Peavey Mark VIII 602 Output Board

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Peavey Mark VIII 602 Output Board

Post by GTS » Sun Feb 14, 2021 5:16 am

I received a Marl VIII Amp for repair and it makes a clicking noise (sounds like a discharge) when the Volume Poti is turned beyond a specific point.

I checked all output Transistors and they are good.
I fed the Amp with a Sinus and followed the Signal, it is totaly screwed after the first OP Amp but it is new and the Voltages on the OP are good.

I found there are 2 Resistors (R159 and R160) in the Schematic which i cant find in the Layout Drawing and not in the Amp itself.

Have they been solderd directly to the collector of the Transistors Q100 and Q112 or where are they located?

Thanks for any Help which will rectify this amp.


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Re: Peavey Mark VIII 602 Output Board

Post by Enzo » Sat Feb 20, 2021 12:56 am

Look at the schematic, note the mystery resistor is between the collector of the Q and the V+ rail (or same on negative side) SO measure resistance between C of Q100 and the C of the main V+ power transistors. YOu either get about zero ohms - as in no resistors, or you get about 4.7 ohms. Either of those is good.

The original board didn't have the parts. Then they updated the board and added the resistors. The original board never got a published schematic. The updated schematic is all there was. On boards with the added resistors, the two transistors Q100. 112 are mounted upside down because the legs are "backwards". If your drivers are 5332 and 5331, then you don't have the resistors.

A note about testing transistors in these amps. Your meter can find BAD ones, like shorted, but your meter cannot tell you the parts are good. Your meter applies a tiny litle current, but these parts conduct high currents at much higher voltages. Your meter cannot do that.

You have a Power AMp In jack, apply a signal ther, does it come through clean?

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