Heat sink, Sound Tech M620 Amp Mixer.

[gary rabbitt]

Hi all, I have been working on a Sound Tech M620 mixer/amp.
Was DOA from the owner, I removed and tested the output transistors, all fine. Found one shorted diode on the amp board. Replaced it, the things sings great now.

The question in, it has a large heat sink on the back of the amp. After playing it with some moderately loud music, one speaker, the heat sink got very hot. I think they make them to run hot. I re-applied the thermal grease on the transistors, so they are transferring heat to the heat sink all right.
There is no factory fan on this unit, so I suspect that the HS is supposed to run really hot, so much you cannot touch it for more than a second. Unfortunately, I don't have a thermal sensor to measure the degrees.
It uses 6 output transistors, and the amp is rated at 250W I believe, and I had it connected to a Peavey SP2 speaker. I want to be sure the thing isn't going to burn up when I return it to the customer. If it were mine, I'd install a small box fan on the thing.
So, are these heat sinks made to run so hot you cannot touch them at all? I say yes, but want to get all your thoughts on it.
Thanks, Gary.
Not my unit but same heat sink arrangement>>>

[The Dude]

My first thought is that it's biased too hot. Do you have the schematic? Have you checked bias? Lots of the Sound Tech stuff does run hot, but usually not so hot you can't hold your finger on the heat sink while it's running at normal levels.

[gary rabbitt]

Thanks! I don't have any schematics I have searched in all the usual places to no avail.
I might put a request in on the schematics column. Yeah this sucker gets burning hot and I fear that even with the heatsink, it's transistors may burn up eventually. I did measure the resistors on the amp board and they all seemed in spec with the marked values. Could be a component on the main board going to the amp module.

If you know of a source for a schematic or service info I'd appreciate it.
Take care, Gary.

[gary rabbitt]

Thanks for moving my post to the schematics column.
I hope that someone has access to schematics of this, or a similar one where I can get some specs on the biasing. As it was, I was going in blind with no documentation, and found the shorted diode.
Other than measuring marked component values, I have no idea on where to measure bias on that particular amp. Something in general, maybe like measuring base voltage at no signal level?
I am guessing that heat sink is well over 200 deg. Normal or not..
Thanks! Gary

[g1]

You can get a rough idea of the bias by measuring the voltage across the emitter resistors of the output transistors.
Then divide by the value of the resistors to calculate the idle current through the output devices.
Post your results.

[gary rabbitt]

Thanks, I'll have to get it back apart this evening if I can.
I'll also get the transistor specs I had wrote down.
Gary.

[gary rabbitt]

Have the amp apart finally, I see the resistors on the emitters. Going to power up in a bit. Those resistors only measure .5 Ohms each. Will be back...
Transistors, pairs of A1694 (2SA4467) and C4467 (2SC4467) Still going in "blind" on the unit, as I have not found a schematic or service manual.
Both are rated at 150 C max, and I don't have an IR thermal meter.

[Enzo]

Typical emitter resistors are under 1 ohm, so that seems OK.

Here is my method: Monitor mains current draw. Now carefully adjust bias watching the current. If it increases, stop and adjust the other way. If the current is decreasing, good, now turn it down until it stops decreasing. Does it get hot now?


You are making faulty assumptions. The fact it has no fan means they expect it to cool on its own by convection and radiation. it does not mean they expect it to get too hot to touch. Under heavy load I am sure it does get hot though.

[gary rabbitt]

Previously posted before your reply Enzo:

Ok, guys, I measured the voltage across the .5 Ohm emitter resistors. .02V
I know I wouldn't see too much across a half Ohm part, but does this tell you anything on the bias? Each bias resistor read the same, and a speaker was connected to the amp, no signal input.
So, would that be .04 Amps, 40ma?
Thanks

Enzo I will get back into it tomorrow. I didn't see any trimmers for bias, but I will look on the main board.
Hard without the diagram. Will see what the mains current is, but is going to be impossible to raise or lower the bias with a trimmer resistor far as I know.

For the 5 or so minutes I had it on, no signal, it barely got warm.
Understand about the no-fan situation and it cools by itself. That's fine, and I would rest assured if this is normal heat, and not an overheat situation that will burn up the parts.. I may be able to borrow a IR thermometer and see how hot it really gets. The Tj Max spec is 150C (305 F) Hard to tell the true temp.
Thank you sir.

[Enzo]

Ohm's law. 0.02v across 0.5 ohms gives me 40 milliamps. That seems pretty high to me. Try adjusting the bias cooler.

[gary rabbitt]

I should have lifted one end of those resistors to measure, I see they are marked 0-22 Ohms.
I measured them In Circuit and they were .5 Ohms.

Ohm's Law across the 22 Ohm part, that would be 1100 ma, which would be really high.
I see no trimpots on the small board before the main amp strip. I will need to recheck behind the front panel, but I doubt there are any trimpots for the amp up there.
That is all I will do tonight, but thanks for any further thoughts.

[Enzo]

That is 0.22 ohm, not 22 ohm. Touch your meter probes together top measure their own resistance. I bet you see something like 0.3 ohms for them. When you are measuring 10k, you can ignore the probe resistance, but when measuring half an ohm, you must subtract that amount.

0.02v across 0.22 ohms gives me about 90ma, which is even worse.

A bias trimmer should be near the power amp circuitry.

1100ma? You said 0.02v and 22 ohms? I = V/R = 0.02/22 = way less than a milliamp. Unless I am screwing something up, I don't see how you got that.

[J M Fahey]

I should have lifted one end of those resistors to measure, I see they are marked 0-22 Ohms.
I measured them In Circuit and they were .5 Ohms.

So they are 0.22 ohms.
Your meter shows that *plus* wire/connectors/switch/probes resistance, hence the 0.5 ohms you see.

Ohm's Law across the 22 Ohm part, that would be 1100 ma, which would be really high.

Neither 22 ohm part but 0.22 Ohm nor 1100 mA .
Ohm´s Law says: 0.02V/0.22 ohm = .09A = 90mA
Somewhat high, I would try to identify bias pot first, triple check I´m not mistaken, and *slightly* move it one way, then recheck voltage drop.
If it rises, move it the other way, always in small steps, until you read, say, 10mV across each resistor.

I see no trimpots on the small board before the main amp strip. I will need to recheck behind the front panel, but I doubt there are any trimpots for the amp up there.

[gary rabbitt]

Ok guys, found only one trim pot hidden on the board.
I know about lead resistance and all, and the resistors and measure .2 Ohms (in circuit).
I will adjust the trimmer later today and report back.

I will be going for 10mv across the resistor while adjusting.
Gary.

[gary rabbitt]

The trimmer did nothing at all to adjust the voltage across the resistors.
No other trimmer on the board. The one I adjusted was T302. Still don't have the schematic.

I doubt there is another trimmer elsewhere like behind the front panel, but I will look.