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Thread: Heat sink, Sound Tech M620 Amp Mixer.

  1. #1
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    Schematic/ Service info Sound Tech M620 Amp Mixer.

    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>>>m620.jpg
    Last edited by gary rabbitt; 07-13-2017 at 04:47 PM. Reason: Edit title.

  2. #2
    Supporting Member The Dude's Avatar
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    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.
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    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.

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    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

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    g1
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    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.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    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.

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    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.

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    Senior Member Enzo's Avatar
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    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.
    Education is what you're left with after you have forgotten what you have learned.

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    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.
    Last edited by gary rabbitt; 07-17-2017 at 07:27 AM.

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    Senior Member Enzo's Avatar
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    Ohm's law. 0.02v across 0.5 ohms gives me 40 milliamps. That seems pretty high to me. Try adjusting the bias cooler.
    Education is what you're left with after you have forgotten what you have learned.

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    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.img_2256.jpg

    img_2257.jpg

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    Senior Member Enzo's Avatar
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    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.
    Education is what you're left with after you have forgotten what you have learned.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by gary rabbitt View Post
    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.
    Juan Manuel Fahey

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    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.

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    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.

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    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?

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    g1
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    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.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Supporting Member The Dude's Avatar
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    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?
    Last edited by The Dude; 07-18-2017 at 10:55 PM.
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    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.

  20. #20
    Supporting Member The Dude's Avatar
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    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/
    Last edited by The Dude; 07-18-2017 at 11:42 PM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    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.

  22. #22
    g1
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    In case that link posted by The Dude is a different soundtech, last I knew they were owned by US Music Corp.
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    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    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.

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    Senior Member Enzo's Avatar
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    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.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    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.

  26. #26
    g1
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    There are schematics around for the PL602. Might give some ideas about the overall topology or may be completely different.
    Worth a look anyway.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    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.
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