Saturday, June 2, 2012

Replaced Prophet 2002 data potentiometer.

This is so stupid...
Whyyyyy!? Sequential Circuits Inc...
They like pissing people off by using potentiometers that are ever so slightly smaller than your standard off-the-shelf potentiometers.
They're called R-235 and seem to be linear 10k potentiometers.
Here are the measurements I got (using a caliper) from the one I removed.

Find a replacement?

The parameter input potentiometer in my 2002 had a broken shaft and missing knob, as you can see in the photo above.

Of course they're not available any longer...
I emailed and they said they had 3 "refurbished" potentiometers for sale...
for $15...
plus $17 shipping cost..

I paid about $140 for my Prophet 2002plus.
I find it hard to motivate spending $32 (+import tax if I'm "lucky") to replace a potentiometer on it.

I dug through my box of potentiometers at home and tried other similar "standard" potentiometers, but they're about 1mm too wide (body diameter).
"Now why is that so important?" you may ask. Well, SCI decided to make little holes in the PCB for the front panel and let the potentiometers sit inside those holes, and of course, they are juuust big enough to let the potentiometer fit. They couldn't have made them 1mm wider, or used standard size potentiometers.

Why?!?!?!?! During the products lifetime, I don't think a lot of potentiometers get worn out.
It's when they're old, like now, that people will want to start replacing them, but they can't.
I hate it. I hate it. I hate it. I hate it. I hate it. I hate it. I hate it. I hate it. I hate it. I hate it. I hate it. I hate it

Did I mention I hate it?


I kept looking in my box of potentiometers and found some strange potentiometers with flat square body.
They happened to be 10k linear as well.
It turned out that they were both small enough to fit in the hole, AND flat enough to be able to be mounted between the PCB and front panel. So they didn't need to sit in the hole at all, even if they fit. Funny...

Of course the pins didn't match the ones on the 2002 so I had to extend them a bit.
Finally, I got the replacement in and .. well.. here's what it looks like:

As you can see, the pot had tabs which I bent for support against the PCB.
You see the hole under the potentiometer. The potentiometer below show that there really is no margin for a potentiometer that is bigger, even if it's just a millimeter. I tried.
Also, the sharp-eyed reader will notice the regular pot below seems to have a bad solder joint on one pin.
That's because I desoldered it to check the pot value :) I even remembered to resolder it before reassembling the thing!

A problem is that the shaft is longer and the SCI knob doesn't fit. It seems the shaft has a slightly different shape. So there's a difference between D and D... somehow. The D is "fuller" on a regular pot but on the SCI one it's more of a half-circle.
Also, it's backwards... The flat edge of the shaft-D is pointing in the opposite direction on my replacement pot compared to the original SCI one. So, I can't really use the original pots anyway.
I assume all pots are like mine and the SCI one is backwards... Another reason to do be annoyed...

Of course, more problems

After reassembly, the display was flickering and the whole thing went nuts.
I was wondering if there was something magical about those R-235 potentiometers that caused it to work fine with them but not with a regular pot. Stupid thought, but all kinds of thoughts fly through my head when I can't understand why something suddenly stops working :)

In the end it turned out to be a bad solder joint on the crappy "connector" for the cable that connects the panel board to the CPU board.
I write "connector" because it's really just a cheap DIP-socket...

Look at pin2. Looks like the pad is missing.

After putting a massive blob of solder on it, I got it to work properly again so I put it back together.
Finally, I noticed that the data potentiometer doesn't seem to work perfectly for some reason.
I don't know why, but the parameters don't go "all the way" when you turn it.
At minimum setting it goes to 0 if that's the minumum.
At maximum setting however, it just goes to 113 or so if the max value is 127.
(I don't remember the exact numbers but a small top segment of the range is not available).
I need to use the inc/dec buttons to step to the value instead.
Perhaps it's a calibration issue. I don't know. I don't care.

Some other time...

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