Optical pedal repair

[Rhodesplyr]

I bought a Farfisa volume pedal to go with my Compact Duo Organ, but in taking it apart to clean it, I ran across a problem.

This pedal uses a blade-like piece of aluminum with a slot in it between the light bulb and the LDR. Riveted to the blade across this slot are two pieces of clear plastic with a thinner plastic aperture clamped between them. Part of this aperture is opaque, but it has a clear center section with an elongated teardrop shape that sets the taper.

The problem: the "clear" plastic is yellowed on all three parts. A common complaint with these pedals is that they don't cut the volume enough, and I think these two observations may be related. Yellowed plastic reduces the amount of light getting to the LDR.

Any suggestions? I drilled the rivets out and tried soaking the outer plastic pieces in Oxy-Clean. It helped a bit, but not that much. The challenge is that the teardrop taper shape is very small and would be hard to copy exactly. If not smoothly cut, I imagine the volume taper would be jagged.

David

[olddawg]

You might want to check out how a Morley volume pedal is done. They are the industry standard for LDR sweep pedals. Most of them use a simple shutter type apperature. I don't see why you would need the clear plastic at all. You could always use a brighter bulb to make up for the opacity though. You might be able to adapt Morley replacement parts to this pedal.

[Rhodesplyr]

I've checked out the Morley pedals. I know them well. I used to play with a guitar player who used a Morley wah. For some reason, he couldn't seem to get organized enough to get a replacement bulb, so he would tape a mini-mag flashlight in it before each show....

The Morleys are so different in design, though, that I don't think I can borrow much from them, but I agree with you in that I don't see why the clear plastic is really necessary. I might be able to cut a design that would work in a thin piece of metal--or get someone who makes jewelry to do it.

[Don Moose]

I wouldn't worry too much about ragged edges on the aperture - you're not lighting up a smaller dot on the photoresistor's surface, you're allowing less light through the gap.

[olddawg]

I've checked out the Morley pedals. I know them well. I used to play with a guitar player who used a Morley wah. For some reason, he couldn't seem to get organized enough to get a replacement bulb, so he would tape a mini-mag flashlight in it before each show....

The Morleys are so different in design, though, that I don't think I can borrow much from them, but I agree with you in that I don't see why the clear plastic is really necessary. I might be able to cut a design that would work in a thin piece of metal--or get someone who makes jewelry to do it.

The only reason I could think of for the plastic is if it is polarized or is a filter.

[Gtr_tech]

Try this:
Retr0Bright - home

I'm sure that would take care of it.

[Rhodesplyr]

The only reason I could think of for the plastic is if it is polarized or is a filter.

I think the reason for the plastic sandwich was to protect the "aperture," which is made of a softer material than the two outer pieces.

But I did find at least a possible solution. I taped a piece of black electrical taper over the hole in the shutter/blade and cut the taper shape in it with a razor knife. It seems to work just fine. Resistance at minimum volume is 395 Ohms, the same as with the bulb on and no shutter at all. At maximum volume (it acts as a shunt to ground) , my meter measures over 200 Megohms, basically an open circuit.