confused about testing potentiometer

Discussion in 'General Electronics Chat' started by Danvitt, Apr 7, 2015.

  1. Danvitt

    Thread Starter New Member

    Apr 7, 2015
    3
    2
    Hi all, new to the forum and electronics.

    I have a guitar amp, (crate gx-140c) that I can't afford to replace. it cuts out, if i gently push on the board with a wooden object it'll cut back in. But it really hard to reproduce the problem at will. Some times jiggling one of the knobs will do it but not always. Anyway, i'm convinced with my limited knowledge that one or more of the potentiometers is bad (least i hope so because its an easy fix).

    I've read quite a few articles and watched every video i can. I'm confused on a few things.

    One, they say connecting an ohmmeter to the outer leads of the pot will tell you the value of the pot. and, that twisting the knob will not affect it, at least not very much. This doesn't seem to be the case. Connecting the outer leads on some of the pots shows a reading that is directly affected by how much it is turned. Just as if you were connected to the wiper and one of the outer leads. On others it has no affect. Is this bad pot? a different kind of pot? Or do i have this wrong?

    2nd, I've notice the pots seem to have different characteristics. eg. one pot when i connect to the center terminal, and then connect to the right terminal, will show a smooth increase in resistance as i turn it, when i connect to the left lead, it remains at 0. On another pot it shows a smooth increase on both sides. Still, other pots seem to be backwards, when I connect to the right lead and turn the knob i get nothing, but when I connect to the left it shows a smooth increase in resistance. Are these bad, or just a different type?

    I'm looking for whatever help you can offer in verifying whether these are defective or not. They don't have any useful markings on them. They have serials such as 70-104-23 9905 roc. all slightly different numbers and I come up with nothing when I search for them.

    I appreciate any help you can offer. Thanks for your time.
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    The best plan is a careful inspection.

    Damaged pots often show outward signs, the tabs that hold the body together may be bulged out slightly, if its just made the wiper a bit loose, setting the tabs back to their original position may do the trick.

    Damaged carbon tracks are a different story - those pots probably aren't cheap, and may be slightly different so you can't use just any old pots.

    Cracks on a PCB can be easy or hard - depending on how bad it is, usually the hard bit is finding it, a jewellers loup or good magnifying glass helps - then just scrape off any varnish and solder a bit of tinned copper wire across the crack.

    You might get really lucky and find its just a dodgy RoHS solder joint that's cracked - just re-make the joint with fresh solder & flux, and have a good search round for any more before putting it all back together.
     
    Danvitt likes this.
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,779
    1,103
    The results you are seeing when testing different pots, assuming you are testing them in circuit, is probably just due to connections to other components, or to the way the pot is configured. For example, a pot used as a rheostat will have the wiper connected to one end or the other.
    If pushing the board gets it working, be suspicious of any edge connectors, attached cables, or mountings of large/heavy components. Guitarists/roadies aren't noted for their delicate handling of kit :D.
     
    Danvitt likes this.
  4. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    The perfectionist test would probably involve connecting a 9V battery across the track of any pot over at least 1k and observe the wiper voltage while its turned with a DMM, another possible test is to capacitor couple the wiper to a test bench amp and see how much it cracles when turned.
     
    Danvitt likes this.
  5. Danvitt

    Thread Starter New Member

    Apr 7, 2015
    3
    2
    You were right Alec. I removed one of the pots I suspected of being faulty and tested it again. It acts completely normal with it off the pcb. There also is no outward signs of damage as Ian suggested. So I'm at a loss of what it could possibly be, everything to my novice eye looks very solid and tight.
     
  6. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    Turn the circuit board belly side up and re-solder everything in sight.
    You don't have to play, "Name that connection" if you get lucky. ;)
     
    Metalmann likes this.
  7. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    When I was in the TV repair trade, we started gettin increasing numbers of Sony sets, the symptoms were always vague and poorly defined, and difficult to pin down to any particular circuit section. Blanket resoldering, though expensive in consumables cured most of them.

    Something similar happened with Hitachi sets - some times if you pressed down on a component from the top, the whole solder fillet detached from the copper side, on the copper land there was a very thin film of black oxide.

    This began before I'd even heard of dodgy RoHS solder.
     
  8. Danvitt

    Thread Starter New Member

    Apr 7, 2015
    3
    2
    Problem is solved! Looked it over real close with a magnifying glass and found a joint that didn't have any solder in it. Don't know if it popped out or they missed it at manufacturing plant or what. But it works, im happy, and I saved $500. Thanks for the input everyone!
     
    Metalmann and RichardO like this.
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,779
    1,103
    Good news!
     
Loading...