Author Topic: Measuring a pot (voltage divider) in circuit  (Read 261 times)

j_flanders

Measuring a pot (voltage divider) in circuit
« on: February 20, 2021, 07:23:47 PM »
I have a big box Deluxe Memory Man that has more than one repeat with the feedback knob at the minimum setting.
I suspected some residual resistance between wiper and ground of the feedback pot resulting in some feedback, even when on zero.
I haven't taken the board out , so I can't see the component side but measuring the solder points of the pot confirms my suspicion.
Before I wreak havoc on this board can anyone confirm that only a faulty pot could give these measurements?


Faulty pot like this? Extra 20k between lug and start of track.


Measurements:


Schematic:
« Last Edit: February 20, 2021, 08:44:04 PM by j_flanders »

PRR

Re: Measuring a pot (voltage divider) in circuit
« Reply #1 on: February 20, 2021, 08:12:32 PM »
Bad solder joint? Wrong resistor pot-bottom to ground?
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Rob Strand

Re: Measuring a pot (voltage divider) in circuit
« Reply #2 on: February 20, 2021, 08:24:28 PM »
Sometimes the rivets on the pot don't connect the terminal to the carbon track properly.   Another one is fine cracks in the carbon track.

 
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j_flanders

Re: Measuring a pot (voltage divider) in circuit
« Reply #3 on: February 20, 2021, 08:43:19 PM »
Bad solder joint? Wrong resistor pot-bottom to ground?
Generally good suggestions but in this case I don't see how they fit in with the measurements or the schematic.

Sometimes the rivets on the pot don't connect the terminal to the carbon track properly.   Another one is fine cracks in the carbon track.
Thanks, I'll check the rivets. Maybe I can press/pinch them.

iainpunk

Re: Measuring a pot (voltage divider) in circuit
« Reply #4 on: February 21, 2021, 07:58:47 AM »
generally, pot is measured in grams, not in K... jk

echo on the bad rivet connection, i think there are some conductive glues that could be usefull in cases like this.
it could also be a partial crack in the trace, or it endured some over current and burned part of the trace, i had this happen with a trim pot once.

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

j_flanders

Re: Measuring a pot (voltage divider) in circuit
« Reply #5 on: February 21, 2021, 09:46:37 AM »
It was not  the rivet but a crack.

Today I dismantled the pedal to have a look at the component side and also inside the pot.
First observation: the faulty pot is a different brand/make from all the others though at first sight it looks stock.
Measurements:


After cleaning the path that leads up to the carbon track the pot reads now open OL.
A strong light and magnifying glass reveals the tiniest hairline crack in the path leading to the carbon track.
By cleaning it I must have removed the 20k dirt that was in the crack. (waiting for Iainpunk here)
The red line in the image below shows where it was. (not the actual pot wafer btw)
I bridged the gap with conductive glue. (in blue on the image)
It's sunday, so I had to make my own. (mixture of glue and magnetised 0000 steelwool dust)


It worked. Pot now reads 9.5k and it goes to 0:


Glued: (grey substance, track at right lug)

iainpunk

Re: Measuring a pot (voltage divider) in circuit
« Reply #6 on: February 21, 2021, 11:30:19 AM »
Quote
By cleaning it I must have removed the 20k dirt that was in the crack. (waiting for Iainpunk here)
i've been watching a lot of gold hunting tv shows on Discovery, they were talking about 21k dollar per cubic yard of dirt, did you remove that kind of dirt from that crack? there could be 1milligram of gold in there then 8)
waiting for me to do what exactly?
with my earlier post i meant that i had a burned track, not a cracked one, burned is worse because its hard to see black burned carbon on black clean carbon. its also harder to ''repair'' since it was like 1/4th of the track.

im glad to read that you managed to fix it tho!

cheers
« Last Edit: February 21, 2021, 11:32:27 AM by iainpunk »
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

j_flanders

Re: Measuring a pot (voltage divider) in circuit
« Reply #7 on: February 21, 2021, 11:44:46 AM »
waiting for me to do what exactly?
To come up with a joke. I gave you pot last time, now you got crack. :p

I'm partially surprised how well the fix worked. (another one for you...)
I read so many people having disappointing results with glue+ carbon/graphite//pencil powder. In the k Ohms for only a couple of mm in track.
Hence my choice for steel wool dust. Someone wrote that magnetising it made a huge difference in resistance. Couldn't hurt to try.
« Last Edit: February 21, 2021, 11:49:20 AM by j_flanders »

iainpunk

Re: Measuring a pot (voltage divider) in circuit
« Reply #8 on: February 21, 2021, 12:04:38 PM »
Quote
I'm partially surprised how well the fix worked. (another one for you...)
sorry no joke here, forum rules prevent my from talking about Joe Biden's win in the US election...

i've read about magnetized iron powder being used, but not mixed with glue, but covered with a small layer of epoxy. also not on pots, but on PCB tracks

cheers
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

Rob Strand

Re: Measuring a pot (voltage divider) in circuit
« Reply #9 on: February 21, 2021, 06:17:01 PM »
Quote
I'm partially surprised how well the fix worked. (another one for you...)
You did well.  Nice pics BTW.
The internet:  answers without the need for understanding.