reduce power to reduce heat from UPC1342V kit

[nine0nine]

Not sure if I am barking up the wrong tree here, but here's my current situation.

My voltmeter is set to 2v range, so I should be looking at lowering the trimpot until it reads 0.015v so its 15mV, correct?

It seems the starting voltage was 440mV, at least with my 'methods'. 440 down to 15 seems...a lot

[AllenB]

440mV/0.25 ohms gives 1.76A. Multiply this by 2 as there are double outputs, by 90V worth of rails is a few hundred watts. Yes, that would be very hot.

[nine0nine]

I'm almost ready to test the amp fully, i lowered the mV down to 50 and it runs hot, but its acceptable. If it actually works I may add a fan to the case later.

[nine0nine]

and what do you know, it works!

really surprised, but after 10 minutes of playing an mp3 from an old phone, it was warm, but actually cooler than I expected, so If needs be I could probably raise the power a little.

Now to do the other amp and try and figure out how I'm gonna fit the other bits.

[FauxFrench]

Originally Posted by nine0nine

and what do you know, it works!

really surprised, but after 10 minutes of playing an mp3 from an old phone, it was warm, but actually cooler than I expected, so If needs be I could probably raise the power a little.

Now to do the other amp and try and figure out how I'm gonna fit the other bits.

The (total) quiescent current you know by measuring the voltage across one 0.25 Ohm/5W resistor, divide this voltage with 0.25 Ohm and multiply by 2 because you have two power transistors in parallel.
That current you can adjust as you like as long as the components and heatsink do not become more than some 45 degrees (Celsius) warm. In principle, the higher quiescent current the less cross-over distortion. But above a certain quiescent current, the cross-over distortion is reduced very little with further increase in quiescent current while the idle power loss increases linearly. The idle power loss is calculated as the total quiescent current times the full supply voltage measured from "+" to "-". It can be quite a lot. Think of "Greta" when you turn the current up .
Anyway, have fun with your newly recovered amplifier/electrical heater.

[AllenB]

The other channel, does it use the same heatsink?

[nine0nine]

Originally Posted by FauxFrench

That current you can adjust as you like as long as the components and heatsink do not become more than some 45 degrees (Celsius) warm. ...

Anyway, have fun with your newly recovered amplifier/electrical heater.

I originally tried to test the temp with a coffee thermometer (hot water probe thing) and I was hitting 70 on the heatsink, that was before I reduced the power though, it seems easily under 45 now.

[nine0nine]

Originally Posted by AllenB

The other channel, does it use the same heatsink?

well, it does...

The other amp was the first one I powered up before learning of the amps great heating prowess, and at the time I didn't have the heat sink anywhere near set up properly, so after a minute or so I got a whisp of smoke before I pulled the plug on it. I'm not sure if it was just some plastic casing that melted or something more serious...but...

I rewired it up just now, with a correctly set up heatsink and after a few seconds, one of the resisters burnt out, with a bright flame and a pleasant aroma of toast. I'm not sure now if that's a repercussion from the first turn on, or something new.

Looks like I'll put the amp on hold again, although I can still work on the preamp section with my one working channel. Will order another kit and give it one last try. The kit itself is pretty cheap as it doesn't come with capacitors, presuming mine are still OK.

[AllenB]

I understand this is frustrating. I think you need to identify the minimum current setting on the pot and set up your case and wiring. High current can come later.

I have a 15x30x5cm heatsink. Two channels, just under 1A each, +/-24V rails so that's about 90W. The heatsinks get very warm but not hot.

Your resistors don't look like 5W resistors (maybe I'm struggling with the photo). 440mV would be creating less than 1W. This could be a problem.

Is there a chance you could get a variac, or make a light bulb tester? What about substituting a different transformer for testing? It is good to see a circuit with reduced voltage so you can see it is in order at low dissipation.

[FauxFrench]

Originally Posted by nine0nine

well, it does...

The other amp was the first one I powered up before learning of the amps great heating prowess, and at the time I didn't have the heat sink anywhere near set up properly, so after a minute or so I got a whisp of smoke before I pulled the plug on it. I'm not sure if it was just some plastic casing that melted or something more serious...but...

I rewired it up just now, with a correctly set up heatsink and after a few seconds, one of the resisters burnt out, with a bright flame and a pleasant aroma of toast. I'm not sure now if that's a repercussion from the first turn on, or something new.

Looks like I'll put the amp on hold again, although I can still work on the preamp section with my one working channel. Will order another kit and give it one last try. The kit itself is pretty cheap as it doesn't come with capacitors, presuming mine are still OK.

If it gets so hot that it smokes, the transistors are probably ruined. Ruined may mean short-circuited. If short-circuited, the power resistors may have much more voltage across them and they burn.
With a modest price and unless you are experienced with amplifier fault finding, you better buy a new module. Else you have to find and order replacement parts, await arrival etc....