Peavey XR-1200 Vintage Powered Mixer

[Guitarman67]

I have a old school XR-1200 that I have a pretty much love / hate relationship with. Hate it's weight but love it's sound. My problem is if I hook up two speakers in standard stereo mode as shown in the user manual it's got a hum but if I use two patch cables and run it in mono with both power amps going and only EG - A then it sounds great. Could it be anything I'm doing wrong? Or does it have a problem inside it?

[Enzo]

Hi
Kinda frown on two threads on the same system...

OK, two patch cords connected WHERE?

Connect the two speakers - left and right - ONLY, no signal input or other connections to the amp. Now do both speakers hum, or just one? Does turning all controls to zero affect the hum?

Does it only hum when the master volume is up?

Plug a guitar into power amp IN jack A, turn guitar volume to zero, does speaker A hum? Now plug guitar into power amp in B, same test, does B speaker hum?Now plug the guitar into Graphic In A, hum? Now graphic in B, hum on that side?

[Guitarman67]

Hi
Kinda frown on two threads on the same system... Sorry I didn't think anyone would see it under the speaker thread.

OK, two patch cords connected WHERE? Channel B power amp in to channel A power amp out, Channel A Graphic in to SUM out. If I hook it up this way there is no hum but this is mono.

Connect the two speakers - left and right - ONLY, no signal input or other connections to the amp. Now do both speakers hum, or just one? Does turning all controls to zero affect the hum? Yes both speakers still hum but if EQ is all dialed down to zero no hum.

Does it only hum when the master volume is up? It hums when A or B is dialed up or the master volume.

Plug a guitar into power amp IN jack A, turn guitar volume to zero, does speaker A hum? Now plug guitar into power amp in B, same test, does B speaker hum?Now plug the guitar into Graphic In A, hum? Now graphic in B, hum on that side?

I tried those things no hum. If I hook into power amp in on the A side there is no hum on A but I can slide B up and hear the hum. Tried on the EQ in on both channels and there is no hum.

[Enzo]

OK, I am going to get picky, because we need to isolate the problem.

Does it only hum when the master volume is up? It hums when A or B is dialed up or the master volume.

I see main A and main B, I don; see a MAster. I see a SUM, but we are not connecting anything at this point. SO: Does bringing up A increase hum in both speakers or just A or B? Likewise does bringing up B increase hum in both or just A or B speaker?

Turning the EQ sliders down reducing hum means the hum comes from before them.

Turning the A and B mains up and down, if that controls hum level, then the hum comes from before them.

Take your two patch cords, plug one from Main B Out to power amp B in. And the other from Main A out to Power Amp A in. We have just bypassed the two graphic EQs. Do you still get hum out of either or both speaker? (specify)

[Guitarman67]

If I bring A up the hum is only in one speaker if I bring B up it's in that side. With the patch cables in bypassing the EQ the hum is still there. If I turn A and B up the hum is in both if I cut one out the hum drops out of that side.

Wired up this way there is no master volume working just A and B if I wire it up in mono the SUM also called the master works.

[Enzo]

We won't be using SUM. SO we have both A and B causing hum if turned up.

Do we currently have all the input channel controls at zero? We should. If any of them were up, did that reduce the hum when turned to zero?

On your master board, check the power pins of the op amps, do both pos and neg pins show about 15-16 volts? And is that power clean of ripple?

[oc disorder]

For those following at home I presume it's this one.. from user manual

[Enzo]

Yep, and that patch panel is a powerful troubleshooting tool right there.

[Guitarman67]

I'm just about ready to scrap this thing and start over. It's got a hum in it rather you use mono or patch it over into stereo. Your way of testing it going straight into the power amps gave no hum. I'm wondering if maybe my garage is just not giving this thing enough juice from the outlet. The three way power switch on the back of it I think may have a problem. If you use this switch it pops the breaker in the outlet. I tried a different outlet on the other side of the garage and still got a hum in it. A month ago in our other house we moved out of it had no hum and was very clear. I don't have a hum in my Fender Sunn powered mixer but I'm sure it's a different animal.

[Enzo]

OK, the tests I described were to do just one job, decide if the hum came from the power amps (apparently not) or the graphics, or the preamps.

If flipping the power switch one way pops breakers but not the other, go get an outlet tester. You ought to have one anyway. They cost $4-$5 and can be had at any hardware store. Plug it into your outlet on the wall and see if the lights are correct. It sounds like your outlet is miswired. Yes, that can cause hum.


Otherwise, you seem to have localized the problem to the preamp part, the main faders turn it up and down.

I also asked about your 15v power supplies, and asking if they had excess ripple. Did you check?

[Guitarman67]

I'm not sure what a ripple is or how to check the power supplies. But I will get an outlet tester today.