Peavey XR 600B power amp noisy

[ljrose]

Hi all

Newbie here--I was looking at older posts and there it was suggested to start new ones which would be more likely to be noticed.

I have an old XR 600 B which is mostly working, just a substantial amount of hiss which sometimes lessens and almost goes away, but eventually comes back. I'm pretty certain it's the power amp, as the noise continues unchanged when something is plugged into the power amp in jack. The mixer section adds its own special hiss when cranked up, but that's not too bad. The amp noise is constant no matter what the front panel settings are.

Catch phrases from my past like "leaky transistor" and "cold solder" are popping up in my head, but I hope to get a real clue here. I'm fixing up this amp so I can give it away to some upcoming rockers--I just don't have the room for it, but would like to see it properly working before it goes.

thanks

LJR

[Enzo]

Which power amp is on your unit? 400B/G? 400BH? Those are VERY similar, but we need to use the correct one - part numbers may not match up otherwise. Or the older "400 series"? That one has the individual plastic transistor covers on the back.


It is going to be a noisy semiconductor. On the 400BG or 400BH, my fist suspect is the TL074 IC. Try a swap there.

If the older 400 series, then one of the differential pair most likely.

[ljrose]

Hi Enzo

Thanks for the quick reply! It's a 400 BH.

LJR

[ljrose]

I'll start shopping for the IC -- wouldn't it be nice if Radio Shack had one... thanks again

[Enzo]

There are more TL074 on the mixer/preamp section. Since they are all in sockets, just steal one from up front, at least long enough to verify it is or is not the problem. You WERE planning to take the mixer panel apart for pot cleaning... weren't you...?

[ljrose]

Now that you've shamed me into it, I guess so. Nothing has been scratchy, but the chance of nailing down the power amp question makes it worthwhile. I'll be back with the results.

thanks

[ljrose]

Yep, that seems to have been the problem--the noisy ongoing crackle has stopped. Now, to find a replacement IC for the channel I hacked; and to put this all back together. Those reverb tank screws are in a tricky place.

great assistance, thanks

[Enzo]

Reverb screws? Isn;t it screwed to the cieling or floor of the wood cab? The rear power amp panel comes out the rear, the mixer panel comes out the front, leaving I thought pretty good access to the reverb pan screws.

You an order a TL074 from most any parts supplier, but are there no amp repair shops near you? One of those would probably sell you a part if you asked them. And a place that fixes stereos or even TVs might even have one. A TL074 is cheap at Mouser or Digikey, but even if a local guy gets you for $5, by the time you order one and pay shipping, you are over that. Of course if you are ordering other things too...

[ljrose]

Hmmm...it didn't seem at the time the mixer came out the front, so I guess I did it the hard way. A fair amount of work for a give-away. In any event, it's back together and still works. I will haunt a TV shop or two and see if I can come up the with part. That would not only save shipping but some time too.

this forum has been a big help--I sure wish this was around 30 or so years ago!! Again, many thanks Enzo

LJR