Need help replacing scratchy pot on Peavey XR-400

[patrouble]

Hello all... new member here.

I just picked up a 1984 Peavey XR-400 pa mixer. Everything works great other than the master volume pot is scratchy and cuts out.

I'm pretty good with a soldering iron, and can easily swap out that bad pot. My question is, how will I know EXACTLY which pot to order? I don't have anywhere local to shop for such an item, and I hate to order the wrong part online.

I can provide pictures of the actual pot, if that would help.

Any assistance would be much appreciated. Thanks!

[52 Bill]

Welcome to the place. Have you tried cleaning the pot with DeOxit? DeOxit will often bring even the scratchiest of pots back to life.

If the pot is really bad and needs replacement, order the schematic from Peavey and you will find the part number for the pot. You can then call Peavey and order it over the phone with a credit card.

[patrouble]

Thanks Bill. Considering I don't work on electronics very often, I figured my $15 would be better spent on a new pot rather than a can of Deoxit. From what I understand, when pots get scratchy, cleaning is just a temporary fix. Maybe I misunderstood that one, though.

I'll have to look it up and see what a new pot costs through Peavey. Thanks for the tip. Have any idea of their turn-around time for ordering and shipping to the Midwest? Are we talking days, or weeks?

Thanka again, Brad

[patrouble]

One more question...

I've been all over the Peavey website, but can't seem to locate anything about schematics. Is that within their main site (peavey.com) or is there a "secret" location for only those "in the know"?

Thanks! Brad

[52 Bill]

If the pot is actually worn or damaged the cleaner will help, but it will not fix the problem. If the pot is just dirty and oxidized the cleaner will work and will last until it either gets dirty again or wears out. I don't know whether your problem is dirty or dead, but cleaning is usually the first step. It's your choice.

Replacing the pot will require the removal of the pc board from the amp. If you are lucky the board will only contain a few sections of circuitry and will be easy to remove and replace. If you aren't so lucky the board will contain all of the preamp circuits and removal will test your skills and patience.

Email the customer service department and ask for the schematic. You should include the make and model and serial number, etc. so that they can email you the correct schematic. This usually takes a day or two.

I usually get parts within a week of ordering, but it sometimes depends on in-stock status of the parts.

[patrouble]

Great! Thanks for the advice. The board is extremely easy to get at. I took everything apart when I first bought it so I could give it a good cleaning. I thought about swapping the faulty pot with another from a channel that is rarely used (hence, why I happen to know how easily the board comes out) but this bad pot is the only one of its type.

Anyway, I feel good about moving forward now that you've steered me correctly.
I think I'll try cleaning first.

Thanks again,
Brad

[Jazz P Bass]

Your best bet for repacement parts is to call Peavey Customer Support at 877-732-8391 ext 1386.

[Enzo]

The master volume pot is part number 71190310, which you get by CALLING the parts department at Peavey. It is a 100k audio taper. The only other same type pot on the mixer is the Monitor Master control. You cannot swap any controls from the channels into the master volume position.

[patrouble]

Thanks Enzo and Jazz P Bass. That should make things much easier. I knew you guys could help me.

Here's a picture of it, by the way.

Take care, Brad

[patrouble]

Good news, guys. Considering I'm a rather impatient person, waiting a few days on a new pot or a can of DeOxit was difficult for me. Thanks to Enzo's accurate information, I successfully swapped the troublesome pot with the main monitor pot (a feature I don't currently use). It works like a dream now. Since it was such an easy procedure, I think I'll just go ahead and order a new one now, and forgo the cost of the cleaner.

I do have another question, though. Any idea why I still have (audible) signal through the main output jack even with all the volume controls at 0? For instance, I was running a backing track from my computer to a channel input, and it was louder than I wanted. After dialing the channel volume all the way down AND the master gain all the way down, it's still too loud. This happens on all 4 channels, regardless if you're using input A or input B. Not a big deal, just curious what I might not be understanding here.

Thanks again for all your help.

[Ronsonic]

Any idea why I still have (audible) signal through the main output jack even with all the volume controls at 0?

A common failure mode for pots is to not go quite to zero ohms. You can measure from wiper to cold lug to check that. In a guitar amp this is not much of an issue, on a mixer, not so good. Sometimes it's a fault in manufacturing. Other times residue and oxidation builds up on the contact at the end of the pot keeping the wiper from cleanly connecting.

You'll find this caused by poor connections to ground. IINM, the mixer section is grounded through the sleeves of the input jacks and the serrated washers. In any case, nail those down with a shot of DeOx on the washer and securing any other ground connections you find. I recall seeing a number of broken solders on those boards so a general reflow might be in order. Anyway check the ground path from wiper on down and you'll find it.

[Enzo]

Plus, especially if the input signal is pretty hot, there is enough crosstalk in the amp to allow a small amout to pass.

Even if you are not using them, make sure to zero out the effects and monitor controls on the channels.