Peavey Standard

[lowell]

Anyone have a schematic for the PA for this? I emailed PV, but it's Sunday and I'm trying to get a head start on this thing. If someone has it that'd be great, otherwise I'll wait till they respond.

It has what sounds like Johnson noise - hissy/crackly noises, constantly. I disconnected the preamp, don't see any bad solder joints, freeze sprayed everything - no help. Will need to scope or signal trace and a schematic would help.

https://reverb.com/item/2703856-vint...74nxoCGDzw_wcB

[Enzo]

http://bmamps.com/Schematics/Peavey/...Schematics.pdf

Most likely a noisy semiconductor, probably the diffy pair.

[vintagekiki]

https://assets.peavey.com/literature/manuals/standard.pdf

http://www.freeinfosociety.com/electronics/schemview.php?id=646

http://www.thetubestore.com/lib/thetubestore/schematics/Peavey/Peavey-Standard-PA-Schematic.pdf

more

http://www.thetubestore.com/Resources/Peavey-Schematics

[lowell]

Ok. So targeting the differential pair to see about the noise. What is the approach here. Is it similar to grounding the grid of a 12ax7 to turn it off? Can I ground the input at the 4.7k junction? I'm gonna try this...but curious on the ideal and most effective approach at isolating transistor stages. I see that the base is at 0vdc so grounding it shouldn't hurt anythin nor will it affect the bias. So maybe this only will tell me if the noise is before the diff pair. Id probably need to bias it off or use a bfc on its collector to see if that kills the noise.

[olddawg]

Ok. So targeting the differential pair to see about the noise. What is the approach here. Is it similar to grounding the grid of a 12ax7 to turn it off? Can I ground the input at the 4.7k junction? I'm gonna try this...but curious on the ideal and most effective approach at isolating transistor stages. I see that the base is at 0vdc so grounding it shouldn't hurt anythin nor will it affect the bias. So maybe this only will tell me if the noise is before the diff pair. Id probably need to bias it off or use a bfc on its collector to see if that kills the noise.

A diff pair by definition have a matched hfe -beta. Normally tickling the diff pair one transistor at a time with freeze mist will let you know immediately if it is unstable. Be careful messing with a diff circuit. If you screw up you will slam the rail voltage across your output.

[lowell]

I did find that the 47ohm on the emitter of 430 driver was toast...burnt to a crisp. Replaced it and the noise is still there...would this lead one to think that this driver has been stressed and could cause this type of noise?

I tried the freeze spray on the diff.. no change. However, I left the amp on for about 10 minutes while troubleshooting and the noise went away. So not sure how to proceed. Gonna keep it off for a bit and try again.

[The Dude]

I did find that the 47ohm on the emitter of 430 driver was toast...burnt to a crisp. Replaced it and the noise is still there...would this lead one to think that this driver has been stressed and could cause this type of noise?......

Did you check it to see if it's shorted or leaky? Resistors most always burn for a reason.

[lowell]

I cant be shorted cause the amp sounds fine. Not sure how to test for leaky. Is that a less than optimal forward voltage drop from b/c and b/e using my meters semi function?

[The Dude]

Most of the time, but not always, if you yank the transistor and check it on a high resistance scale, a junction will read similar both ways/polarities indicating that it is leaky. A kind of "rule of thumb" is that it should measure AT LEAST 10 times the reading reverse biased as it does forward biased (talking probe orientation). It could also be failing in circuit and check perfectly fine out of circuit. Are the voltages on the transistor what you would expect? It might just be easier to sub it out if you have one.

Most of the time a simple test using the diode check function suffices. I have found transistors that check ok on diode test, but show leakage using high resistance scale readings.

[Enzo]

cant be shorted cause the amp sounds fine

Never assume. Dead shorts are not the only failure mode for transistors.

[lowell]

True and I agree... but, wouldn't this thing sound bad if it were shorted? I guess you're saying "leaky".

[olddawg]

Are you sure you aren't hearing crossover distortion? Has the unit been worked on before? It took some current to burn that resistor. Did someone replace the driver and miss the resistor maybe? If there is a bias pot... mark, check, and clean it. And always remember that in circuit, ANY working silicon bipolar transitor will measure .6-.7 v across a forward biased emiter/base junction.