20K dual Variable Resistor - centre indent - Resources?

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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by bassbass » 20 Mar 2018, 10:54

Sounds like the hot and earth are around the wrong way. In the centre, an MN blend pot has no resistance between the wiper and one outer lug - clockwise for one gang, and anti-clockwise for the other. Trouble is, there's no real standard. The clockwise short could be the top gang, or the bottom, and vice-versa of course for the anti-clockwise! That short is the hot side of the circuit. You really need to test the pot before wiring it.

If you've mounted a standard type of dual gang pot on wire legs on your preamp board, I think you'll have to swap the outer two lugs on both gangs. IOW, top-left should be swapped with bottom left, top-right should be swapped with lower-right. If the pot rotation is otherwise correct (rolling forward gets neck pickup, and back for bridge) then you'll have to leave the middle lug wires where they are. You may need to slip a length of insulation over one wire from each side so there's no chance of a short.

Another approach is to mod the board, but cutting tracks etc is not recommended for the less-experienced. :/
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by sheeny » 20 Mar 2018, 11:00

Hey Rob, the pickups are fed to the centre lugs. Does that help?

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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by sheeny » 20 Mar 2018, 11:03

Thanks for that informative post too, I'll give that a go tonight!
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by sheeny » 20 Mar 2018, 13:20

Hey Rob, just wired it up as you advised. It now behaves exactly as the original blend. This basically indicates its s pcb issue and not the pot. Oh well, I'll just wait for the replacement pcb and be done with it.
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by bassbass » 20 Mar 2018, 23:18

Bummer.

Hey, what actually was the original problem? I'm not sure you stated this...
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by sheeny » 21 Mar 2018, 12:49

When I turned the blend control to the neck pickup it went dead, Bridge pickup was fine. Swapped out pots, still did the same thing. Pickups fine, because when I swapped them around on the board, the issue was mirrored. PCB issue?
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by bassbass » 21 Mar 2018, 18:24

Yeah that sounds like a circuit board issue. Probably just a dry solder joint or something. Or maybe a dodgy connector from the pickup...

If the signal is working from the bridge pickup, it means that the problem must be around the pickup connections or the blend pot and its directly related pcb tracks.

If you've swapped those two white pickup connectors and found that the neck pup suddenly worked and now the bridge pup was dead, then I'd suspect the connector (or its solder joint, track to the pot, or pot solder joint).

(FWIW, those solder joints don't exactly look, ah-hem... amazing ;) )

So I'd simply cut the white wire (the Nk pup hot) and solder it directly to the pot.

Also I guess by now the pot lugs have been soldered and re-soldered a few times. There's a chance that you've damaged the pot. I still always use heatsink clips on pot lugs, and I've been hand soldering since the 80's!
Last edited by bassbass on 21 Mar 2018, 18:26, edited 1 time in total.
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by sheeny » 21 Mar 2018, 18:26

Okay Rob, sounds like a plan! I'll let you know how I go.

thanks again!
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by bassbass » 21 Mar 2018, 18:28

Good luck man! PCB, especially surface mount boards, can drive you crazy...
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by bassbass » 21 Mar 2018, 18:30

Oh, and be sure to cuut the wire a couple of inches back from the connector so you can re-wire it when the new pre arrives.
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Re: 20K dual Variable Resistor - centre indent - Resources?

Post by sheeny » 21 Mar 2018, 20:11

I really think Ozbass forum needs to introduce feedback scores for guys like you Rob. You certainly have my vote! Very helpful :)
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