Home › Forums › Help/Technical Questions › Repairing a vintage electric mistress

Viewing 4 posts - 1 through 4 (of 4 total)
  • Author
    Posts
  • July 2, 2014 at 6:23 pm #83549
    DaveC22
    Participant

    Hi all
    I have a vintage electric mistress that I’m trying to fix, here is the schematic:
    http://www.metzgerralf.de/elekt/stomp/mistress/images/1980-electric-mistress-v5-schematic.gif
    I’ve been following the troubleshooting steps found here:
    http://www.metzgerralf.de/elekt/stomp/mistress/repair.shtml
    Step two is where my pedal goes wrong. Basically, my v+ should be around 12-13v, but instead I get about 0.3v. There’s definitely 18v entering the voltage regulation section and at pin 7 of the lm741 I get about 15.8 volts. At the 2n4354 I get 18v at the emitter, just over 16v at the base and 0.3ish volts at the collector.

    So my question is, is it the chip that has gone, or the transistor? I don’t want to be replacing parts unnecessarily hence my question. I’d greatly appreciate any help with this.
    Thanks in advance
    Dave

    July 2, 2014 at 6:28 pm #120067
    The EH Man
    Moderator

    I’d start with replacing the transistor.

    July 2, 2014 at 7:30 pm #120068
    DaveC22
    Participant

    Cheers eh man for the quick response. My instinct was to go for the tranny but I want to keep repairs to a minimum ideally.
    Thanks again
    Dave

    July 5, 2014 at 9:41 am #120082
    DaveC22
    Participant

    No dice!
    Ok, so I replaced the transistor with a 2n2907a, I also replaced the 33uf cap with a higher voltage tolerance one. Now, when I hook up two 9v batteries, I get 18.96v going into the circuit and at pin 7 of the lm741, at pin 2 I get 6.86v, pin 3 I get 9.44 volts and at pin 6 I get around 3v.
    At the transistor emitter I get the full voltage, at the base I get 18.16 and at the collector 18.75v.
    What the heck is going on here? Appears as if there is little to no regulation going on.

  • Author
    Posts
Viewing 4 posts - 1 through 4 (of 4 total)
  • You must be logged in to reply to this topic.