Need a replacement potentiometer for a Boss ME-25

[g1]

Not sure how hard those are to take apart, but is it possible there is the proper taper available in a different format (other shaft for example) that you could cobble together one from the two?

[nevetslab]

[QUOTE=g1;522701]Not sure how hard those are to take apart, but is it possible there is the proper taper available in a different format (other shaft for example) that you could cobble together one from the two?[/QUOTE

I'll find out in the morning, as the Blackstar Series One Model 200 is chock full of these pots. Is the part you're looking for 10k Log Taper?



I'll also have the mechanical dimensions & disassembly photos from the surgery....this PCB is from my boneyard.

[olddawg]

You may be able to disassemble both pots and transplant the wafer or wafer and housing assembly onto the new shaft and wiper assembly. It’s tedious but it can be done.. even if you have to encase it in a block of epoxy. You have them already. Why not try?

[eaelec]

Not sure how hard those are to take apart, but is it possible there is the proper taper available in a different format (other shaft for example) that you could cobble together one from the two?

I'm very confident I can take one apart, putting it back together not so much.

If the mod I'm thinking about works, there may be not need to do that.

[nevetslab]

Pot Dissambly & dimensions

I'm very confident I can take one apart, putting it back together not so much.

If the mod I'm thinking about works, there may be not need to do that.

What's on the Blackstar Model 200 front panel board are Alpha, 11mm size, mounting between the support tabs is 11mm, and between the center line of the support tabs to that of the four pot leads is also 11mm. 8mm dia threaded bushing, 5mm long, shaft length on these are 17mm, shaft dia is 6mm, D-shaft knob detail at the end. PCB Lead pitch is 2.5mm (between the four leads). Photos of the pot & disection:



Prying up the four fingers set down into the body's grooves and straightening them allows the resistance module to be removed. The shaft/rotor assembly slides out of the bushing/mounting bracket assembly. Leave the grease on the shaft/bushing insides, as that's your viscosity feel compound. Normally you would clean the resistance track with some alcohol, then apply some Caig DeOxit 5 for lubricant, and brush on a little bit onto the rotor assembly's wiper fingers. I didn't clean this one until after I had taken all the photos, and have more images, only I've used the max 9 images on this post. With the shaft/rotor assy back onto the body, I connected my Fluke 8060A in 200k Ohms mode, showing Min rotation between wiper and CCW terminal (0k), 50% rotation (15k) and full rotation (98k)...this being a 100k Audio Taper pot.

[J M Fahey]

Oh well.

[nickb]

Here's the key voltage levels at the wiper:
.3 volts - "toe-down", the FX pedal light toggles when you drop below this level
.5 volts - Volume reaches max, does not increase as voltage goes lower
3 volts - max attenuation, no volume.

With the pot attached to the pedal mechanism, the voltage range is limited from .16v up to 1.17v volts.
So there is some effect on volume, but the way the human ear works it's not noticeable.

Now if I were to feed the pot with a higher voltage i could fix this, the pedal runs on a 9V adapter so I could tap off of that.
PS
I made a real rookie mistake yesterday, voltmeter leads in the wrong way, toe-down voltage is actually 3V, vol. max is 2.8v, min vol. is .3V.

Good idea. Alternatively, it looks like the PCB in the picture doesn't do anything other than connect the ribbon to the pot. So would it be possible to connect the ribbon directly to a common 16mm pot mounted in the plate. Is there room?

[eaelec]

I got it working, but with a little more effort than I originally thought. After I found the mistake in post 13, i realized I needed to pull one end of the pot down below ground with a negative voltage.

So, I tapped +9V off of the back of the adapter jack, then used a simple linear regulator to bring it down to +5V. The +5V feeds into a switched capacitor voltage inverter (ICL7660) which changes +5V to -5V. Then I cut a track to disconnect the end of the pot which was connected to ground and connected that to the -5V (that's the white wire in the picture). Now it would be prudent to add some kind of clamping circuit on the wiper. If the wafer in the pot breaks, it could send -5V into the main board, which would not be good. I tried a simple 3V zener between +3.3V and the wiper but it caused problems, I'll have to investigate that some more. Here's what it looks like:

[eaelec]

What's on the Blackstar Model 200 front panel board are Alpha, 11mm size, mounting between the support tabs is 11mm, and between the center line of the support tabs to that of the four pot leads is also 11mm. 8mm dia threaded bushing, 5mm long, shaft length on these are 17mm, shaft dia is 6mm, D-shaft knob detail at the end. PCB Lead pitch is 2.5mm (between the four leads). Photos of the pot & disection:

[ATTACH=CONFIG]52539[/ATTACH] [ATTACH=CONFIG]52540[/ATTACH] [ATTACH=CONFIG]52541[/ATTACH] [ATTACH=CONFIG]52542[/ATTACH] [ATTACH=CONFIG]52543[/ATTACH] [ATTACH=CONFIG]52544[/ATTACH] [ATTACH=CONFIG]52545[/ATTACH] [ATTACH=CONFIG]52546[/ATTACH] [ATTACH=CONFIG]52547[/ATTACH]

Prying up the four fingers set down into the body's grooves and straightening them allows the resistance module to be removed. The shaft/rotor assembly slides out of the bushing/mounting bracket assembly. Leave the grease on the shaft/bushing insides, as that's your viscosity feel compound. Normally you would clean the resistance track with some alcohol, then apply some Caig DeOxit 5 for lubricant, and brush on a little bit onto the rotor assembly's wiper fingers. I didn't clean this one until after I had taken all the photos, and have more images, only I've used the max 9 images on this post. With the shaft/rotor assy back onto the body, I connected my Fluke 8060A in 200k Ohms mode, showing Min rotation between wiper and CCW terminal (0k), 50% rotation (15k) and full rotation (98k)...this being a 100k Audio Taper pot.

The only RK11K series pot that you can buy off-the-shelf with a log taper is a 20K pot, not a 10K.