Peavey XR 8600

[lowell]

I'm attempting a fix on one of these. One channel works but there's a hum and it seems less powerful than it should be. The other channel simply hums.

With the output switch and set it to mains/monitor that is.

No fuses are blown. After disassembling and attempting to run it I saw smoke and immediately powered it off. I made an attempt at being sure the grounds were still connected using alligator clips... but maybe I missed one.

This sort of PA is still a mystery to me. It is not an emitter output stage. It seems that the filter caps for the rails are the output? The topology also, after consulting Doug Selfs book, seems to be Class G with the stacked transistors.

Any help in understanding how the output works would be appreciated. Also any books or links to education would be great. Thanks!

[nosaj]

I'm attempting a fix on one of these. One channel works but there's a hum and it seems less powerful than it should be. The other channel simply hums.

With the output switch and set it to mains/monitor that is.

No fuses are blown. After disassembling and attempting to run it I saw smoke and immediately powered it off. I made an attempt at being sure the grounds were still connected using alligator clips... but maybe I missed one.

This sort of PA is still a mystery to me. It is not an emitter output stage. It seems that the filter caps for the rails are the output? The topology also, after consulting Doug Selfs book, seems to be Class G with the stacked transistors.

Any help in understanding how the output works would be appreciated. Also any books or links to education would be great. Thanks!

Can't help you with it, but which one of Self's books did you consult?

nosaj

[Enzo]

Look up "flying rail" amplifiers or "grounded emitter" amplifiers.

The speaker is wired between ground and the transformer center taps and their filter cap commons. That may seem funny, but look at it this way. In amps you are familiar with, the speaker connects between the output emitters and the grounded filter commons. The only difference here is that we ground the other end of the speaker. Everything else is pretty much the same.

The 40v and 90v are steady DC voltages across the caps, but with respect to ground (chassis) the rails have signal on them.

[Enzo]

Look up "flying rail" amplifiers or "grounded emitter" amplifiers.

The speaker is wired between ground and the transformer center taps and their filter cap commons. That may seem funny, but look at it this way. In amps you are familiar with, the speaker connects between the output emitters and the grounded filter commons. The only difference here is that we ground the other end of the speaker. Everything else is pretty much the same.

The 40v and 90v are steady DC voltages across the caps, but with respect to ground (chassis) the rails have signal on them.

[lowell]

Nosaj it's the book on Designing Solid State Amplifiers.

Enzo thanks. Does the 1k resistor from B to E work in a stacked Class G topology when trying to see if the circuitry pre-power amp and drivers are working correctly? I have one NPN and one PNP drawing more current than the rest. And they are running away. The transistors in parallel are fine... so I'm guessing it's not the circuitry but the transistors.

[Enzo]

So pull them and find out.

[Jazz P Bass]

Here is a good read on the 'grounded collector'/ Class H operation.

RMX_Service_Manual.pdf

[lowell]

Thanks Jazz I'll read that.

Enzo I had tried it...but got no voltage readings. I then discovered my variac fuse had blown. Oops.

I currently do not have the heatsink mounted. Is it possibly ir even likely that they are running away with quiescent current because of no heat sink? I wouldn't think they'd runaway just sitting there with no signal...but maybe that's the reason. I have my bulb limiter in circuit and slowly increases brightness. 2 Re .22ohm have 130mv across em. 1 PNP 1 NPN. Others have 0v.

[Enzo]

YES!!! NEVER run an amp without its heat sink, the transistors rely on it, it only takes seconds to cause damage. How long would you run a car engine with no oil in it?

[lowell]

Gotcha. I put it on. Everything seemed kosher with the limiter in place and heatsink mounted. Removed limiter. Had about 5A mains current at power on... 1 minute later it smoked again. I think I'm chasing my tail and may need to put a brand new FULL set of transistors in here.

These ballast resistors, Re, are very thin tube-like 5watt resistors. Your typical 5w cement resistor will not physically fit between the heatsink. I think they're metal oxide. Any tips on finding replacements? Do they have a more specific name? A fee of em have cracked from smoking.

[lowell]

Here's a pic of the slim 5watt resistors

[nosaj]

Here's a pic of the slim 5watt resistors

Call Peavey their customer service is really good.

nosaj

[lowell]

The resistors are CA5100 Vishay. I cannot find them online. Had to order from Peavey. They're 3mm diameter 5watt resistors!!

[lowell]

Chased my tail with this for 6hrs straight today. Replaced one thing...then another component smokes on and on.

I have the 1k/2w resistors from B to E in place of all power transistors.

Hopefully I'm getting closer. The 79L15 regulator that feeds the PNP driver base opened...so had to order it.

[lowell]

The .22ohm ballasts are

Part Number#30241102

Description .22--5W--5%--WW--AXL

In case anyone stumbles onto this thread and is in the same predicament.