Fixing a Power Supply

Discussion in 'General Electronics Chat' started by matpec, Jan 13, 2016.

  1. matpec

    Thread Starter New Member

    Aug 25, 2015
    Hello and thank you for your help, first and foremost! This post is broken into two parts .. the back story, and the following steps.

    Last Sunday, I was putting back-together a synth, which I was trying to restore. I wanted to test the synth, so I connected it as follows: The line out of the synth (no ground) was plugged into a channel in a sub mixer (no ground), the sub mixer main outs were plugged into another (grounded) mixer's stereo line in, and finally this mixer's main outs were going into an audio interface (grounded).

    I made a huge mistake. I had not yet secured the AC mains connector to the synth chassis, but I had never-the-less plugged it into the wall socket to test the device. Don't ever do that!!! As you can imagine, the connector became dislodged from where it fits in the chassis, and the legs that are soldered to the cables that connect to the power switch touched the chassis of the synth.

    A bright white light emerged from the contact point, followed by a loud buzz from the loud speakers, followed by the smell of smoke, followed by panic from yours truly. Miraculously, I didn't get hurt ... for which I am very grateful. Unfortunately, this wasn't a victim-less "oopsie."

    I expected everything mentioned in the chain above to be fried - from the synth all the way through to the interface .. yet, only the second mixer down the line - the grounded one - bit the dust. Everything else seems unscathed, at least to my untrained eye.

    So - what happened?

    My best guess is that when the prongs touched the chassis, the voltage of the ground on the synth was simply raised by 120V. Everything that wasn't grounded ended up with no voltage, but the grounded mixer did have the reference needed to "feel" the 120V at the line in. Because this voltage exceeds pretty much every possible component's max voltage, the mixer experienced extreme "clipping" to the point where stuff got burnt up, but ultimately could not send 120V out of its line out to cause harm to the interface. Can this be what occurred?

    And what about the mixer? Is it gone forever? I've opened it up and can make a few observations. The first is that a lot of the components are surface mount. The second is that there were 8 through hole resistors, and these were all cooked up. The third is that the power supply is still smoking if I turn the mixer on.

    I have the schematics for the all the boards in the mixer, so replacing the 8 resistors was painless ... all things considered. The problems are that - #1, with a power supply that is still busted, I can't test the mixer and #2, while only a few components were visually broken, my gut says everything got burnt to a crisp.

    I would never the less like to find out, so how do I test a power supply that is still smoking when turned on? .. Do I let whatever is in there fail completely until it's obvious it was X, Y, or Z component? .. Are some components more likely to smoke, and should I just attempt to replace these first? .. Or should I just cut my losses before I burn up something else or myself?
  2. Veracohr

    Distinguished Member

    Jan 3, 2011
    If you have it, post the schematic that has the mixer's power supply and channel input.
  3. profbuxton


    Feb 21, 2014
    Buy a new one!
  4. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    I would pull the supply out and rig it so the unit turns on and then start with some basic tests to see what else failed... The unit will be repaired when you get all the right voltages with a load on the circuit..Then install the power supply and see what you get with the mixer unit and work from there..
  5. matpec

    Thread Starter New Member

    Aug 25, 2015
    thank you. I've pulled the supply out (and discharged the caps). Should I just take it outside, plug it back in, and "let it smoke?" I'm a little nervous about measuring any voltages with something catching fire in the vicinity!!! I'm hoping whatever is failing fails fully before I put a multimeter on the circuit. Is there a "best practice" for this type of situation?

    Also, is it imperative that I plug the mixer back into the power supply to provide a load for the psu circuit? Schematic for the supply is attached.
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Its a classic 3842 switch mode psu, if you have the patience of a saint, and a dvm, we can take you through fixing it,,,with hope.
    GopherT likes this.
  7. matpec

    Thread Starter New Member

    Aug 25, 2015
    I've got a digital volt meter and I'm trying to be patient with the loss of this piece of equipment :) I certainly would like to try to repair it.
  8. marcf


    Dec 29, 2014
    Can you spring for a bench power supply?

    You can then apply a controlled voltage (of your choosing) and limit the current as well.

    Shorts and bad components stand a good chance of showing up under a controlled environment, rather than an exploding one.

    It's much better to spot problems that become warm, rather than ones burning up.

    A fairly good one may set you back $150.00 or so, and you will probably find all kinds of uses for it in the future.
  9. matpec

    Thread Starter New Member

    Aug 25, 2015
    got myself a used bench power supply, which can provide three voltages simultaneously ( ! ) ... I need some banana plug for it, which I'll probably be picking up this evening.

    We'll see what happens when I send the voltages into the mixer. I'm confused as to what VP and VM mean on the schematic, and Google isn't yielding any useful results. I'm guessing they are just regular DC rails?

    Any guidance on how to tackle the power supply repair will be welcome with utmost gratitude