Monoblock amp hot at idle compared to its brother

[240z4u]

I have a pair of Professional Studio Engineering monoblocks. One is running quite a bit hotter than the other at idle. They both sound good, but the extra heat from the one amp is disconcerting.

Anyone have any thoughts where I should be looking. I am pretty sure this is a setup issue and I need to get the bias/rail voltage dialed in. I can also adjust the quiescent current but am not sure where/how to test to see what it actually is.

Thanks everyone! If this turns out to be real messy I will take it in but wanted to give it a shot first since the tech is over an hour away.

[Andy5112405]

Initially try measuring the voltage drop across each and every SOURCE or EMITTER resistor.

This will indicate whether (1) Both amps have been set-up similarly and (2) Is one output device is dawing more current than it should be.

[240z4u]

I did check that in one location on each amp, I can check all. The voltages were the same between the two I checked though. ASSuming that is okay, what's next? I will check it anyway.

Thank you.

[Andy5112405]

If the voltages are the same then both amps MUST BE drawing the same idle current.

Have you got a blockage (dust) in the heatsinks ?

If they are VERY OLD, then you might have OLD heatsink goo between the output devices and the heatsinks. Might be worth cleaning everything and re-gooing them.

Are they fan cooled ? One fan may not be running at the intended speed.

[240z4u]

They aren't that old. Prob. 1995.

The amp that is running cooler is the one that has been re-gooed
within the last few years.

I will recheck those voltages to make sure, my memory is foggy on that
one. Should I also check the rail voltages?

No fans, the whole front of the amp is a heatsink.

[240z4u]

Okay I was wrong. Measured .017v across the emitter resistors on the hot amp and .001vdc across the cool amp. Emitter resistor is .15 ohms rail voltage is 51.2 on the hot amp and 51.9 on the cool amp.

Now, I am starting to wonder if the amp that is showing .001 is actually too low. I did put outputs/drivers in this amp when I got it. It's the one that's running cool. I had to replace a burnt resistor as well that was in the protection circuitry.

Thank you.

Evan

[gizmo1961]

Problem may lie in the power supply or the output. Sounds like a parasitic on one or more transistors.

[240z4u]

Gizmo, sorry to sound dense. Are you talking about the running hot amp or the running cool amp? The one that's cool is the one I had repaired.

Thanks!

[kevinkr]

This isn't that complicated, the cool running amp is biased very cold at less than 10mA, whilst the hotter running unit is a little over 100mA which is probably about right..

If you have the service manual or can get it it should be comparatively easy to rebias whichever amp is not running close to specification. My suspicion is that the cold amp having been repaired was not adjusted after the repair. There should be a bias pot in there associated with a VBE multiplier or similar.

No need to over-complicate or make negative prognostications, the amps are running after all.

Edit: I'd say this is probably a job for a tech as a serious misadjustment could blow up the amp.. (I spend 3 hrs a day getting to and from work so an hour to the tech seems not so bad..)

16yrs old for a solid state amp is pretty elderly actually, and it is probably time to consider a recap of both amps. (Caps from Digikey or Mouser to avoid issues with counterfeit parts.)

[240z4u]

Kevin, I know the amp wasn't re-biased properly because I didn't re-bias it.

There is a pot clearly labeled "bias". There is also a pot labeled "quiescent current".

I don't have a service manual and it's unlikely I can get one since the company is out of business. I do have the owners phone number, but I am hesitant to call him at home with a tech question.

Evan