PV 1500 Thermal issue

[Wes]

I've got one of these on the bench and it fires up but it doesn't have any output. So I open it up and it has to be the filthiest piece of electronics I've ever seen. Everything is coated in dust and trash. I blast it out, clear everything outta there, clean and lube the level pots, and it works fine. As I let it run for a bit I hear the fan kick in high gear so I check to see how hot the sinks are getting. Channel B's sink is smokin' hot. I'm hardly running any signal through and even with no signal it gets this way. Channel A's sink is cool as a cucumber in comparison; room temperature.

What could be the cause of this? The channel sounds fine.

Most of the heat is being generated at one end too. That end has Q215, Q214, and Q213 strapped down.

Is this cause for concern?

peavey_pv-1500.pdf

Thank you for your time.

[Enzo]

So do not apply any signal, does that side get hot even at idle? Probably does.

Look at the schematic you posted. Each channel has a control marked BIAS pot. On the hot channel, start by turning it down, that could be all it needs. I don't know which way to turn the control, but it needs to go towards the end resulting in lower resistance.

A simple trick is to monitor the mains current draw. Turn the bias control and watch the current. Turn it the direction that lowers the current. When you get to the point where further turning stops lowering current, STOP, it is adjusted.

If that doesn't help, we start finding why that side is using too much current.

[52 Bill]

What could be the cause of this? The channel sounds fine.

Most of the heat is being generated at one end too. That end has Q215, Q214, and Q213 strapped down.

Is this cause for concern?

Yes, it is a problem. Do you have a load connected to the amp? If you do disconnect it until you get the heat under control.

[Wes]

Yes, it is a problem. Do you have a load connected to the amp? If you do disconnect it until you get the heat under control.

Gotcha. Thanks, Bill. I have transistors on order for replacement. Hopefully that will take care of it.

[Jazz P Bass]

Is there a reason for ordering 'transistors' or are we guessing?

[Enzo]

Agree, those transistors are not probably getting hot because they are bad.

[Wes]

Sorry, when i replied to Bill, for some reason, I did not see your first post Enzo.

Yeah, she's gettin' hot at idle too, no load on. I will look into a bias adjustment.

I ordered transistors on a guess for sure. Thank you guys for the guidance. I will try what is recommended here and look into it further.

[Wes]

So I got back into the amp. Reading the resistance across the bias pot- channel A (which was running cooler) was about 90 ohms while Channel B
(the smoker) was about 120 ohms. I dialed that resistance down to match channel A's and they match now thermally. Pretty much right down to the
degree. Idling no load and running a load with a light signal through- the sinks don't get any more than about 115 degrees F.

I would like to get an ammeter that I can use for Enzo's recommendation- it'd be nice to measure thataway.

Also- do you guys know of a good resource/book for servicing solid state gear like this? There's so much about tube amps, but hardly anything for SS it seems.

Thanks for your help!

[52 Bill]

I would like to get an ammeter that I can use for Enzo's recommendation- it'd be nice to measure thataway.

Another method of measuring the idle bias is by reading the voltage across the emitter/ballast resistors connected to the output transistors.

This would be more accurate than measuring the resistance of the bias pot.

[Wes]

That's kind of like reading across a tubes cathode resistor, right?

How would you determine/what would be the target?

[g1]

Use the number from the "good" untouched channel as your target.

[52 Bill]

That's kind of like reading across a tubes cathode resistor, right?

Exactly right! The very same concept, but because the ballast resistor will be tiny by comparison to a tube's cathode resistor, the voltage will be measured in millivolts.

Sometimes the manufacturer will publish the voltage on the schematic, sometimes you just guess and like G1 says, you can match the value that you measure on the good channel.

[Wes]

Gotcha. I checked the hot channel against the ok one and the draw is matched within a millivolt or two after having made the adjustment.

thanks guys!

[nsubulysses]

Also- do you guys know of a good resource/book for servicing solid state gear like this? There's so much about tube amps, but hardly anything for SS it seems.

Thanks for your help!

I always throw this one out there -- Teemu Kytalla "Solid State Guitar Amplifiers"
Not necessarily service-minded in writing style but lots and lots of info on design, so then if you know how it works you can troubleshoot it