Issue with a vintage Maestro phaser - should it be an easy fix?

[Delicieuxz]

I'm not sure if this is the proper place to be asking about this. If there's a better place, please let me know.

I recently bought a used Maestro MP-1 phaser. When I got it, I found that the Balls / intensity control doesn't work. The wheel for it also spins very resistance-free, compared to the Speed control.

I can't contact the seller about it due to the platform it was purchased through, and the fact that they don't have any listings currently up which would enable me to message them.

It seems to me like a broken control might be an easy fix, but I'm not very versed in these things. I've only built a bunch of pedals from kits with detailed instructions, and don't have engineering knowledge to diagnose thing other than by noticing a physical break or burn or busted cap somewhere.

[theehman]

The wheel is held in place by a set screw adjustable with an Allen wrench.  Have you checked that it's tight?

[Delicieuxz]

Thanks for the suggestion. It turns-out that was part of the problem. I took the wheel off and I was able to make some intensity changes with a screwdriver into the potentiometer's split-shaft.

While doing that, I discovered another part of the problem. Starting from the lowest setting, the first half-turn of the potentiometer makes no difference at all. Then after the half-turn point, there's an audible click heard through my amp's speakers and then there's and adjustable difference for the last 1/4-turn of the potentiometer.

[Marcos - Munky]

Do you think the range from minimum and maximum intensity is ok? What you described seems like a potentiometer with the wrong kind of taper. You know, when you expect the adjustment to be smooth along the potentiometer rotation but it is really crammed on just a small part of the rotation.

[theehman]

I'm wondering if the resistive track is broken or if the tab to limit the shaft rotation has broken off
  With the wheel off, does the pot turn a full 360 degrees?

[Delicieuxz]

Do you think the range from minimum and maximum intensity is ok? What you described seems like a potentiometer with the wrong kind of taper. You know, when you expect the adjustment to be smooth along the potentiometer rotation but it is really crammed on just a small part of the rotation.

It might be the full range, but I'm not sure. The potentiometer makes a pop sound sound when it passes that 1/2-turn threshold and start making a difference.

I'm wondering if the resistive track is broken or if the tab to limit the shaft rotation has broken off
  With the wheel off, does the pot turn a full 360 degrees?

The potentiometer turns about 3/4 of the way around, which should correspond to 0 - 9 on the knob.


Here's the 1/2-turn pop issue.
https://youtu.be/CkQzrUC4T0Q


The range of the MP-1 at maximum Balls, lowest Speed.
https://youtu.be/9gdExXEhZXE

[iainpunk]

welcome to the forum

i guess that its a broken potentiometer track, it should be a 500K LOG potentiometer, good luck finding that specific style of pot.

cheers, Iain

[Delicieuxz]

welcome to the forum

i guess that its a broken potentiometer track, it should be a 500K LOG potentiometer, good luck finding that specific style of pot.

cheers, Iain

Thanks.

The parts-list in the manual describes it as "Resistor, Rotary, Reverse, Audio, 1 Megohm"

I found this other thread where someone is looking for one for a Maestro phaser that uses the same pot:
https://music-electronics-forum.com/forum/effectification/guitar-effects/52010-

And some more resources on the topic:
http://www.ilovefuzz.com/viewtopic.php?f=151&t=46996

[Delicieuxz]

Should this pot work as a replacement?

https://www.ebay.com/itm/Alpha-1-Meg-1000k-Reverse-Audio-Pot-Potentiometer-CCV-Cameron-mod-1-5-tolerance/324293523286

[theehman]

You really need a full sized pot. 

[Delicieuxz]

I might have been fortunate and found a suitable replacement.

According to this thread, the regular Alpha 1Meg reverse audio taper pot's part-number is 313-1540F-1M
https://www.talkbass.com/threads/1-meg-reverse-audio-potentiometer.609219/

Mouser carries that part:
https://www.mouser.ca/ProductDetail/Alpha-Taiwan/RV16AF-20-15S1-C1M-LA?qs=8%252br4Hz5Xir86CJNGaAj%2Fnw==

The stock image looks like it has a split-shaft thread, but the product specs say it has a smooth/slotted shaft, which is what the Maestro MP-1 has.

The mouser specs say it has a 6mm-diameter shaft, and the original on the MP-1 is also 6mm in diameter.

I just don't see a confirmation on the Mouser page saying that it's reverse audio, rather than just audio. Is there a way to tell from the Mouser product details? The RV at the front of the product part-number, does that mean it's reverse?

You really need a full sized pot.

The Mouser spec says the shaft is 15mm long, which is the same I measure the original as being. Is there anything else I should be aware of? If it fits into the chassis hole, the shaft should be the right length and diameter for the knob. If the leads don't fit the circuit board, I could connect them to the mounting holes with wires.

[Marcos - Munky]

The potentiometer is a 1M reverse audio (log) taper. If you can't really get a reverse audio, just get a regular 1M log pot and wire it in backwards. It will work, but with the max setting on min pot rotation and with min setting on max pot rotation. Anyway.

What really matters for the shaft spects are the length and the original knob. The length matches? Good. Now check for the knob, does it have a set screw to fix it? We already know the answer is yes, so it'll fit any shaft as long as the shaft is long enough for the screw to be fixed.

If you get a split shaft, you have to "fill" the space between each half of the shaft, or else one of them will bend when you turn the screw and this bent half may break. Just get something that's hard enough but at the same time easy enough to be shaped, like a wooden toothpick. Cut a piece and shape it so it fits tightly on the space between each half of the shaft.

[PRR]

...I just don't see a confirmation on the Mouser page saying that it's reverse audio, rather than just audio.

https://www.mouser.ca/new/taiwan-alpha/taiwanalpha-rv16af-potentiometers/
RV16AF 16mm Reverse Audio Potentiometers