Equalizer W50K Potentiometer

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X100Ver10
Posts: 83
Joined: Mon Sep 18, 2017 2:49 pm

Equalizer W50K Potentiometer

Post by X100Ver10 » Mon Jul 30, 2018 1:15 am

Hello all,

I am trying to fix a Rockman Equalizer. The W50K potentiometers at 500, 700, & 2K in the Mid Band section are not working. I took the 500 pot out of the board and measured it with a multimeter. I get 50k ohms when measuring leads 1 & 2, no matter where the slide knob is set (I believe this is normal). I get nothing when I measure leads 1 & 3 and leads 2 & 3. Again, no matter where the slide knob is set. For purposes of clarity, lead 1 in the below picture is top left. Lead 2 is bottom left. Lead 3 is bottom right.
IMG_20180729_190616.jpg
I took it apart and cleaned the carbon tracks. No luck. The 3 contacts on the white knob were cleaned also but they look fine and are making contact with the carbon tracks. They also show continuity between them. I'm going to go out on a limb and say the pot board is bad, somewhere in the shorter carbon track with the 1 lead.
IMG_20180729_191041.jpg
As usual, a replacement W50K pot is proving difficult to find. I found a W100K pot that I'm pretty sure will fit. I'm waiting for some to arrive from China. My questions are:

1) If it fits, can I just put a W100K pot in without any issues?
2) If not, based on some internet research I did, can I solder a 100K resister to the leads of the W100K pot to make it a W50K pot? If so, to what leads? 1 & 2, 1 & 3, or 2 & 3?

Thanks.
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RockmanCentralBob
Posts: 240
Joined: Fri Feb 06, 2015 2:10 am

Re: Equalizer W50K Potentiometer

Post by RockmanCentralBob » Tue Jul 31, 2018 11:30 pm

"If not, based on some internet research I did, can I solder a 100K resister to the leads of the W100K pot to make it a W50K pot?"

I don't think that's going to work since it's variable.

X100Ver10
Posts: 83
Joined: Mon Sep 18, 2017 2:49 pm

Re: Equalizer W50K Potentiometer

Post by X100Ver10 » Wed Aug 01, 2018 2:13 pm

Hi Bob,

Thanks for replying. A few days ago, I played around with a new A100K slide pot I had in stock. I soldered a 100K resistor to the 1 & 2 leads and got 50K across those leads using a multimeter, no matter where I slid the knob. I can't remember now what kind of measurements I got with the multimeter across leads 1 & 3 and 2 & 3 though. I figured I'll just wait until I get the W100K's in and try soldering a resistor to different leads to see if I can get it to match one of the old, working W50K pots. I was hoping I could use a circuit board from a new A50K or B50K pot, but the W taper is definitely different. I'll report back, if I have any success.

GuitarBuilder
Posts: 196
Joined: Sat Aug 30, 2014 7:14 pm

Re: Equalizer W50K Potentiometer

Post by GuitarBuilder » Mon Aug 06, 2018 7:42 pm

Check your wiper contacts again! They may need to be bent out a little.

X100Ver10
Posts: 83
Joined: Mon Sep 18, 2017 2:49 pm

Re: Equalizer W50K Potentiometer

Post by X100Ver10 » Tue Aug 07, 2018 2:41 am

GuitarBuilder wrote:Check your wiper contacts again! They may need to be bent out a little.
Hi GuitarBuilder,

Thanks for the reply. I did try that. I even pulled the board and knob out of the metal case and visibly made sure the double wipers were contacting the longer/wider trace and the single wiper was on the shorter/thinner trace. If I put the wipers on the round part of the metal leads of 2 & 3(riveted side near the traces), I could get 50K with the multimeter on leads 1 & 3 and 0 with it on leads 2 & 3. However, once I move the knob and and the wipers make contact with the (carbon?) traces, nothing. It should go from 50K down to 0 when the mm is on leads 1 & 3 and 0 up to 50K when on leads 2 & 3. The mm just stays at 1. There is definitely something wrong with the shorter trace. It is like the carbon trace is not making contact with it's riveted lead, a break somewhere I can't see (underneath the rivet?) or the trace is just worn out.

BrightKJ
Posts: 4
Joined: Mon Oct 05, 2015 2:52 pm

Re: Equalizer W50K Potentiometer

Post by BrightKJ » Tue Aug 07, 2018 7:33 pm

Placing a 100k resistor in parallel with a 100k pot will NOT give you a 50k pot. It will only be 50k when turned all the way up. Otherwise, the resistance is calculated as (50k*R2)/(50k+R2), where R2 is the position of the pot. This is basic electronic resistance theory.

X100Ver10
Posts: 83
Joined: Mon Sep 18, 2017 2:49 pm

Re: Equalizer W50K Potentiometer

Post by X100Ver10 » Tue Aug 07, 2018 7:51 pm

BrightKJ wrote:Placing a 100k resistor in parallel with a 100k pot will NOT give you a 50k pot. It will only be 50k when turned all the way up. Otherwise, the resistance is calculated as (50k*R2)/(50k+R2), where R2 is the position of the pot. This is basic electronic resistance theory.
Thanks for the reply BrightKJ. Bummer. It looks like I have a Rockman Equalizer for parts then. :( At least I have 9 good W50K pots if I ever get another Equalizer with a bad pot. ;)

Unless someone has a source for these W50K pots. Anyone?

GuitarBuilder
Posts: 196
Joined: Sat Aug 30, 2014 7:14 pm

GuitarBuilder

Post by GuitarBuilder » Thu Aug 09, 2018 5:12 pm

X100Ver10 wrote:
GuitarBuilder wrote:Check your wiper contacts again! They may need to be bent out a little.
Hi GuitarBuilder,

Thanks for the reply. I did try that. I even pulled the board and knob out of the metal case and visibly made sure the double wipers were contacting the longer/wider trace and the single wiper was on the shorter/thinner trace. If I put the wipers on the round part of the metal leads of 2 & 3(riveted side near the traces), I could get 50K with the multimeter on leads 1 & 3 and 0 with it on leads 2 & 3. However, once I move the knob and and the wipers make contact with the (carbon?) traces, nothing. It should go from 50K down to 0 when the mm is on leads 1 & 3 and 0 up to 50K when on leads 2 & 3. The mm just stays at 1. There is definitely something wrong with the shorter trace. It is like the carbon trace is not making contact with it's riveted lead, a break somewhere I can't see (underneath the rivet?) or the trace is just worn out.
Here's a wild idea: using one multimeter probe at one end of the carbon trace, probe along the trace to see where the discontinuity is. If you find the spot, use some conductive shielding paint (such as the type used for guitar electronics cavities: https://www.stewmac.com/Pickups_and_Ele ... Paint.html). Just a tiny dab will do! It may just work!

X100Ver10
Posts: 83
Joined: Mon Sep 18, 2017 2:49 pm

Re: Equalizer W50K Potentiometer

Post by X100Ver10 » Fri Aug 10, 2018 2:38 pm

GuitarBuilder wrote:
X100Ver10 wrote:
GuitarBuilder wrote:Check your wiper contacts again! They may need to be bent out a little.
Hi GuitarBuilder,

Thanks for the reply. I did try that. I even pulled the board and knob out of the metal case and visibly made sure the double wipers were contacting the longer/wider trace and the single wiper was on the shorter/thinner trace. If I put the wipers on the round part of the metal leads of 2 & 3(riveted side near the traces), I could get 50K with the multimeter on leads 1 & 3 and 0 with it on leads 2 & 3. However, once I move the knob and and the wipers make contact with the (carbon?) traces, nothing. It should go from 50K down to 0 when the mm is on leads 1 & 3 and 0 up to 50K when on leads 2 & 3. The mm just stays at 1. There is definitely something wrong with the shorter trace. It is like the carbon trace is not making contact with it's riveted lead, a break somewhere I can't see (underneath the rivet?) or the trace is just worn out.
Here's a wild idea: using one multimeter probe at one end of the carbon trace, probe along the trace to see where the discontinuity is. If you find the spot, use some conductive shielding paint (such as the type used for guitar electronics cavities: https://www.stewmac.com/Pickups_and_Ele ... Paint.html). Just a tiny dab will do! It may just work!
No continuity/conductivity at all on the shorter trace. I think its worn out.

Thanks for the idea GuitarBuilder. I forgot that I actually have a small container of black conductive paint that worked well on a headphone amp trace. It came out kind of rough though but I used a toothpick to apply it. I think I have a bag of small paint brushes I bought at Harbor Freight that I can try to get a smoother coat on the pot trace. I'll give it a go this weekend and report back. I can't make it any worse. ;)

X100Ver10
Posts: 83
Joined: Mon Sep 18, 2017 2:49 pm

Re: Equalizer W50K Potentiometer

Post by X100Ver10 » Mon Aug 13, 2018 3:32 pm

Using a product called Wire Glue from Anders Products that I already had, I applied it to the entire shorter/thinner trace. I still couldn't get any continuity to show at any points along the trace. I put the pot back together, however, and could finally get a reading with the multimeter connected to leads 1 & 3 and 2 & 3. The only problem is at 0% travel, the readings were at around 3.5 ohms. They wouldn't go down to 0. At 100% travel, they were at 53.5 ohms. Also, the wipers are making a groove in the conductive paint. Although it is still working, I think eventually it will wear out too quickly. Based on the weird reading at 0% travel and the groove, I wouldn't trust it. Thanks again GuitarBuilder for the suggestion though.

I did receive the W100K pots from China. I connected a 100K resistor to leads 1 & 2 and, surprisingly, it may actually work. The mm reading at leads 1 & 2 was a constant 50 ohms. The mm readings with the leads on 1 & 3 or 2 & 3 were a little higher then they should be at 25% & 50% travel (compared to an old, working one) but could work. The readings do vary from 0 to 50 ohms as you slide the knob.

I did find some W50K pots from China. They have 4 leads, instead of 3 but I'm hoping I can just snip one of them and get it to work. The knobs are too long but I should be able to use the old ones. I'll report back when I get them in a few weeks.