1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

Thermistor in a MC240 blew, violently

Discussion in 'McIntosh Audio' started by EvanF, Mar 9, 2018.

Tags:
  1. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,098
    Location:
    Eastern Bamastan
    I was consulting an online schematic, which appears correct, the power supply is incorrect on yours re that elyticap.
    Check this one;
    https://goo.gl/images/ez8KdT
     

     

    Please register to disable this ad.

  2. x3workshop

    x3workshop AK Subscriber Subscriber

    Messages:
    1,983
    Location:
    Harlem
    Here's the setup with one can isolated from the chassis, if it helps you. It is as the original setup was. Tying the two negative leads directly seems to be your issue.


    L1120384.JPG
     
    Pio1980 likes this.
  3. EvanF

    EvanF AK Member

    Messages:
    289
    Location:
    Kirkwood, Missouri
    This is information I could not find unless I went back in time and took better pics! Knowing my schematic is wrong is saving me big time. Thanks dude!
     
    Pio1980 and CruzBay like this.
  4. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,098
    Location:
    Eastern Bamastan
    Sorry for the confusion, we'll get it going!
     
  5. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,098
    Location:
    Eastern Bamastan
    I can see, now that we are off the road, that that is a solder joint, not a diode. Turn the elyticap around, replace the inrush limiter and fuse(s), check that it is set, wired for the higher line voltage.
     
    Bill Ferris likes this.
  6. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,098
    Location:
    Eastern Bamastan
    How are you doing on this?
     

     

    Please register to disable this ad.

  7. EvanF

    EvanF AK Member

    Messages:
    289
    Location:
    Kirkwood, Missouri
    Letting it sit.... :(

    Can I use a 100Ω power resistor and then short it to replace the thermistor before I power the unit on again? All else I need to do is test and rotate the e-cap. The 2/10 amp fuse blew so I'm also gonna use two .1A fuses in parallel.
     
  8. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,098
    Location:
    Eastern Bamastan
    Fuse(s) 3.2 Amp slo-blow.
    The resistor will likely overheat almost immediately, so, no. A 100 Watt 120 Volt incandescent lamp bulb would be fine, but do be sure the amp power is set for the higher 125 Volt option.
     
  9. x3workshop

    x3workshop AK Subscriber Subscriber

    Messages:
    1,983
    Location:
    Harlem
    I believe OP is referring to the fuse on the small board with SR1, SR2, SR3. It will blow if there is a B+ problem. L1120380.JPG
     
  10. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,098
    Location:
    Eastern Bamastan
    That fuse doesn't show on the schematics, but is usually present. Afaik, it was also spec'ed at 3.2 Amp slow blow. Your take?
     
  11. x3workshop

    x3workshop AK Subscriber Subscriber

    Messages:
    1,983
    Location:
    Harlem
    It is indeed not on the schematic and it is a 3.2 Amp fuse. The older fuses are marked 3-2/10A.
     
    Pio1980 likes this.

     

    Please register to disable this ad.

  12. x3workshop

    x3workshop AK Subscriber Subscriber

    Messages:
    1,983
    Location:
    Harlem
    So, to the OP, you can use another 3.2 amp fuse and a variac to do a slow start on the amp if you don't have a thermistor. The 100ohm resistor in place of the thermistor will do nothing for you except burn up.
     
    Pio1980 likes this.
  13. EvanF

    EvanF AK Member

    Messages:
    289
    Location:
    Kirkwood, Missouri
    Tested the backwards ecap out of circuit. 281µF, esr .25Ω, Vloss iss .4%. Any chance it could still be bad? Drew at least 3.2 A when connected backwards.
     
  14. x3workshop

    x3workshop AK Subscriber Subscriber

    Messages:
    1,983
    Location:
    Harlem
    Looks like it's fine by the readings.
     
  15. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,098
    Location:
    Eastern Bamastan
    You can try it with the proper fuses in place, it may be ok.
     
  16. mcgjohn

    mcgjohn Active Member

    Messages:
    122
    Location:
    Chicago area

    yep, agree. replace the thermistor, double check the diode orientation. then bring her up on a variac and watch the current draw as you bring the AC voltage up from a few volts. If current draw stays low, then move up a bit more. by the time you get to 80-90VAC, current draw should be minimal.

    dont feel bad, mistakes happen. A few months ago, I was rebuilding an MC60. was late, was tired, should have gone to bed but decided to solder in the bias cap first. confirmed the orientation and still soldered the cap in backwards. did not notice it til I was bringing up the AC lines on the variac. sure enough at around 50VAC input that darn magic smoke started to appear. Shut it down. realized what a bone head move I had done, tossed the newly installed diode, cap and resistor in the bias section with all new parts, and brought it back up on line slowly. The second time no issues.
     

     

    Please register to disable this ad.

  17. EvanF

    EvanF AK Member

    Messages:
    289
    Location:
    Kirkwood, Missouri
    Lastly, to (for now) replace the thermistor,
    I wired up two 60W bulbs in parallel where the thermistor was. They glow dimly but decently. Where 435 should be, there is 260.25VDC. All ecaps get proper voltage direction. I can list the other voltages and compare them to the proper rail level. Can I short the bulb or get a proper thermistor for closer voltage? The line setting is at 125, close to the 122.2VAC where I am in Missouri.
     
  18. EvanF

    EvanF AK Member

    Messages:
    289
    Location:
    Kirkwood, Missouri
    Current draw through the bulb is at .5845AAC.
     
  19. EvanF

    EvanF AK Member

    Messages:
    289
    Location:
    Kirkwood, Missouri

    Using 200W worth of bulbs in place of the thermistor, still get 325.7VDC where I should get 435VDC.
     
  20. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,098
    Location:
    Eastern Bamastan
    Try it, mind your fuses.
     

Share This Page