SOLVED 6-Band EQ (slider) - Potentiometer issue

[DougieFresh]

I've encountered a very frustrating issue with the leftmost (100 Hz) slider pot and I've completely run out of ideas. After assembling everything and soldering the potentiometer and main boards together, I noticed that the leftmost pot was not boosting the low frequencies as expected when above the midpoint. When I would gently flex the shaft of the pot to the RIGHT (towards the other pots) while sliding it up and down, it worked fine, but otherwise it wasn't working properly. I verified later that it also appeared to work correctly when I applied pressure to the housing of the pot in the same direction (to the right, by gently squeezing the row of pots together with my hand) rather than flexing at the shaft.

Here's what I've done so far to troubleshoot:

- Verified that there was no contact between the pots and the enclosure. The problem persists with the boards completely out of the enclosure.
- Verified that the pots were sitting flush and even in the PCB, and that they were not touching each other or the PCB in a weird way.
- Separated the two boards and verified all solder connections on the potentiometer board (pots and pin headers), ensuring there was no excessive flux (removed with alcohol), no bridges and reflowing all solder joints.
- Tested the pots using the pin-headers. Other than the 100 Hz one, the pots all showed smooth changes in resistance along their travel length. The values were not at all what I expected (going from 0 at one end to 15k at the midpoint, and then going back DOWN to 10k on the other end...), but I presume that's from the way they are connected to each other? In any case, they were functioning normally in the circuit so I presume this is not an issue. For the 100 Hz pot, however, I noticed the values were different, ranging from 10k to 60k from one end to the other, unless I flex the shaft to the right, in which case I see the same values as the others. So at this point it seems obvious that the problem is a janky pot, right?? Nope!
- I removed the 100 Hz pot and tested it. When not connected, it seems to work perfectly, smoothly ranging from 0 to 50k without any weirdness even when I torqued the shaft to the left or right, or when applying pressure in different ways to the housing. I re-installed it, and the problem re-appeared.
- To be extra sure the problem is not with the pot, I removed it again and then also removed the rightmost pot (3.2 kHz) that was working perfectly before, and I swapped them. Lo and behold the same issue occurs with the (new) 100 Hz slider.

So now I'm at a complete loss and have not clear sense of what to try next. As always, any help is greatly appreciated!!

 

[Harry Klippton]

Have you checked the surrounding components for any mistakenly incorrect values?

 

[Brett]

Tested the pots using the pin-headers

Can you clarify on how you did this?

I ask because all of these slide potentiometers share the same net for pin 1 (shown green below). They also share the same net for pin 3 (shown red below).

 

[DougieFresh]

Have you checked the surrounding components for any mistakenly incorrect values?

There are no additional components on this PCB... only the six pots. All the components are on the main board connected to this one via 9 header pins.

 

[DougieFresh]

Can you clarify on how you did this?

I ask because all of these slide potentiometers share the same net for pin 1 (shown green below). They also share the same net for pin 3 (shown red below).

I tested between each of the top 6 pins (which connect separately to pin 2 on each pot) and one of the bottom pins.

 

[Harry Klippton]

There are no additional components on this PCB... only the six pots. All the components are on the main board connected to this one via 9 header pins.

I mean look at the schematic and check the values of the components on the other board that are connected in circuit to the slider in question

 

[Brett]

I tested between each of the top 6 pins (which connect separately to pin 2 on each pot) and one of the bottom pins.

So, when you measure between the top two pads on each potentiometer, it does not range from 0Ω to ~50KΩ on each potentiometer?

 

[DougieFresh]

I mean look at the schematic and check the values of the components on the other board that are connected in circuit to the slider in question

The problem appears to be on the slider board itself (when not attached to the main PCB with the components)... The measured resistance is very different (ranging from ~10k to ~60k) unless I torque the pot a bit, in which case it reads the same values as the others (oddly, ranging from 0 to 15k from one end to the middle and then back down to 10k).

 

[DougieFresh]

So, when you measure between the top two pads on each potentiometer, it does not range from 0Ω to ~50KΩ on each potentiometer?

With the pots removed (I only removed the first and sixth pot in order to test and swap them), it reads 0 through 50k. But when they are installed, they go from 0 to 15k at the middle, and then back down to 10k at the other end. It's odd (to me), but I figure that has to do with how they are all wired up in parallel?

[Edit to add that the other pots all seemed to be functioning as expected, attenuating the expected frequency below the midpoint and amplifying above. The 100 Hz pot affected the low frequency, but only reducing it below the midpoint. I heard no amplification above the midpoint, unless I applied some torque to it. This all seems to fit what I was observing at the level of measured resistance]

 

[jwin615]

It sounds like it may just be a bad part. Not making proper contact with the resistive tracks unless pressure is applied.
You could try some deoxit/fader lube.

 

[Brett]

It sounds like it may just be a bad part

I initially thought the same, but he says he swapped places with another (known working) fader and experienced the same issues.

 

[DougieFresh]

I initially thought the same, but he says he swapped places with another (known working) fader and experienced the same issues.

It sounds like it may just be a bad part. Not making proper contact with the resistive tracks unless pressure is applied.
You could try some deoxit/fader lube.

Yes I swapped the parts and it didn't help... Maybe it's a batch of faulty parts combined with some kind of physical interaction with the PCB? The faders seem to work fine when removed from the board. I tried spraying contact cleaner and it didn't help, unfortunately.

 

[Brett]

But when they are installed, they go from 0 to 15k at the middle, and then back down to 10k at the other end. It's odd (to me), but I figure that has to do with how they are all wired up in parallel?

I plugged the circuit into a simulator to get an idea of what to expect in-circuit and the behavior you describe is inline with the results of the simulator.

To be extra sure the problem is not with the pot, I removed it again and then also removed the rightmost pot (3.2 kHz) that was working perfectly before, and I swapped them. Lo and behold the same issue occurs with the (new) 100 Hz slider.

When you swapped pots, did you put the entire circuit back together again to verify that you needed to squeeze the pots together to get the replaced potentiometer to work correctly?

With both pots giving you the same behavior on a PCB that has several verified builds, I think it's worth doing as @Harry Klippton suggested:

I mean look at the schematic and check the values of the components on the other board that are connected in circuit to the slider in question

 

[DougieFresh]

I plugged the circuit into a simulator to get an idea of what to expect in-circuit and the behavior you describe is inline with the results of the simulator.


When you swapped pots, did you put the entire circuit back together again to verify that you needed to squeeze the pots together to get the replaced potentiometer to work correctly?

With both pots giving you the same behavior on a PCB that has several verified builds, I think it's worth doing as @Harry Klippton suggested:

Thanks for the suggestions. No I did’t put it back together. I only noticed the effects on the PCB with the faders as measured using my DMM (weird resistance values unless I torque it a bit). Guess I have some more exploring to do…

 

[jwin615]

Maybe when you are torquing it, you're actually making unintended contact internally?
We're you careful to have all other sliders down/up/same when comparing

 

[DougieFresh]

Thanks everyone for your suggestions. As always, I find these troubleshooting experiences (and discussions) very educational.

Just a final update... After trying everything mentioned here with no luck, I decided to remove all the faders, desoldered all the contacts on the PCB, sprayed contact cleaner in all the faders, and then replaced everything taking care to align all the faders and not use too much solder. It now works without any issues (and hopefully will stay that way). Not too satisfying since I still don't know exactly what was to blame, but I hope this can be of some help to someone else who runs into this same issue in the future!