Peavey xr600b distorted output

[Alan]

Here is my method

First, ground the neg probe to the chasis
set meter to dc voltage and test for 1.17v at base of Q2 get 1.2v
next lead to the right is the collector test for 52v and get 52.4
last lead to the right is the emitter test for .56v and get .52

decide to check for .56v at R26 get -1.4mv, thinking this must be meter fluctuation, i test the other end of R26 and get the same -1.4mv. It just looked suspisious to me so, I turned the amp off and using a test lead at each end of R26, I get .1 ohms and the meter rings for continuity. R26 has to be bad right?
I see the jumper wire, it is to the left of the inductor leads. I'm definately not hitting that. I apoligize if I'm trying your patients. These are honest results and I repeated the tests before I typed this note........alan

[Enzo]

No, I am not impatient, I just want to make sure we are communicating. SOme details here are not fitting into place for me. And generally I can fix these old Peavey amps easily.

OK, I have turned my layout image upside down. You seem to be looking at your board that way. SO the wire jumper is to the left of the inductor, then R26 is to the left of the wire. And R32 is just to the right of the inductor, is that correct?

I am concentrating on this detail because I am having a hard time making sense of R26 measuring shorted end to end as well as to ground yet leaving the correct voltages on Q2. That would be very difficult to do looking at the schematic.

If you were looking at R32, that would measure shorted because it is in parallel with the inductor next to it. And the inductor has almost zero ohms resistance. However those would not be shorted to chassis, they would measure 4 ohms or 8 ohms or whatever your load resistance might be. Unless the load was shorted.


I asked if you had tried a different speaker cord and speaker, and you replied you had indeed tried a new cord. I didn;t note if you tried a different speaker as well. Did you?

Has the board been bolted down all this time? Have you inspected the solder side? IN particular for the traces between the R26 and the Q2? And to make sure no loose piece of hardware is not wedged under the board grounding stuff out.

[Alan]

You are correct, I am working with the board folded down off of the back of the amp. Yes I was measuring R32, what an idiot. I got the .52v at R26. The inductor reads 0.1 ohms as you suggest I still get 0 ohms end to end on R32 and -1.4mv to ground. I'll pull the board and look for bad soldier or traces in that area next. Definately no hardware floating around under the board.

The PA speakers were a set of Peaveys and I can get clean sound out of both through my Kustom 100W

off to look at the bottom of the board

Thanks, Alan

[Enzo]

The burnt traces were more a thought when we were looking at R26 as weird. it isn;t, so those traces are likely OK.

R32 in on the output bus, so 1 or 2 mv there is just fine. Earlier however you said you also measured zero ohms to ground from what we now know is R32. That doesn;t sound right. Normally that measurement shouold yield what the load resistance might be. SO zero ohms to ground still sounds like the oputput is shorted to ground - either from a bad jack, a shorted speaker cord or a short in the speaker cab. If the speakers and cord work elsewhere, then they wouold seem OK. Look closely at the output jacks when the speaker cord is plugged in. Do any of the elements of the jack touch chassis?

Better yet, remeasure from R32 to ground. is it really zero ohms, or could it be just a few ohms, like 3 or 6 ohms, masquerading as zero? If it really is zero, unplug the wires to the speaker out jacks and remeasure again. If the jacks themselves are shorted, this will remiove them from the circuit.

[Alan]

Remeasured R32 to ground with speaker connected...got a fluctuating reading but settled at 6 ohms. It still rings continuity when measured end to end though. Acts the same with another speaker and cord too. The jacks are ok. I even tried using test clips to hook up the speaker....same behavior.

I loosened up the boards and found the main board to look fine through the traces, but, someone has been in here before. All 8 main output transistors have different solder on them. There were cracks in the solder and the joints didn't look clean. I got all excited and cleaned them all up hoping that would be the culprit....no dice the problem persists

However on that board full of output Transistors, I took readings on the 10W resistors. R60,61,62,63 all read -1.4mv . Shouldn't we see 52V in there somewhere?

[Enzo]

R32 has the inductor in parallel - a coil of wire - so it will indeed measure continuity end to end. OK, the 6 ohms is your speaker, 6 ohms is what an 8 ohm speaker usually reads.. I bet if you unplug the speaker it would go up to a much higher reading. Don;t bother, it is not your problem area.

By the way, a speaker can act as a microphone, if the coil moves, it will generate a little voltage. SO it is important he speaker cone does not move when reading its resistance. Try it. Connect your ohm meter, get a steady reading, then push on the cone with your hand. See?

Look at your schematic. R60-63 are all connected directly to -52v. If you are getting close to zero, then either your meter is set for AC volts or there is a loss of continuity between those resistors and -52v on the main board. And that would certainly explain a distorted out put.

There are two 3-pin molex connectors on the power transistor board and three corresponding ones on the main board. Make sure the two short 3-wire cables are connecting the power transistor board to the main board. Furhter make sure the pins in the connector have not spread open and losing contact. One of those pins carries the -52v from main board to the power transistor board.

[yourgeeman]

Peavey - Distorted Output - xr600b/400b

Hey guys, didn't see any fix on this issue. I have followed the troubleshooting steps indicated by Enzo. I get a reading of -.49 on pin 14 of the IC TL074. What should I be looking for next?

Thanks,
Jerry

[Enzo]

Hi, welcome. We don;t know what you are fixing yet. "Distortion" is not specific.

If all those DC voltages on the schematic are reasonably close, that is good. Take the load off the amp and apply a steady signal. Scope the output. Is it clean or distorted? If it is clean, does it then distort only when a load is connected? Of it is distorted either way, describe the distortion. Is one half of the waveform missing? Or is it clipped on both top and bottom? Or something else?

There are two ICs on this powr amp. The small one is the 87478 compressor IC. Remove it. Does the amp sound OK now? And the other is the TL074. Just try a different TL074 in its place. Hint: there are other TL074s in the mixer section, try a swap.

[yourgeeman]

Enzo,

I know I should be starting a new post, but thought this thread was exactly at the same place I'm at in the sourcing of the issue. My unit is clipping, distorting with little input signal. I removed the 478 and had same response. Removed the limiter from the circuit, by disconnecting the module connector. No difference. I called Peavey to get two new Filter Caps and the tech told me that the old caps are most likely the issue (saw you mentioned it in a thread, too). Ordered the caps and will try that. FYI caps are c35, 36 5000/55 - for those of you wondering) Also, Peavey replacement for part70320039 is 70320120 - $11.70 each, not sure if anyone can source these elsewhere cheaper.

I was reading the wrong pin on the tl074. Pin out readings are - 1-0v, 2-0v, 3-0v, 4=+14.8, 5=0.52, 6=-0.41, 7=+14.25, 8=+14.28, 9=-0.49, 10=-0.44, 11=-15.10, 12=0v, 13=0v, 14=0.80

If the caps don't fix the clipping, I will replace the tl074 - if you think that is the path.

FYI - board is a 400b/g - old board from another model, tech stated. Got schematics if anyone needs them.

Thanks

[Enzo]

Well maybe, all we know is he had distortion, it may or may not be for similar reasons to yours.

With the 478 removed, none of that stuff in the upper left corner does anything.

If the filter caps were bad, you'd have low main power voltage with lots of ripple (hum). Your complaint is not hum, is it?

OK, let's get something straight. What board do you have and what schematic are you going to use? The board will say 400BH or 400B/G right on the solder side. You say 400B/G, but when you say the filter caps are C35,36, those are the numbers from the BH board, the main filters on the B/G board are C34,35. The boards are similar enough you can use the same schematics for them, but you get in trouble refering to part numbers then.

SO you are clipping. About the same on both tops and bottoms? My question remains, does it matter if there is a load or not? If it distorts without a load that tells us something different from distorting with a load but clean without.

At what voltage does the output clip? I mean when the output clips, what voltage does the peak lop off?

SCope your input signal right at R35, is it clean? If that is clipped, so will the output be.

How are you getting signal into this board? Through the patch jacks out front or right at the corner molex pins? Pull the molex connector to the front panel off the connector in the corner. The pin closest to the corner should be the input. Touch it with a finger, if hum comes out the speaker, then it is the input. Connect a test signal there with clip wires, don;t go through the front of the mixer.


I don;t know that the TL074 is especially suspect, but it is in a socket, they are cheap and common, most shops have them in stock, and even if you don't have them, there are a bunch of them on the mixer panel you can borrow. SO it is a quick question that can be answered. If swapping the 074 fixes it, you saved some time. If it makes no difference, at least you didn;t order some part and wait for it.

Unless times have changed, Peavey has not has the 5000uf 55v caps for years and years. What they generally send out is a common 4700uf 63v cap. You may have to drill a different hole for its legs to match up - careful. Most have 10mm or 12mm lead spacing. Search Mouser: "4700uf 63V" and from the results under aluminum electrolytics you can use either "leaded" or "snap-in". leaded has the wire leads. snap-in has the two short little bent legs so they snap into the holes. None will have the four solder down moutning tabs of the original caps.

Mouser sells leaded ones from $3-5, and snap ins from about $4-10.

However, I THINK the Peavey price includes shipping, is that the case? Minimal UPS charge from Mouser or anyone will be like $8 or something, so for two caps that adds like $4 each to their cost. Peavey prices are not discount, but they are reasonable. Of course if you are ordering a bunch of other stuff from Mouser the shipping cost gets spread around.

[yourgeeman]

Enzo,

You are correct. I was looking at the wrong schematic when quoting the caps. Thanks for the info. I saw an IC for .20 online. They had a $5.00 min order. Wish Radio Shack was still an Electronic Parts store...

I don't have a scope - I'm probing around with a VOM and alli clips. I'll install the caps from Peavey and go from there. Could I not bend the leads to match the spacing?

I really appreciate your willingness to guide us newbies. Thanks

[Enzo]

There is no lead to bend in a snap-in. I am not sure what PV is sending out at present. A leaded cap, yes, you could bend the leads to fit the hole spacing. But if the hole space is not th same as the cap lead space, then the cap will sit up off the board on its skinny wire legs. Vibration will break it off soon enough. SO drilling a small hole and sanding the coating off the copper so it will take solder allows me to set the cap dead square flat on the board, so it will be stable.

There is no absolute. You could mount the cap off the board in a clamp, and run wires from it over to the circuit board. The modern caps will be substantially smaller than the original - technology has advanced - so if there is room, you might be able to lay them on their side on th circuit board and run their leads to the holes. A bed of hot glue or silicone sealer to anchor them in place makes that work. And leaded or snap-in, you could glue the cap to the board upside down - leads sticking up in the air, and run wires from the leads down to the board. Use contact cement for that.

20 cent IC, $5 minimum, welcome to real life. Even if there was no minimum, you still have to include shipping. But that is why I suggested swapping for one on the front panel. If you find that it makes the difference, then you do need to get a new one, but if you find that changing the IC makes no difference, then the whole point is moot.

[yourgeeman]

Just want to give an update to the issue...I hate it when people want all kinds of information for free, then don't have the "time" to update the post with a conclusion.
I installed the two filter caps from Peavey. Had to drill three new holes on the PCB to fit them in. Powered up the puppy and what do you know, no distortion. Not only no distortion, but a significant reduction in the noise on power up and power down.
Thanks for all of your help, Enzo. I really appreciate the time you take to assist others with troubleshooting. FYI - got a lead on a signal gen and a scope for an old techie.
Later...

[dmaddux]

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?

Enzo, I know this is an old thread, BUT as I was reading through it you just fixed my XR-600B and didn't even know it.

Back story: I got this amp from a friend. he stated that a fuse holder had broke and after he fixed it the amp was now putting out a distorted signal through the main signal path. Well, I did some basic testing and discovered that the PWR. AMP connection worked fine (main amplifier working great), I discovered that I could send a signal through the GRAPHIC IN and get equalized output. Next I was able to send a signal through the mixer, use the MON control, then patch the MON. OUTPUT to GRAPHIC IN, GRAPHIC OUT to PWR. AMP. and have a usable system.

at this point I have determined that my MAIN control on the preamp board (not the mixer board) was the problem. Reading your post (quoted above) led me to the 2 diodes above the MAIN OUTPUT jack. Tested them and both were bad. put in some new ones and it's working like a charm!

THANK YOU, THANK YOU, THANK YOU, for your wealth of knowledge and sharing with us.

BTW, my friend told me that if I fixed it I could keep it, otherwise he was going to put it in the shop and pay someone $100 for a 10 cent diode.

[Enzo]

Glad you fixed it, but please don't describe our profession as paying $100 for a 10 cent diode

I actually pay more like one penny for those diodes. But what you paid the $100 for was me knowing how to find the problem, having the facility in which to do it, and having the parts on hand to effect your repair promptly, and the skill to do it correctly and not do further damage. For your $100 you got the amp back working in a day, rather than paying 10 cents for a month online.

So welcome to the forum, and what else can we fix?