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Does this sound like a bad pot to you?

Discussion in 'Shock Brother's DIY Amps' started by TelZilla, Mar 25, 2020.

  1. TelZilla

    TelZilla Friend of Leo's

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    Hello. Hope everyone is staying safe (and able to pay their bills).

    I had a recent thread about my homebrew 5F6A- I had a bad recto tube. Weird. In any case, while I had the chassis on the bench I had another issue surface, and I wanted to gather opinions about it. I'm thinking it's a bad pot, but wanted to get the opinions of the heads here.

    Details on the Amp:
    • It's based on the 5F6A, built on a Hoffman board (layout below), with some of Rob Robinette's mods involved (specifically a switch to lift the tone stak and switchable negative feedback).
    • It's built in a BFDR style chassis
    • It's only got two inputs- one regular and one Hi- both with their own volumes. Vol 1 is an unmodified 1 Meg Audio. Vol 2 is a 1 Meg Audio pot with a bright cap

    Here's what's happening:
    • When I got it on the bench and put in a new GZ34, it roared to life
    • Channel 2 works great, but there's an issue with channel 1- the one with the vol pot with no cap.
    • At first, I got no response at all- totally quiet with a guitar plugged.
    • With some gentle (and safe) chopsticking, I got it to come on.
    • Specifically , what worked worked was tugging a bit on the wire (marked "this wire" below) from Lug 1 of the volume pot to the .022 cap coming from pin 1 of v1. A gentle tug would make the signal appear again
    • The signal stays "on" for a period of time, but typically shorts out again at some point
    Here's what I tried:
    • Reflowed the solder on the first lug of the volume pot
    • Verified connectivity between the first lug of the volume pot and the lug on the board holding the end of the .022 cap
    • Verified (with a multimeter set to ohms resistance) that the pot still "worked"- i.e. the resistance changes when turning the knob
    • Verified that the first lug of the volume pot did not have connectivity to ground.
    • I used shielded wire for this part of the amp, so I cut the shields connection to ground, thinking maybe I had an inadvertent connection between the shield and the wire proper. you can see that cut shield wire hanging off the ground buss in some of the pix below
    I added some Pix. The crust yellow junk is mostly solder flux, but you probably knew that...

    None of this had any effect. What's your guess?

    I wonder if I have a bad pot. As you can see, it's pretty tight in there, and all the pots are wired together, so I'm not real excited to try to get another one in there. Anything else I should try? I thought about just putting another wire (or an alligator clip wire) in there?

    Which Wire.png
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    Last edited: Mar 25, 2020
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Could be - it's three connections and two minutes, so not a hard thing to find out :D
     
    dan40 and King Fan like this.
  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    FWIW... you might try it with a non-shielded wire just for testing. Reflowing the solder was good -- and your shielded cable looks good -- but it's way easy to melt the inner jacket on shielded wire while soldering, and a variable short to shield can be the very devil to track down. Go ahead -- ask how I know. :) I thought I was good at building shielded cables, but built two in a row that would short out -- invisibly -- from soldering. That, and a stray whisker of shield can also be a common and subtle gremlin. Sometimes continuity testing can help, sometimes not so much.
     
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  4. TelZilla

    TelZilla Friend of Leo's

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    Yeah, I'm gonna get out some alligator clips and wires and get to work...
     
  5. Cruisin Home

    Cruisin Home Tele-Meister

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    couple things might help. (I love 5f6As and have an Original and a DIY):

    1. Appears lot of solder globs everywhere and heat damage. May not come into play here but try and clean that stuff up, you dont want it loose and bouncing around. But the bypass cap/resistor are touching the orange drop coupling cap where that wire goes. The cap doesn't look good and may have been heat violated? When you are chop sticking try and determine what is responding.

    2. spray the POT with Deoxit. Then try.

    Overall your soldering a little on the hot and excessive side. Back off just a tad on both. Clean with isopropyl alcohol.

    Good Luck! These are awesome amps (my favorite)
     
    dan40 likes this.
  6. dan40

    dan40 Friend of Leo's

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    I also agree that the wire may be your culprit. Could be broken internally or an internal short as King Fan alluded to.
     
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