Peavey XR 680C Hum Problem

[misterc57]

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).

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!

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Today, 07:27 AM #17 (permalink)
Enzo
Senior Member


Join Date: May 2006
Location: Lansing, Michigan, USA
Posts: 7,358 Welcome to the forum.

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
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Thank you Enzo. Started a new thread and will report back on findings! Misterc57

[misterc57]

On mixer board alone...

Tested all the diodes "in circuit" using the diode setting on my DMM. Not sure about this test procedure...I probed them all in one direction and then reversed direction and there were no shorts.

Identified pins
pin 1 : ground
pin 6 : -15V
pin 7 : +15V

resistance pin 1 to 6 = 1.6 to 1.7 ohms
resistance pin 1 to 7 = 74K ohms

resistance input channel 1 ground to pin 6 = 11.4 ohms
resistance input channel 2 ground to pin 6 = 11.4 ohms
resistance input channel 3 ground to pin 6 = 11.3 ohms
resistance input channel 4 ground to pin 6 = 11.3 ohms
resistance input channel 5 ground to pin 6 = 11.2 ohms
resistance input channel 6 ground to pin 6 = 11.2 ohms
resistance input channel 7 ground to pin 6 = 11.2 ohms
resistance input channel 8 ground to pin 6 = 11.2 ohms

resistance input channel 1 ground to pin 4 of the (16) RC4558P op amps (two op amps on each channel) = 10.7 ohms to 11 ohms (ohms increased as I got farther away from channel 1 ground)

resistance input channel 1 ground to pin 4 of the (4) other RC4558P op amps = 11.5 ohms to 11.2 ohms (ohms decreased as I got farther away from channel 1 ground)

Tested the two small caps at each channel and they were all consistent.

[Enzo]

OK, so you have about 1.6 ohms across -15 up at the molex conector, but about 11 ohms over by Ch1 input jack.

We did want to know the resistance from pin 4 of a nearby IC to the Ch1 ground, now try the Ch8 ground to pin 4 of the IC next to it.

I bet it is lower.

What we are doing here is reading the resistance from -15 to ground at various spots, looking for where it is lowest. The lowest resistance point will be the closest to where the short is located.

So we need to measure from a ground point near the -15 point in each case. If we use a ground poiont across the board, we have the added resistance of the copper traces crossing the board.

The little pairs of caps at each channel, you say they are consistent. COnsistent how? What do they read? ALL 11 ohms? Does the one closest to the Molex - I assume Ch8 - still measure 11 ohms?

You currently have all the ICs removed? Note which way they aim and remove them if not. I assume the short will still be there. So far it appears your short is closer to the MOlex pins than the other end of the board.

Inspect really close at the Molex pins - no solder bridge? Did you solder anything on this board at any time? If so, inspect that solder work closely.

Those two little caps in the groups at each channel? Unsolder one end of each and lift it from the board. Does the short remain?

Go down the long row of diodes, look on the solder side for any solder bridges or little blobs of solder. When you tested them with your meter, I assume you would have spotted any debris or solder dripped on them from above.

And then there is always the old standby - follow the -15 trace from the Molex pin along its path, looking for clues.

[misterc57]

ch 8 ground to nearby IC pin 4 = 10.8 ohms

>The little pairs of caps at each channel, you say they are consistent. >COnsistent how? What do they read? ALL 11 ohms? Does the one closest to >the Molex - I assume Ch8 - still measure 11 ohms?

I measured resistance of these pairs from Ch1 ground....
The top of each cap all measure around 10.5 to 10.7 ohms.
The bottom of each left cap would measure very high resistance and then start dropping rapidly.
The bottom of each right cap measured around 11.2 to 11.6 ohms.

All ICs have stayed removed during these tests.

It does not look like this board has ever been worked on. I am not ready to start pulling soldered components as I do not want to mess up the board. I have looked all over the board and especially the molex pins and everything looks clean and good.

I am still waiting on a schematic from Peavey. If anyone has one they can pass along that would be great. Will then try to trace the -15V path.

Thank you!!!

[Enzo]

Those little paired caps. You measure 10.5 ohms to ground from the top, and 11.2 ohms from the ground. That means to me there is about .7 ohms across them. measure right across each one - end to end.

Remember here we have the ground and the -15v shorted together. If the short is at one end of the board, we could measure right at that short and get the lowest reading on an ohm meter. Let us say that is 1.6 ohms. Now let us go to the far end of the board and measure between the ground and the -15v. Since all the grounds are wired together, and all the -15v points are wired together, at the far end of the board we would still measure that 1.6 ohms, but over there we would also get a little added resistance from the copper traces themselves. Even if it is only 1/10 of an ohm, it is measurable.

SO that is why we want to read at various points on the board, looking for the minimum reading. SO we want to measure directly end to end on the small caps, not from the ground down in the corner up to them.

Go right down the row of them measuring resistance across each. I will assume the left ones are OK, and the right ones are across the -15v line. SO concentrate on the right ones. Notice the reading carefully, since a tenth of an ohm could be the difference. I'd bet the lowest reading one is shorted. AM I certain? No. But it makes the most sense so far.

And if they all read about the same, then as I mentioned, go down the row and unsolder one end of each of those caps, and pull it up from the circuit board so it no longer sticks through the hole. That way it is disconnected from the circuit. Now check between ground and -15v.

You are going to have to solder on this board eventually. The ICs are in sockets and we already know they are not the problem. Everything else is soldered. It is only a very small portion of failed components that look any different from good ones. We have to unsolder things to test them, often as not.

I checked my dealer site, and the XR680C is not in the database. I have large paper copies. They may have scans that are not yet in the database, they certainly have the drawings.

I fear a schematic won;t help much. You have a short between the ground and the -15v rail. The schematic won;t tell you where.

[misterc57]

Here is the resistance across those right side little caps.

ch 1 right cap - 0.7 ohms
ch 2 right cap - 0.6 ohms
ch 3 right cap - 0.7 ohms
ch 4 right cap - 0.8 ohms
ch 5 right cap - 0.9 ohms
ch 6 right cap - 0.95 ohms
ch 7 right cap - 1.0 ohms
ch 8 right cap - 1.1 ohms

[Enzo]

OK, see the pattern? Over on Ch8 which is next to the Molex connector, you get 1.1 ohms, which is not far from that 1.6 ohms you got at the Molex itself. Then each channel further to the left the resistance gets lower and lower. Until you get to Ch2. Then after that it starts to rise again at Ch1.

The cap at Ch2 seems to be the closest to the problem.

Unsolder the right cap in Ch2 and remove it. Did the short go away? Check the part by itself, is it still showing shorted?

[misterc57]

Hmmm.... I did not think to mention this but Ch 2 is the only channel with a broken pot stem. The monitor pot stem is snapped off. I have different readings on that pot compared to the ones next to it.

I have all the pots turned up to their max position. I am reading resistance from center tap to top tap and center tap to bottom tap...

Ch 1 center to top = 5.45k ohms
Ch 1 center to bottom = 40.5 ohms

Ch 2 center to top = 5.09k ohms
Ch 2 center to bottom = 4.2 ohms

Ch 3 center to top = 5.02k ohms
Ch 3 center to bottom = 6.3 ohms

Ch 4 center to top = 5.02k ohms
Ch 4 center to bottom = 5.1 ohms

Ch 5 center to top = 5.21k ohms
Ch 5 center to bottom = 4.2 ohms

Ch 6 center to top = 5.28k ohms
Ch 6 center to bottom = 130 to 150 ohms (jumps around)

Ch 7 center to top = 5.37k ohms
Ch 7 center to bottom = 3.9 ohms

Ch 8 center to top = 5.34k ohms
Ch 8 center to bottom = 4.8 ohms

[Enzo]

Unless the pot is physically broken so that parts of it are touching the circuit board or something, it isn;t involved in your -15 short. SO solve one problem at a time. FInd that short.

You can order replacement pots from Peavey. Should be a part number printed on each pot, or at least a partial number. The real part number would be something like 71190536 (I am making that up), the pot could have that on it or it might just say 536 or #536.

[misterc57]

I removed the small cap, now have the following readings at the molex connector

prior readings...
resistance pin 1 to 6 = 1.6 to 1.7 ohms
resistance pin 1 to 7 = 74K ohms

new readings with cap removed...
resistance pin 1 to 6 = 86K ohms
resistance pin 1 to 7 = 6.1M ohms

resistance across the cap by itself is 0.6 ohms
dc volts across the cap by itself = 0

[Enzo]

OK, you found and removed the shorted cap. Reconnect everything, fire up the mixer, and see if it works now. It will work fine without that cap, at least during tests. If the amp works well without it, then I probably wouldn;t bother replacing it.

[misterc57]

WOW... a quick test and no more hum and all the channels work. Amazing you were able to guide me to the exact little component causing the trouble. Thank you!!!

I would say that the channel two volume response is not as smooth as the other channels, would that be due to the missing cap? Should I replace the cap? Do you know the cap values?

Thank you Enzo!

Misterc57

[Enzo]

I doubt that cap has anything to do with any performance issues in that mixer.

More likely a dirty volume control.

I don;t know the value offhand, I'll have to dig out the schematic again. I refiled it when I knew we had it solved.

Glad I could help. This is a CLASSIC example of troubleshooting.

[misterc57]

I was able to read it...

473
Z5E

I think it is 47,000pF

I will need to order a couple of pots from Peavey. I wonder if they will have the cap available also.

What is the purpose of the cap in this circuit?

Thank you.

[Enzo]

In many digital circuits, EVERY IC has a cap across the power rail right at the IC itself. In audio circuits like this, that would be overkill in most cases. So each subsection they put a cap across each power rail to ground. This will bleed off any transients from the stage and keep them out of the power rails so they don;t get into other stages. ALso bleeds off any high frequency crap that might get into the circuitry. There are 8 channels here, and each one has the same caps across the same powr rails, so there is a lot of redundancy.

Yes, those caps are .047uf, same thing as 47,000pf. I am sure PV would have them, as would any general electronics parts supplier.

[misterc57]

I have a bunch of these sitting around...

.047Z
Y5S
16V

Not sure how to read the value but if the .047 is uF I guess I can use these. There is no polarity on these, right?

Thanks again.

[Enzo]

No polarity, right.

you will probably get away with it, but 16v is marginal in my mind for a 15v rail.