Need help trouble shooting peavey xl-500 mix amp

[Enzo]

YOU ABSOLUTELY MUST HAVE THE INSULATING MICA WASHER UNDER THE POWER TRANSISTORS. The mica allows thermal energy - heat - to conduct from the transistor to the heatsink, but is electrically insulating. After installation, check with an ohm meter to make sure the case of the transistor is not conducting to the metal heat sink.

[captainlee]

Hello Enzo,

Ok So I got the capacitors in the mail but they don't have a + or -
indicator on them. But they do have a gray bar with o's going down the height of the cylinder is this the negative side? Do you know where I can get a slider pot for the equalizer on the front with a square knob to replace the broken one on this unit? The slider pot works but the plastic is broklen and won't
except a new knob. thanks Cap.

[Enzo]

Sliders? Knobs? From PV.

Seems to me those sliders came in long shaft and short shaft version, but one of those was no longer available. SO a new one might be longer than the others. C'est la vie.

Look at your caps. or look at the image in the Digikey link. The light gray bar down the side of the black cap doesn;t have "0" in it, those are hyphens or negative signs. That indicates the negative lead.

[captainlee]

It Works But Still Needs Help

Enzo here is the update......Hi

So to recap I installed 4 power transistors
2 power capacitors
2 high filter capacitors w/6800uf caps
5 cement resistors
2 heat sink transistors
1 14 pin rectangle thing

Any way now I have a ac hum not real loud until I turn up the master
volume. if the master volume is at four can barley hear and i can still crank up the channel volume with no problem.
but higher than 4 it gets very loud.????
also the compression light comes on at start up then go s off, my other xr-500 does not do that,
The effects work as do all the pots on the front of the amp mixer.
with no load there is no voltage at speaker jack output.
So I am very happy with the progress so far you have done a great job helping me I really thank you. Cap I hope it will be a quick find. also has one poping sound from speaker when turned off.

[Enzo]

There is a four wire cable from the corner of the power amp board up to the front panel mixer. Disconnect it. Now is there hum in the speakers or not? If there is still hum, them the power amp is not yet ready, but if it is quiet that way, the power amp probably works.

There will be the four pins sticking up where the cable was. One of those is the inpit to the power amp. Touch it with your finger, and a hum should come out if the amp is working.

SO if the amp works, then your mixer panel has issues. That is the whole front panel.

COnnect the cable to the power amp back up - it poweres the mixer panel. Now measure for DC voltage at each of the patch jacks - lower right on the front. A simple way is to plug a cord into each jack, and use the other end of the cord as a test point for your meter. If you see +15v or -15v at any jack, there is a shorted protection diode near the jack.

If those are OK, check the +/-15v power supplies coming from the power amp. Are they both the same voltage? Are they clean? (No AC voltage present?)

Go down the ICs in the master section and check for DC voltage on the output pins of each.

Is the hum present when ONLY the master is turned up and ALL other controls set to zero?

[newbywankenobi]

Scratchy pots

An old electronics repair guy buddy, old guy who worked on old analog electronics ;-), said to use a syringe and inject a little denatured alcohol in the back of the pots and actuate them several times to clear up the scratchiness.

(same guy who gave me a cigarbox full of various nos vacuum tubes, including 1950-60's GE and others 12AX7's !! for my other guitar pre-amp) Now that is a buddy! Also showed me around Wuxi China while we were there starting up a new plant for a month! Good to have old friends, coworkers for 15 years so far.

Had an old auto repair buddy at church say to tie a foot of string on the pot, with a little tape added to help hold it on the knob, and wrap it around so that pulling the ends of the string each way will rotate the pot quickly both directions. Do this enough times and the friction and abrasive corrosion coming off the contacts will clean the pots without alcohol, and without disassembly of the unit. He is a wise old dude too, who works on old cars.

[captainlee]

The tests I have made.

BTW I'M NOT SCREAMING AT YOU JUST BEING DIFFERENT FROM YOUR TYPESET.

There is a four wire cable from the corner of the power amp board up to the front panel mixer. Disconnect it. Now is there hum in the speakers or not? NO HUM
If there is still hum, them the power amp is not yet ready, but if it is quiet that way, the power amp probably works. IT IS QUIET

There will be the four pins sticking up where the cable was. One of those is the inpit to the power amp. Touch it with your finger, and a hum should come out if the amp is working.
I GET A HUM ON THE 1 ST TERMINAL
SO if the amp works, then your mixer panel has issues. That is the whole front panel. YES I THINK SO

COnnect the cable to the power amp back up - it poweres the mixer panel. Now measure for DC voltage at each of the patch jacks - lower right on the front. A simple way is to plug a cord into each jack, and use the other end of the cord as a test point for your meter. If you see +15v or -15v at any jack, there is a shorted protection diode near the jack. NO VOLTAGES AT ANY PLUG IN JACK ON THE FRONT PANNEL

If those are OK, check the +/-15v power supplies coming from the power amp. Are they both the same voltage? Are they clean? (No AC voltage present?) DON'T KNOW WHERE TO CHECK THESE?

Go down the ICs in the master section and check for DC voltage on the output pins of each. PLEASE DESCRIBE THIS PROCEDURE IN MORE DETAIL

Is the hum present when ONLY the master is turned up and ALL other controls set to zero?
YES HUM GETS LOUDER WHEN MASTER IS TURNED UP

THE SIXTH CHANNEL SEEMS TO HAVE MORE VOLUME THEN THE OTHERS?
WHEN TESTING THE GRAPHIC INPUT JACK FOR DC VOLTAGE THE COMPRESSION LIGHT COMES ON THEN GOES OFF? NOT NORMAL!!

[Enzo]

If those are OK, check the +/-15v power supplies coming from the power amp. Are they both the same voltage? Are they clean? (No AC voltage present?) DON'T KNOW WHERE TO CHECK THESE?

The four-wire cable in the corner? The four wires are power amp input, ground, +15, -15. Ground your meter and check right at that connector. Otherwise, check at the power pins of an IC.

Look at the schematic, all your ICs look to be 14-legged. The TL074s have power at the center leg on each side. The 4741 types have power on legs 4 and 11. Oops, one little 4558 8-legger on the master for the reverb. It gets power on oins 4 and 8.

Isolate the problem. There is a small cable connecting the master section circuit board to the channel circuit board - a short 5-wire cable. Unplug that cable from the channel card, so now the master board is the only board connected to the power amp. Is the hum still there? If it is gone, then the channel board has trhe problem, but if it stays, then the master board is at fault. This is an important step.

Go down the ICs in the master section and check for DC voltage on the output pins of each. PLEASE DESCRIBE THIS PROCEDURE IN MORE DETAIL

Look at the schematic. All the TL074 ICs have the output pins on the corners. (pins 1,7,8,14) All the ICs on the channel board are TL074. On the master board, there is the one 4558, with outputs on pins 1 and 7. There is a TL074, and there are some 4741s. The output pins of the 4741s are on the drawing.

Isolate the problem. Plug a signal into the Graphic In jack. Does that come out clean from the speaker? No hum? If so, all your 4741s are OK.


DOn;t worry about the peak light when you conected the volt meter.

Isolate the problem. YOu said the hum goes up with the master control when all other controls are at zero. Then you said channel 6 is louder. If the thing hums with all controls at zero, we need to get that fixed before we concern ourselves with the channels. Then bring each channel up one at a time and see if any of them add hum.

[captainlee]

pre amp board

Is it necessary to unscrew all the nuts for the plug in jacks from the front panel to get at the circuit board?

I put a drum machine signal into the graphic in jack and heard the drums that were playing.



Ok Enzo [I] will work on them tomorrow Thank You Very much. Cap.

[Enzo]

What you see is what you get. To take a board off the panel, the jack nuts and pot nuts have to come off, and of course all the knobs. Might even be a small screw up by the graphic, I forget.

Some of these tests can be done from the back side of it, you do what you have to do to get at it.

I had to service a LARGE mixer for a church near here not long ago, if I recall it was something like 60+ channels plus all the master section. Every board had to come off. I counted 896 knobs that had to come off - and go back on with the colors in the right place. Over 250 jack nuts. Piles of XLR jack screws. SO let's not gripe too loud about a 5 channel mixer.

[captainlee]

Master pre amp panel

Actually I was wondering how get those nuts off with out using a pair of needle nose pliers. I have a set of nut drivers but not that big and my sockets
are not thin enough. I just don't like to mar the nuts but I'll get them off. Thanks.... Cap.

[captainlee]

Master Board update

Hi Enzo,

Removed jumper from channel board to master board hum still there.

power amp board to master board checked voltage on pins 1 and 4 +15- vdc
ok.
The master board has 4 ic's

3 TL 074's which test at the middle leg on both sides +15- vdc all 3 ok
1 7942 / 5532 8 pin pins 4 and 5 check +15- vdc pins 4 and 5? ok dcv.
I'm not sure what pins this above one has .reverb works.

still has hum and gets louder when turned up on the master.

If you think the problem is in one of these ics I'll order them and we can see
what happens. but I need to order them from mouser because i have a credit there will the part numbers be the same?? or do you know what they are?


thanks Cap.

[Enzo]

Standard 1/2" nutdriver for the jacks and large pots. I forget which size is for the pot nuts, but pot nuts are not recessed anyway.. My 1/2" driver has a screwdriver handle (my smaller sizes are an Xcelite kit with snap-in blades) and is a Sears Craftsman. Has always fit in those little holes just fine. A good Sears or well stocked hardware should have also "thin-wall" sockets for standard socket wrenches.

Um... is this an "XR500" or an "XR500C?" All TL074 on the master board doesn't agree with my XR500 schematic. In the center of the master board, written in solder, does it say XR500? Or XR500C?

Since you seem to have good power to the board, I would check the output pins - the four corners of the TL074s - for DC voltage. Ignore a fraction of a volt, we just don;t want to see several volts of DC.

Before you order parts, find out if they are bad. If you think one might be bad, then swap it for one from the channel board, or even just swap places between two on the master board. They are in sockets aren;t they? Make sure to note which way each socket faces. yes, a bad IC will still be bad if we move it, but the problem will move. That is how we tell. If two different ICs still hum, then chances are neither was bad.

[captainlee]

Master Board update #3

Hi Enzo, The master board reads XR-500

The TL074 4u ic has no voltage on four corners and has +15- vdc at cnt pins

The TL074 U2 IC has 1.5 vdc at all four corners and +15- vdc at center pins

The TL074 U3 IC has 1.5 vdc at pins 1,7,8. 14 has no voltage and +15- at c pins

The 5532 U1 has 1.5 vdc at pins 4 and 5 the rest of the pins show no voltage.

I have a new TL 074 and I changed out all of these ICs and they all read the same voltages before and after the exchanges.


still have hum. Thanks Cap.

[Enzo]

OK, disconnect the speakers from the amp. Run a signal cord from the MAIN out jack to the input of some other amplifier and speaker. Is the hum still there through the other systen? And does it still turn up and down with the master?