Home › Forums › Help/Technical Questions › Weird grounding/bad noise issue with vintage DMM

Viewing 13 posts - 1 through 13 (of 13 total)
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  • October 7, 2009 at 10:22 pm #79095
    davidawheeler
    Participant

    So, I committed the unthinkable sin of modifying a friend of mine’s vintage DMM. In my defense, he asked me to do it, and it worked brilliantly for about 2 years. Today, he tells me it has been making all kinds of terrible noises, so I went to check it out. Sure enough, it is jacked up. I’ve put back to original, with a new A10k pot for the feedback control, and the issue still exists. I think it may be a grounding problem, but I can’t seem to pinpoint.

    When the unit is on and you touch the chassis, it thumps and pops and makes all kinds of terrible noises, and the overload light is full on.

    I forgot to mention that since I modified this baby, my friend had put a screw through the AC cable. He said it shocked him and it smoked up good. I went over there shortly after and repaired the damaged cable, but I didn’t replace it completely. Is it possible that there is residual damage to the power unit or something else?

    October 7, 2009 at 11:05 pm #102086
    The EH Man
    Moderator

    I’d check to make sure the ground wire from the jacks to the board is still intact. I’d also check for bad solder joints in the power supply area, especially on the big filter caps.

    October 9, 2009 at 4:32 pm #102120
    davidawheeler
    Participant

    The solder joints seem to be fine. I’m confused. I wonder, would it be okay to send an audio sample of what’s going on?

    October 9, 2009 at 4:33 pm #102121
    The EH Man
    Moderator

    That would be helpful.

    October 25, 2009 at 9:53 pm #102670
    davidawheeler
    Participant

    Okay, sorry it’s taken so long.

    Here’s the deal. I finally got the sound file, and it’s 2.2 mb, however, I can’t seem to attach any file larger than 75 kb. That’s bit small for an audio file. I can e-mail this.

    When you get it, you’ll hear the guitar plugged in through the unit with the DMM on but not engaged. Then, you hear it on. At about 35 seconds into it, you’ll hear what happens if I gently tap the enclosure. That’s when it all goes south. You’ll hear all kinds of terrible noises. If I tap around the overload light and turn some knobs in arbitrary directions, the feedback noise and overload will stop. At which point, I may or may not be able to continue playing with out this noise. It varies. With some patience, I can get the unit to sound right and not make the feedback, but if I tap it again, it’s over.

    So, I’ve checked the solder joints, and everything seems kosher. I’m stumped at this point. Could one of the op-amps have mailed it in? Or, does this have to do with grounding like I first suspected?

    Thanks,

    Dave

    December 5, 2009 at 6:39 am #104859
    davidawheeler
    Participant

    Have you had a chance to read over my last post. I have this file, but I can’t send it…too big.

    Thanks!

    David

    December 5, 2009 at 2:50 pm #104865
    SanquiFlerb
    Member
    Quote:
    Have you had a chance to read over my last post. I have this file, but I can’t send it…too big.

    Thanks!

    David

    Upload the file somewhere else, like goear, or one of those sites. I can’t recall anymore right now.

    December 5, 2009 at 11:56 pm #104884
    The EH Man
    Moderator

    The symptoms you describe are indicative of a bad solder joint/connection somewhere. Take the back off, plug it in, hook it up to an amp and run the end of a Sharpie across the back of the circuit board and listen for where it causes a noise.

    December 7, 2009 at 10:10 pm #104956
    davidawheeler
    Participant

    Forgive the clueless response, but when you say “the end of a Sharpie” do you mean the felt part where the ink comes out or the other end made of plastic? This is a new thing to me, and I’m sorry to bother.

    Thanks!

    Dave

    December 7, 2009 at 10:25 pm #104958
    davidawheeler
    Participant

    Here’s the link to the file on goear:

    http://www.goear.com/listen/0462c95/Memory-Man-Issues-david-wheeler

    Just to warn you…don’t have your volume up too high. When the bad sounds start happening, it’s pretty unpleasant. So even if you have the volume pretty low to start, you’ll be able to hear when it goes to pot. Thanks for the help. This pedal is really one of the coolest sounding delays ever, and I really hope to get it back to working order.

    Thanks,

    David

    December 7, 2009 at 11:40 pm #104966
    The EH Man
    Moderator

    Use the plastic end, not the writing tip. Unless, of course, you want to mark all over the board.

    Your sound file sounds like a bad solder connection somewhere.

    December 10, 2009 at 12:35 am #105107
    davidawheeler
    Participant

    Ron,

    Thanks so much for pointing me in the right direction here! I have never heard of the sharpie trick before. I’m going to file that one away and use it for years to come. When I get home this weekend, I’ll sort this thing out.

    Much obliged,

    David

    December 13, 2009 at 12:01 am #105255
    davidawheeler
    Participant

    Well, the only place that it seems to make any noise is on every ground solder joint or track. The others seem fine. So, I’m going to re-solder the ground wires to the board and to the jacks to begin with. I’ll see what it sounds like after that.

    Thanks,
    Dave

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