example that -.87V at pin 14 of the TL074?

[Alan]

I pulled out the compressor ic and it did not affect the issue. The inductor seems good and tight.

I went back in and dug around some more with the meter.(The -.87v on pin 14 checked out ok) Pretty sure I finally found a clue. If you look at the schem in the upper right hand corner you find a dotted box. In the left most dotted line mid way down you see+.56v that should be able to be read at the end of R26 correct? its not there. there should be 1.17v on the base on Q2 5331 and it is there. The .56v(actually measures .52V) is there from the Emitter. we should see that at R26 and its not there. As a matter of fact, there is 0v on both ends of R26 to ground and end to end R26 measures 0 ohms.......it would have to be located under the inductor coil...... This is also the first time I've ever seen a resistor notted as F.P. .....is that just a fancy way to say use ceramic?

[Enzo]

The dotted boxes represent the parts that are on the separate power transistor circuit board.

Yes, the .56v that should be on the base of Q3 should also be at the emitter of Q2 and of course on the top end of R26.

I don;t know how R26 could be shorted, and if Q3 shorted dead across, there would still be 1 ohm in parallel with R26. You sure it is not open?

Yoiur reading report is suspicious. I wonder if your probe was just not making proper contact with the resistor lead?


Hmmm. R26 is near the inductor, yes, and there is a wire jumper next to it and between it and the inductor.

resistors marked FP are to be "flame proof" types - metal film. R26 and the other small FP resistors are film types.