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Peavey XRD 680S Plus
- Thread starter joeyd999
- Start date
Yup. That's the preamp and memory effects only. Not power output.
Preset potentiometer aka trimmer.
Measure the bias by removing a fuse connecting a current meter across the fuseholder. If the fuses are before the main reservoir capacitors this might blow the fuse in your meter On switch-on.
A better way is to connect a low-value resistor in place of the fuse and measure the voltage across it.
Most presets are wired so that clockwise increases the bias. Some were designed by engineers who were looking at the board from the wrong side. . . .
I found that one as well, but it’s just the preamp
Lucky.
the Bias is done By a “telepathic thermal link”
schmitt trigger
- Joined Jul 12, 2010
- 2,090
Ian, at first I thought you were joking, but indeed it is done via a telepathic thermal link. 
You are lucky, Joey. There is no additional adjustment.
You are lucky, Joey. There is no additional adjustment.Okay, now I'm intrigued. Explain please. Is this an engineering joke, or some ingenious technique?Ian, at first I thought you were joking, but indeed it is done via a telepathic thermal link.
Makes you wonder whether the bloke in the drawing office was having a laugh, or whether the engineer was winding up the bloke in the drawing office.Ian, at first I thought you were joking, but indeed it is done via a telepathic thermal link.
View attachment 344293
It uses a diode to sense the temperature of the output transistor. It is probably stuck to the heatsink with something that looks like old chewing gum. It might even have fallen off, in which case you can blame that for the failure. Watch out for it when you replace the output transistor, and make sure that the diode is in thermal but not electrical contact to the heatsink.
Wouldn't that add distortion at high output power?
I'll take pictures when I have it disassembled and post them here before I proceed with repairs. I'll probably need help sourcing the transistors.
No. The aim is to keep the standing current at zero output power (the “bias”) constant at about 50mA. Because Vbe of a transistor reduces by 3mV/°C the bias has to reduce by the same amount, otherwise the bias would increase as it gets hot, and It increases exponentially by about 1 decade for 60mV extra Vbe, or alternatively 1 decade for 20°C increase in temperature, so you can easily get a thermal runaway effect.
Crossover distortion approximates to a step of fixed size in the output waveform. As the waveform gets bigger, and the step says the same size, the percentage reduces. Crossover distortion is always worse at low volumes.
Like "blinker fluid"?
schmitt trigger
- Joined Jul 12, 2010
- 2,090
Blinker fluid?
WTF is that?
To answer your question, read the following article. You can scroll down and skip until the paragraph
Biasing a Class AB Amplifier
https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html
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For your car, to run the turn blinkers.
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schmitt trigger
- Joined Jul 12, 2010
- 2,090
Ooooh!
So the windshield wiper fluid is the one that powers the wipers?
So the windshield wiper fluid is the one that powers the wipers?
Exactly.
