Home › Forums › Vintage EHX › 18v Electric Mistress Troubleshooting Questions

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  • July 30, 2016 at 6:00 pm #84517
    nedblau
    Member

    I just bought a broken 18v mid-’70s Electric Mistress. I assumed it needed a new BBD chip, but when I asked the previous owner about what happened when it crapped out, he said he’d tried using a 12v power supply. It worked weakly for a while, then stopped. I’ve never heard of a power supply producing LESS than the pedal needs damaging the pedal. On the other hand, it doesn’t seem like it would have worked at all if the polarity on the wall wart was switched. Can anyone here suggest where to start looking for this sort of damage? I’m fine replacing parts, but I’m not experienced with solid state and pedals especially. Is there a particular IC or area of the board, diodes, etc. that would be suspect after a power supply issue? BTW the pedal passes no signal at all at any setting of the knobs or interior pots. Thanks!

    July 30, 2016 at 6:35 pm #121782
    Scruffie
    Member

    Post voltages of all the ICs, only way to debug it.

    July 30, 2016 at 6:52 pm #121783
    nedblau
    Member

    Do you mean voltages at each pin of each IC? Sorry, more of a tube guy.

    July 30, 2016 at 10:47 pm #121784
    Scruffie
    Member

    Yes, each pin, numbered like this, notch or dot indicating the top of the chip 220px-Pin_numbering_01_Pengo.svg.png

    July 31, 2016 at 10:30 pm #121786
    nedblau
    Member

    I just had the foot switch and output jack leads break off the boards. I guess moving it around was just too much for them old wires. Where does the brown wire go on the board? I’ve been unable to find a schematic except at that German site that won’t load.

    July 31, 2016 at 11:38 pm #121787
    nedblau
    Member

    Never mind, got it. The wire broke off so cleanly, I didn’t spot the end in the solder blob ’til I got out the loupe.

    August 4, 2016 at 12:59 am #121795
    nedblau
    Member

    I finally had time to get voltages on all the ICs. I’m not sure about the names of the chips. Some of these values really varied a lot as I was measuring.

    741

    1 .1 mv
    2 6.6 v
    3 8 mv
    4 0
    5 0
    6 20 mv
    7 18.8 v
    8 5 mv

    4558

    1 3.7 mv
    2 1 mv
    3 0
    4 0
    5 0
    6 & 7 7 mv
    8 .2 mv

    SAD1024

    1 0
    2 5 mv
    3 1.5 mv
    4 0
    5 .1 mv
    6 0
    7 .1 mv
    8 2mv
    9 .1 mv
    10 2 mv
    11 .1 mv
    12 0
    13 0
    14 1.5 mv
    15 5 mv
    16 0

    7611

    1 18 mv
    2 17 mv
    3 0
    4 .1 mv
    5 0
    6 0
    7 0
    8 0
    9 2.8 mv
    10 0
    11 .1 mv
    12 2.8 mv
    13 1.2 mv
    14 .1 mv

    LM339

    1 .2 mv
    2 .2 mv
    3 .2 mv
    4 .1 mv
    5 .1 mv
    6 1.2 mv
    7 .1 mv
    8 0
    9 0
    10 .2 mv
    11 0
    12 0
    13 0
    14 5.5mv

    August 4, 2016 at 11:04 am #121212
    Scruffie
    Member

    Right, there’s a problem with your voltage regulation circuit by the look of it as nothing is getting proper voltage.

    It was quite common on earlier models and a fix was released by EHX so you need to check if it was done already before we go on, on the board there should be 2 transistors, one will be a 2N5087 or BC309 and if the mod was done the other should be a 2N4354, if they’re both 2N5087/BC309 then the mod needs doing.

    Or you can just send a photo of the board.

    August 4, 2016 at 3:51 pm #121797
    nedblau
    Member

    Thanks Scruffie, will do!

    Both transistors are 2n3563.

    August 4, 2016 at 4:52 pm #121798
    Scruffie
    Member

    Right, sounds like that might be the issue then as it wasn’t upgraded. You should have another 2 transistors on the board, both of which should be a 2N5087 or BC309, one in the power section and the other is tied to the 339 and should be ignored.

    So, replace the 2N3563 from pin 2 of the 741 with a 6.8V zener diode, cathode (striped end) to pin 2, anode to ground.

    Replace the 2N3563 from pin 6 of the 741 in to the base of the 2N5087/BC309/2N4354 with a 47k resistor.

    Replace the 18k resistor from pin 3 of the 741 with a 22k resistor.

    If the transistor with its emitter connected to the 18V input is a 2N5087 or BC309, replace it with a 2N4354 (don’t forget to check the datasheet for the pinout).

    And that should be it.

    August 4, 2016 at 4:57 pm #121799
    nedblau
    Member

    Awessome. Looks like I have my work cut out for me! While I was waiting for your reply I FINALLY found a schematic. It wasn’t showing up in my searches.

    August 4, 2016 at 11:02 pm #121801
    nedblau
    Member

    Is this a good schematic for me to refer to?
    http://www.freeinfosociety.com/electronics/schemview.php?id=383

    August 5, 2016 at 10:57 am #121802
    Scruffie
    Member

    That’s fine and you want to convert the power section so it looks like the one in this schematic;

    EHXElectricMistress18vFactoryEH-5100.jpg

    August 6, 2016 at 6:47 pm #121814
    nedblau
    Member

    Hi Scruffie,

    Woo hoo, progress! I made all the changes suggested, and now the pedal passes signal when engaged. No effect at any adjustment of the internal pots, but engaging the effect does cause a volume drop. I understand that’s a big complaint about these pedals, but right now it sounds pretty good.

    Two questions: what should my next diagnostic step be? Should I go back through and check all the pins on all the ICs? The previous owner used a 12v power adapter with it — that’s what broke the pedal — but would that necessarily fry the SAD1024? I have a replacement, but I want to A. Be sure the old one is dead, and B. Be sure the rest of the circuit isn’t going to fry the new one.

    Can I also just bitch about the incredible flimsiness of EHX boards? I’ve worked on a lot of old circuit boards, and traces break, come away from the substrate, etc., even on high quality products. But holy crap I only have to breathe on this board to destroy the traces!

    Thanks!

    August 6, 2016 at 8:55 pm #121815
    Scruffie
    Member

    Great! Your next step is indeed to take all the voltages again, now that’s fixed they’ll all have changed and hopefully they’ll offer some useful insight now.

    It’s a common thing to assume the BBD (especially SAD1024) is dead first as it’s the most expensive part but blindly assuming that and replacing it isn’t the way to go and nine out of ten times in my experience (and I know other techs who have had the same experience) it’s not the problem… not chastising you but hope if I say it enough the idea will disappear :) except for when it’s SAD512D, they do seem to die more often but that’s neither here nor there.

    You may indeed bitch about them, they are pretty bad, some more so than others (especially when the wires are wrapped round) you get used to them after a while of working on them but even then a trace still lifts now and then.

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