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Power amp fan trouble

Discussion in 'Troubleshooting and Repair' started by calmsea, Jul 7, 2015.

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  1. calmsea

    calmsea

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    Jun 3, 2014
    I'm trying to fix a Crest CA2 poweramp with what seems like a pretty straight forward problem. The two cooling fans are supposed to run at a low idle most of the time but these ones kick in to high speed as soon as the amp is turned on and stay there.
    I figured there would be a temperature sensor somewhere that's controlling them, maybe on the heat sink, but I can't find anything. I found a schematic for the closely related CA6/9/12 and located the Darlington transistors that drive the fans, but I can't find anything driving the transistors (on the schematic there's just a tag that says fan_driver but doesn't seem to go anywhere).
    Can anyone suggest what might actually be driving the fans?

    (There are temperature leads coming from the transformer, but I assumed a transformer overheating would be a more critical failure and would open the protection relays. The fans are nowhere near the transformer. )
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    how about sharing the schematic, so we are all on the same page, so to speak :)
     
    Arouse1973 likes this.
  3. Gryd3

    Gryd3

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    Jun 25, 2014
    If you can. eyeball the actual board.
    The temperature sensor could be present inside another IC on the board.
    following the traces from the transistors that control the fan should point you in the right direction.
     
    ADRT likes this.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Another issue could be the fans themselves. It is not unusual for fans to have a tachometer output indicating the true speed of the fan to enable a closed fedback loop for controlling fan speed. If the tachometer output doesn't work correctly, the control circuit assumes the fan's speed is too low and tries to increase the speed by outputting more power to the fan.
    Are the fans' connections soldered or plugged? If plugged try unplugging and re-connecting the fans to improve the contact. Also have a look at the fan's datasheet to see whether my assumption can possibly be right at all.
     
    Gryd3 likes this.
  5. Gryd3

    Gryd3

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    Jun 25, 2014
    ha! Forgot about that little detail. Very nice.
    You can probably get a really good idea by looking at the wires from the fan. You will always have power and ground. Then the 3rd and possibly 4th wires are commonly Tachometer output, and PWM control input.
     
  6. Martaine2005

    Martaine2005

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    May 12, 2015
    I just found this in the manual.
    It should point you in the right direction.

    "The “intelligent” variable-speed DC fans are controlled by heat sink temperature-sensing circuits. When the
    amplifier is turned on, the fans briefly “rev up,” then slow to an idle; this indicates that the temperature
    sensing circuits are operating normally."

    Martin
     
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  7. Martaine2005

    Martaine2005

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    May 12, 2015
    This is the schematic that Peavey emailed to me.
    They have taken over Crest.
    Scroll down to your section that says Fan driver. Continue left and maybe down a little. It gives the Fan drivers and board location. I tried to copy that one page but it wouldn't paste here for some reason.

    Martin
     

    Attached Files:

    Harald Kapp likes this.
  8. dorke

    dorke

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    Jun 20, 2015
    From the schematics Martin posted:
    The Temperature sensor is IC5 at location A-6(bottom left of page) of the "output module", type is LM35DT .
    It is a TO-220 type and thus looks like a power transistor :) look for it on the heat-sink probably with thermal grease.
    Here is it's datasheet
    http://www.ti.com/lit/ds/symlink/lm35.pdf

    It is very easy to check you will need :
    1.a " heat gun " (an hair dryer should do fine ).
    2.a laser temp meter(or a contact one).
    3.a mili-volmeter to check the sensor output.

    Heat the sensor up and measure it's temp. and output voltage:
    The sensor is linear 10 milivolt/degree Celsius.
    Hope this helps
     
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  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Well done Martin, as helpful as always.
    Adam
     
    Martaine2005 likes this.
  10. dorke

    dorke

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    Jun 20, 2015
    The schematics Martin posted seems to be missing the path to the FAN from the sensor.
    I found other schematics which have holes in them but together may give he full picture(jpg simplified drawing).
    The path then goes to the Arlington driving the FAN itself on the power supply schematic location B-5.
    If this is indeed the case than we have a linear voltage control drive of the FAN.
    It can be traced easily and and tested by heating the temp sensor.
    The schematics is the PDF file.
    BTW,This kind of "jumping " schematics are terrible to service and trace :(


    sense to fan drive.jpg
     

    Attached Files:

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  11. Martaine2005

    Martaine2005

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    May 12, 2015
    I enquired about the CA 2 and they referred me to that.
    My 5 minute look at it seemed right.
    Thanks for pointing that out Dorke. I will email them again and ask!

    Martin
     
  12. calmsea

    calmsea

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    Jun 3, 2014
    Thanks everyone for all the excellent information. I apologise for the for the late reply. I'm only in here a couple of times a week now, so I haven't had a chance to take another look until now.
    Thanks dorke for that missing piece of the schematic. I spent a lot of time puzzling over the incomplete one trying to figure out where those jumps went.
    I found the temperature sensors as soon as I had another look at the heat sinks. One of those packages is not like the others. They both seem to be behaving normally under the heat gun though.
    However, I realized thinking about it later that it is unlikely to be the sensors, or the drivers, or any other part of the individual fan circuit that is at fault, since BOTH of the fans are behaving the same way (something I obviously should have noted at the outset.)
     
  13. calmsea

    calmsea

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    Jun 3, 2014
    Now here's something interesting I just discovered. When I probe the voltage on the base of the driver transistor (the input from the temperature amplifier), the fan for that side suddenly drops down to it's normal idle speed.
     
  14. Martaine2005

    Martaine2005

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    May 12, 2015
    When you probe?
    with a DVM? Are you using chassis ground too?
    If so, your meter could be acting as a 'pull down'.
    What happens if you leave the probes in place and power on the amp?

    Martin
     
  15. calmsea

    calmsea

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    Jun 3, 2014
    Yes. Well, the signal ground. There is a switch that separates the signal and chassis ground.

    The fan runs at idle speed from the moment I turn on the amp.
     
  16. Martaine2005

    Martaine2005

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    May 12, 2015
    Can you replicate this with both fans/transistors?

    Martin
     
  17. Alec_t

    Alec_t

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    Jul 7, 2015
    I'd guess the base of the driver Darlington transistor is floating instead of properly connected to the temperature amplifier. Check for dry solder joints and hairline cracks in traces in that area.
     
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