Peavey XRD 680 PA (dual power amp, one side bad) diagnostics

[The Dude]

Forget EBay even if they cost 50 cents and they send Miss Alabama in a tight bikini to deliver them.

Apparently, I need to become a better shopper!

[Jazz P Bass]

Woohoo.
Miss Alabama, Katherine Webb.



Honey, UPS is here.

[The Dude]

My 50 cent transistors are on order.

[misterc57]

Make sure the 0.33ohm/10 watt ballast resistors are still good.

These all tested good on the DMM.

[oc disorder]

I had one of these recently (small world) with a couple of those o/p transistors shorted. A little more length on the internal hookup wires would have made it easier to service.
I tested it both channels driven into a dummy load just under clipping for half an hour and worked fine.
Then , after cool down did it again into 4 ohms both channels driven worked fine.
Then when connecting it to a speaker the next day It didn't work and unfortunately some transistors in the other channel had failed.
So I guess on powerdown after the 4 ohm test something tossed in the towel !
I also found some of those little 1N 4148 shorted , easy to find with a diode testing setting on a multimeter.
Attaching a PDF of the unit which has a clearer version of the driver board.

304S POWER MODULE USED IN THE XRD680+, XRD680S+, XR1200 AND 1600D, XR886. 6 PAGES. .zip

[misterc57]

Where did you order your transistors from? Thank you.

[oc disorder]

Where did you order your transistors from? Thank you.

I already had some Motorola/On semiconductor MJ 150 24/25's on hand but if I have to buy them I get them from WES components an Australian
company with whom I have an account.

Trade here I pay about AU$8.75 ea per transistor plus courier freight which is usually about 10 bucks.

[misterc57]

I have my new transistors and fuses. But I just noticed that I lost one of the transistor insulators (looks like a clear plastic) that sits between the transistor and heat sink body.

I have heat sink compound. Is this insulator needed? If so is there any material I can make one from?

Thank you.

[The Dude]

Yes, the insulator is needed. It isolates the tab or body of the transistor from the heat sink, which is normally grounded. If they didn't need them, I doubt they would go through the trouble to install them.

[misterc57]

Is there anything I can use to make one? Thank you

[The Dude]

I've never really had to or thought about it. I have plenty of extra stuff laying around the shop. Maybe someone else will chime in.

[Jazz P Bass]

Ding.
If you are only testing the amplifier you can use anything that is thin & nonconductive.
Although I wouldn't drive the amp into full rated output power.

To finalize the repair you must obtain the correct mica isolater.

The mica serves two purposes.
1: it isolates the transistors from the heatsink. (which in this case is at ground potential, while the collector is at the high voltage rail.)
2: Because the mica is isolating, it is also insulating. As mica conducts heat very well, it is not perfect. Neither is the interface between it & the transistor case nor the heatsink.
That is why you use a small smear of silicone grease on the transistor, the heatsink & both sides of the mica. To fill in the microgaps.

[misterc57]

Finally got all the parts needed. Amp is now working after replacing the two bad output transistors. Thank you to everyone for the help! MC

[kk4msa]

Finally got all the parts needed. Amp is now working after replacing the two bad output transistors. Thank you to everyone for the help! MC

can you replace just the ones that are bad, or do you have to replace all due to matching?

[Enzo]

You can usually replace just the bad ones, they are not matched. However, you DO need to use the exact same types, Do not mix in NTE replacements or something.