Home › Forums › Vintage EHX › Faulty EHX Mistress V2

Viewing 9 posts - 1 through 9 (of 9 total)
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  • October 6, 2020 at 8:21 am #86455
    wsg1985
    Member

    Hello,

    I recently purchased a faulty EHX Mistress V2 with the hopes of repairing it. I’ve resoldered some wires back (which had broken off) and carried out some initial checks on the voltage regulation circuit (which appears to be functioning correctly, but I’m going to recheck all the voltages to the chips later).

    When the unit is plugged in and switched on I get a very loud and glitchy sounding signal. Has anyone encountered anything similar that could point me in the right direction for fault finding.

    I’ve rechecked the values and it appears there is something wrong with the voltage regulation as i’m only getting 537 mv at 4558 pin 8 (I’m sure this was previously reading 12.79v).

    LM741
    1 1.4 mv
    2 6.54 v
    3 275.7 mv
    4 0 mv
    5 1.2 mv
    6 17.61 v
    7 18.35 v
    8 1.5 mv

    4558
    1 178.2 mv
    2 177.4 mv
    3 167.3 mv
    4 0 mv
    5 167.4 mv
    6 178.4 mv
    7 178.7 mv
    8 537 mv

    Cheers

    October 9, 2020 at 2:59 pm #126517
    puppiesonacid
    Participant

    you may be better served going here:

    http://www.metzgerralf.de/elekt/stomp/mistress/repair.shtml#ts2

    October 9, 2020 at 3:58 pm #126520
    wsg1985
    Member

    Thanks, I’ve already been using that website (and a few other forum posts). I’ve just not heard of anyone encounterung my specific issue (the distorted tone volume boost).

    Ive just replaced the Lm741, so I’ll retest the voltages and see how I’m are fairing after that.

    October 9, 2020 at 4:34 pm #126521
    puppiesonacid
    Participant

    no problem. glad you know about that resource. could it be something a stupid as a bad jack? my other thought is a bad opamp (4558)

    October 9, 2020 at 10:48 pm #126522
    Scruffie
    Member

    Can’t help much until you have a supply voltage but yes, it’s either the 741 or transistor in the regulator circuit.

    Will need a full set of voltages once it’s going.

    October 11, 2020 at 9:15 pm #126524
    wsg1985
    Member

    I’ve replaced the 4558, as I was getting some bad readings on the original. Things are looking better but I’m still not getting any effect. Below are the new voltages.

    LM741
    1 0.2 mv
    2 6.54 v
    3 6.54 mv
    4 0 mv
    5 1.2 mv
    6 8.91 v
    7 17.47 v
    8 0.9 mv

    4558
    1 6.21 v
    2 6.22 v
    3 6.16 v
    4 0 mv
    5 6.161 v
    6 6.22 mv
    7 6.22 v
    8 12.79 mv

    LM399
    1 8.6 v
    2 0-5.7 v
    3 12.79 v
    4 6.54 v
    5 6.54 v
    6 4.81 v
    7 9.2 v
    8 89.7 mv
    9 7.47 v
    10 0-5.9 v
    11 431.2 mv
    12 0 mv
    13 13.8 mv
    14 4.809 v

    4013BE
    1 6.27 v
    2 6.68 v
    3 0
    4 3.86 v
    5 0
    6 14.6 mv
    7 0
    8 0
    9 6.42 v
    10 0
    11 5.25 v
    12 6.42 v
    13 6.38 v
    14 12.79 v

    SAD1024 (Removed)
    1 0
    2 4.072 v
    3 6.39 v
    4 0
    5 12.79 v
    6 0
    7 12.79 v
    8 6.42 v
    9 12.79 v
    10 6.42 v
    11 12.79 v
    12 0
    13 0
    14 6.39 v
    15 4.072 v
    16 0

    Any assistance would be much appreciated. Starting to think the SAD1024 might be bad though.

    October 14, 2020 at 11:50 am #126531
    wsg1985
    Member

    I’ve built myself an audio probe and I have output at the SAD1024, but when I tweek VR2 to get a output it is very distorted.

    I’ve checked the output (Pin 1) of LM4558 and the distortion (almost fuzz tone) occurs here as well.

    Any ideas on what the cause could be? Any other places I could probe? there is some noticeable noise at the input jack, could the Op amp just be boosting this signal during the gain stage?

    Edit: I’ve just checked it at pin 7 and the distortion is there aswell, I’ve changed the LM4558 and it hasn’t made any difference.

    October 15, 2020 at 4:19 am #126363
    Ned Flanders
    Moderator
    Quote:
    I’ve built myself an audio probe and I have output at the SAD1024, but when I tweek VR2 to get a output it is very distorted.

    I’ve checked the output (Pin 1) of LM4558 and the distortion (almost fuzz tone) occurs here as well.

    Any ideas on what the cause could be? Any other places I could probe? there is some noticeable noise at the input jack, could the Op amp just be boosting this signal during the gain stage?

    Edit: I’ve just checked it at pin 7 and the distortion is there aswell, I’ve changed the LM4558 and it hasn’t made any difference.

    Personally, I feel that the best place to ask for help is here:

    https://www.diystompboxes.com/smfforum/

    If you provide all the voltages, work you’ve done, things you’ve checked, etc. etc, then some of the guys will chime in and help you fix it.

    They are a very knowledgeable bunch over there (Some are electronics engineers with decades of experience) and also willing to help anyone.

    October 15, 2020 at 3:23 pm #126533
    wsg1985
    Member

    Thanks, but I think I’ve solved it. Looks like it was a bad ground on the input jack.

    I need to properly test it when I find some time, but hopefully another vintage pedal has been saved from the scrap heap.

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