A mystery — “fine” pot not working on a MOTM-300

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DrReverendSeance
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A mystery — “fine” pot not working on a MOTM-300

Post by DrReverendSeance » Sun Nov 25, 2018 12:03 pm

I have a used MOTM-300.

I noticed today that the fine pot is not working. When I rotate the pot, there is no change in pitch at all. The Coarse pot on the same module is fine.

I checked the resistance on the Fine pot on both sides of the wiper, and it is about 100K.

The only other component in the circuit that could be an issue is a single 3.32M resistor. I doubt that this is the cause of the problem?

This is pretty mysterious to me and my limited understanding of electronics. Has any one else seen this issue before?

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Post by unrecordings » Sun Nov 25, 2018 1:40 pm

If coarse is working then it should be easy to figure out. Is +15V getting to one side of the pot and -15V to the other side. Can you measure any voltage coming out of the wiper ?
My guess is the pot itself or a dry joint

I'm assuming you've only just got this module ?

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Post by DrReverendSeance » Sun Nov 25, 2018 4:20 pm

Hi unrecordings

I am getting +15V on 1 side and -15V on the other. I am also getting voltage from the wiper. I think the pot is OK.

I noticed that on R3, the resistor just after the pot, I get 15V on one side, and 0V on the other. Can a resistor “die” and block voltage? The resistor is 3.32M Ohms.

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Post by DrReverendSeance » Sun Nov 25, 2018 4:21 pm

I got his module some time ago, but I had never noticed that the fine was not working. Maybe it was working before and recently failed.

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Post by Dave Peck » Sun Nov 25, 2018 5:05 pm

DrReverendSeance wrote:Hi unrecordings

I am getting +15V on 1 side and -15V on the other. I am also getting voltage from the wiper. I think the pot is OK.

I noticed that on R3, the resistor just after the pot, I get 15V on one side, and 0V on the other. Can a resistor “die” and block voltage? The resistor is 3.32M Ohms.
Is that the resistance you MEASURED across the resistor? Or is that what the color coded markings say? It's a pretty high value, so maybe it's supposed to be much lower, but has changed.

JohnLRice

Post by JohnLRice » Sun Nov 25, 2018 5:17 pm

DrReverendSeance wrote:Hi unrecordings

I am getting +15V on 1 side and -15V on the other. I am also getting voltage from the wiper. I think the pot is OK.

I noticed that on R3, the resistor just after the pot, I get 15V on one side, and 0V on the other. Can a resistor “die” and block voltage? The resistor is 3.32M Ohms.
It's rare but possible that a fixed resistor might fail. It could have been slightly defective when manufactured but it still worked initially but then through vibration and/or repeated heating/cooling it 'broke" inside? :hmm:

You could quickly test if it's 'bad' by just temporarily holding another resistor over the top of the existing one to see if the Fine pot comes to life. Use a 3.32M one if you have it but anything that is approximately 150K or greater would safely work just to test the theory. If this works the first thing I would do would be to reflow the resistor's solder joints to see if that fixes the problem, and then if it doesn't I'd replace the resistor.

If you aren't up for soldering you should write to Paul at Synth Tech and see what he says. I think he used to offer free repairs on MOTM modules "for life" so maybe he'll fix if for you? 8-)

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Post by DrReverendSeance » Sun Nov 25, 2018 6:03 pm

JohnLRice for the win!

I bridged the 3.32M resistor with a 1M resistor I had, and although the overall pitch shifted signficantly the fine knob was now adjusting pitch as expected.

I’ll try reflowing the solder and if that doesn’t work I’ll replace the resistor.

The resistor doesn’t measure at 3.32M ohms, but I think this is because it is in the circuit? The spec document shows a 3.32M ohm resistor at R3, and the resistor at R3 does say 3.32M on it (no little lines though, just 3.32M written out).

Thank you everyone for your help.

Free reapirs for life! That is quite the guarantee.

JohnLRice

Post by JohnLRice » Sun Nov 25, 2018 6:32 pm

DrReverendSeance wrote:JohnLRice for the win!

I bridged the 3.32M resistor with a 1M resistor I had, and although the overall pitch shifted signficantly the fine knob was now adjusting pitch as expected.

I’ll try reflowing the solder and if that doesn’t work I’ll replace the resistor.

The resistor doesn’t measure at 3.32M ohms, but I think this is because it is in the circuit? The spec document shows a 3.32M ohm resistor at R3, and the resistor at R3 does say 3.32M on it (no little lines though, just 3.32M written out).

Thank you everyone for your help.

Free reapirs for life! That is quite the guarantee.
:party: :tu: Yeah, it's not so simple to measure a component in circuit, I'm too dumb but there is a way to do it by calculating all of the other components that are connected to it to know what it should read in circuit or something like that. :hmm: Me, I'd just unsolder one side of it then measure it so I don't hurt my brain trying to figure it out! :hihi: :ripbanana:

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Post by DrReverendSeance » Mon Mar 11, 2019 6:54 am

Thank you everyone for your help on this.

I replaced the 3.32M resistor, but it did not fix the issue. I checked the pot again and found that the voltage from the wiper was not changing when I moved the knob. I replaced the pot and now the module is working as expected.

A great learning experience.

unrecordings you had the right diagnosis at the start.

Thanks again

JohnLRice

Post by JohnLRice » Tue Mar 12, 2019 12:28 am

Glad you got it working DrReverendSeance! :party:

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Post by pugix » Thu Mar 28, 2019 6:47 pm

DrReverendSeance wrote:Thank you everyone for your help on this.

I replaced the 3.32M resistor, but it did not fix the issue. I checked the pot again and found that the voltage from the wiper was not changing when I moved the knob. I replaced the pot and now the module is working as expected.

A great learning experience.

unrecordings you had the right diagnosis at the start.

Thanks again
I had to replace the FM2 pot on a MOTM-300. Paul has said that the pots used in that era of production are prone to failure.
Richard
https://www.pugix.com

"Everything in our world is actually always modulated by everything else." - Peter B

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