Repairing/Replacing Vintage Maestro Parametric Filter Pot

[Muaar]

Hey, I have a maestro parametric filter (designed by moog in the 70's) that has a faulty pot. I took the pot apart and it appears the wafer has cracked. I have the service manual listing the replacement parts but obviously its very old and the company no longer exists. So what I'm wondering from you wizards is how can I find a replacement for this part?

Number R-23 in this image:


"Resistor, Rotary Pot, Dual FREQUENCY, AUDIO, 100k Ohms"


Can I find a replacement on mouser? Or would I be able to somehow replace the wafer?  I don't know how to look for a specific part like this.

Thanks!

[RickL]

I bet it's a dual gang 100k audio (log) pot. They're quite common, even Radio Shack carries (carried?) them. Small Bear does too.

[Muaar]

Could you help me find one specifically? I'm having trouble finding what I'm looking for. http://www.smallbearelec.com/Detail.bok?no=114 seem to be right?


Only problem with that is I'd need terminals that go through a PCB instead of the circle ones that has...

or maybe letter "E" on this page.
http://www.mouser.com/catalog/635/667.pdf
(http://www.mouser.com/Search/Refine.aspx?Ntt=313-2420F-100K)



Do those look like the right thing?

[RickL]

The Small Bear part you've linked is a linear pot. It will work electronically but you probably won't like the way it "feels" (all of the control will be concentrated in a small part of the pot's rotation). Small Bear sells an audio 100k dual pot http://www.smallbearelec.com/Detail.bok?no=736

The Mouser page also has a 100k audio dual pot.

You may have to use a pot with solder lugs and join each lug to the pcb with a short length of wire. I won't be completely original but at least it will work.

[Muaar]

since the small bear you linked is 16mm, would the mouser be a better choice?


http://www.mouser.com/Search/ProductDetail.aspx?qs=UxSkNpifYSl3VS2eqVcG%2fQ%3d%3d


I'm not sure the 16mm will fit in the crazy housing this pedal uses.

[Mark Hammer]

Is that one of the Maestro pedals with a sort of corrugated top and the controls are mounted on the each side with numbered wheels?

It may be tricky finding a physical replacement.  Alternatively, it may be as easy as making good use of washers so that a smaller unit can be used.  There is also the matter of the shaft, too.  Those big wheel-type knobs use a slotted shaft, not a solid shaft with a set screw, if memory serves.  So make sure to pay attention to that.

The good news is that state-variable filters of this type have a tremendous amount of tolerance for different pot values.  So, yes, the original is dual 100k, but you could just as easily stick in a dual 250k or 500k and add some fixed resistors in parallel to bring the max resistance down and customize the taper.  In the end, it will be possible to come up with a fix that your ears and hands can tell the difference with.

[RickL]

My phaser from the same series uses set screws. One more consideration is to make sure the shaft is long enough. IIRC you need almost an inch of shaft to mount the wheel securely.

[Muaar]

Cool, thanks for the info. The shaft is kina ling, but I just want it working asap. Is the link i posted in my last post the right thing before I place my order?

[Muaar]

Ok guys, thanks for the help so far but I have another problem now.

I installed that pot and it seems to be working except for a small problem.

Now it seems the "range" of the pot is condensed into a very small area of turn on the pot - i.e. turning the pot only makes a difference in sound in a small part of the "turn".


Is there any advise you guys can give me to fix this?


Thanks! and I can't wait to get this thing working 100%

[Muaar]

The Small Bear part you've linked is a linear pot. It will work electronically but you probably won't like the way it "feels" (all of the control will be concentrated in a small part of the pot's rotation). Small Bear sells an audio 100k dual pot http://www.smallbearelec.com/Detail.bok?no=736

The Mouser page also has a 100k audio dual pot.

You may have to use a pot with solder lugs and join each lug to the pcb with a short length of wire. I won't be completely original but at least it will work.

This seems to be what the problem is.... I thought that the pot I bought wasn't linear?


"24mm Carbon Potentiometers DUAL Audio 100K"


Am I missing something?

[nelson]

The Small Bear part you've linked is a linear pot. It will work electronically but you probably won't like the way it "feels" (all of the control will be concentrated in a small part of the pot's rotation). Small Bear sells an audio 100k dual pot http://www.smallbearelec.com/Detail.bok?no=736

The Mouser page also has a 100k audio dual pot.

You may have to use a pot with solder lugs and join each lug to the pcb with a short length of wire. I won't be completely original but at least it will work.

This seems to be what the problem is.... I thought that the pot I bought wasn't linear?


"24mm Carbon Potentiometers DUAL Audio 100K"


Am I missing something?

Yes, the dual pot in the maestro filter is 100K Rev Log.

With a Log in there the bunching will be even worse than with a Linear pot.

My advise?

Wire the 100K Dual Log Pot Backwards, (swap lugs one and three on each pot) you will get the same range of control. However the settings will be in reverse.

If that bothers you, look for a 100K Dual *Rev* Log Pot.

[Muaar]

The Small Bear part you've linked is a linear pot. It will work electronically but you probably won't like the way it "feels" (all of the control will be concentrated in a small part of the pot's rotation). Small Bear sells an audio 100k dual pot http://www.smallbearelec.com/Detail.bok?no=736

The Mouser page also has a 100k audio dual pot.

You may have to use a pot with solder lugs and join each lug to the pcb with a short length of wire. I won't be completely original but at least it will work.

This seems to be what the problem is.... I thought that the pot I bought wasn't linear?


"24mm Carbon Potentiometers DUAL Audio 100K"


Am I missing something?

Yes, the dual pot in the maestro filter is 100K Rev Log.

With a Log in there the bunching will be even worse than with a Linear pot.

My advise?

Wire the 100K Dual Log Pot Backwards, (swap lugs one and three on each pot) you will get the same range of control. However the settings will be in reverse.

If that bothers you, look for a 100K Dual *Rev* Log Pot.

OK thanks a lot for helping me understand. Any tips on sites to look for the 100k dual rev log pot? smallbear and mouser seem dry =(

\

[nelson]

You could get a larger dual linear pot and use tapering resistors.

http://www.diystompboxes.com/analogalchemy/emh/emh.html

Say, for example you got a 500K linear dual pot.

You would want to put a 10K between lugs 1 and 2 and 2 resistors valuing 140K between lugs 2 and 3 of each pot in the dual pot.

This would give you a close approximation of a rev log pot.