Heat sink, Sound Tech M620 Amp Mixer.

[gary rabbitt]

Is it possible that there is a fixed method of bias on this amp? Without the diagram it is hard to say which resistor(s) would be the one to suspect.
Maybe a voltage regulator device?

[g1]

It is most likely fixed bias. As you say, it is probably a resistor that needs to be tweaked. You would need to either find a schematic or draw one up.

[The Dude]

Going back to post #1 you replaced a diode. Did you use an exact replacement? Cross referenced part? SWAG? I'm wondering if that wasn't a diode having to do with bias and possibly your substitute part threw the bias out of whack. For instance, if it was a dual diode for bias or zener regulating a supply and you stuck in a common 1n4148, 1n4007 or something of the sort, it might cause bias to be wrong. Can you tell us what was there and what you replaced it with?

[gary rabbitt]

The diode that was shorted was a 1N5402 3A and 200Vmax PIV
I replaced with a diode (1N5404) in the same exact series, however rated at 3A and 400V max

The other characteristics remain the same between the two parts. I would doubt the diode is a factor.

(I have drawn out a diagram for a vintage radio, but this amp would be a test in patience. There has to be a schematic out there someplace. Is Soundtech still in business?)

Today bought an IR thermometer and did some temp readings.
Despite the current draw I measured, everything might be ok with the heat issue. Let me know your thoughts.

Driving the amp pretty high, bass and volume I measured the heat sink , and the individual transistors on the back of the heat sink.
Medium to moderate volume, the heat sink averages 125F and 180F (51 C - 82 C)
components averaged 125-200F
During a short time of low volume, the parts cooled off down to the 100 F range.

Driving almost to full volume, I mean, really thumping, the max temp I saw on the parts was about 250 F (121 C)
Heat sink felt like it did before, could not touch, but read only 140 F (60 C) at the most.

Transistor part ratings for both transistors is 150 C (302 C) max.
I was not really close to maxing out the temps to the spec. The amp has some kind of "protection circuit" and that light never came on.
With that, do you think the amp is performing as it should? Despite the readings on the current draw we figured out above?

The owner will not be driving the amp as hard as I was, using it for more mellow church type use.
Maybe I am chasing my tail, and thinking the heat was greater than it really was?

I still would like to find a schematic, and the little trimpot on the board made no audible difference if adjusted. (It was not a bias control). I put it back where it was.
Thanks for your thoughts guys!
Gary.

[The Dude]

Your diode sub should be just fine, but thought I'd ask to be sure. It's hard to say what that trimmer is without a schematic. It could be for DC offset, or it could be intended for bias and is not working due to a problem in the bias circuit (or any number of other things). I might check that trimmer itself and make sure it is good/working.

Edit: I believe this is their contact page. Maybe they will send a schematic to you.

http://www.sound-tech.com/contact-us/

[gary rabbitt]

Thanks Dude>
I did Ohm the trimmer out, and it was responding as one should. Thanks for the link too.
I knew they were out of IL, but with corporate changes they could have moved. I think I will button the unit back up, and wait to see if they can send info on the unit, or someone here may have a diagram. The thing may be fine for what my friend wants to use it for.

Appreciate your replies.
Gary.

[g1]

In case that link posted by The Dude is a different soundtech, last I knew they were owned by US Music Corp.

[gary rabbitt]

Thanks G1
I heard someplace they moved 'down under' so possibly some firm in Australia took over. My google searches didn't turn up much.
No diagrams from Sams Photofact, and only one place supposedly had a manual, but it was a "download free, pay to join" type site. No thanks.

I'll let you know if I get an answer from the above ST link.
US Music is still in business, and I think the merger with Soundtech was in 2003 so their site said.
Tracking something down is half the 'fun". Take care.

[Enzo]

Sound Tech used to have a fax back schematic number. You called it up, entered the number of the schematic you wanted, and it faxed it back to you in a couple seconds.

[gary rabbitt]

Sound Tech used to have a fax back schematic number. You called it up, entered the number of the schematic you wanted, and it faxed it back to you in a couple seconds.

That would be great. I have not received any answer from a couple of contact messages from various places that 'own' the ST name. (US Music).

It may be the way this thing is designed, reaching 75-80% of the transistor's design limits in a worst case (driving amp very hard)scenario.
Mellow lower volumes show about 40-45% of design limits , if my calcs are right.
For now, I am stuck as far as finding the one bias resistor to lower that a bit.
Take care.

[g1]

There are schematics around for the PL602. Might give some ideas about the overall topology or may be completely different.
Worth a look anyway.

[gary rabbitt]

Completely different animal than the PL602. I am going to look at some of their other mixer amps. I had the 4150 here for the same guy, (and the diagram too) but the boards were different.

[Tumriel]

Can you give me some pictures from board? I need the value of potentiometer. Reply me as soon as possible.