Tips for fixing a DOD FX25 Envelope Filter?

[Hal]

I recently got an fx25 for free, the one in the green case with with 2 knobs, and I think there are 2 problems with it.  The first is the switch is broken - does anyone know where I can find an replacement that would fit?  Its a large NO flat tactile switch...hard to explain if you don't have one/never seen it, but I could take a picture if that would help.  

The second problem may not be a problem.  The knob on the right, sensitivity, seems to be more like a volume control than anything else.  Still, the effect seems to be working pretty well.  Fairly subtle, but sounds good.  Is this normal?

Thanks, everyone :-D

[Mark Hammer]

That's weird behaviour.

The sensitivity control is not part of the audio path.  All it does is set the gain of the envelope follower stage so that softer picking yields more sweep.  On the other hand, the range control adjusts the Q of the two OTAs in the LM13600-based filter, and that WILL tend to increase apparent volume.

[toneman]

first tip is 2 get a schemo....
T

[william]

I have a small site for it here.

http://216.250.178.122/will/circuit/fx25/index.html

The schematic has one or two tiny errors, the buffers on the LM13600 are displayed the wrong way.  I have an update to the schem to post soon.

[Hal]

william - thats an excellent place to start.  However, I have the revision G board, and it seems like you're working from the F.  

To explain what the sensitivity control does more in depth, the effect seems to be more of a volume swell than a wah sweep.  When I put the sensitivity up, the volume swells higher, and it _seems_ like a volume control.

Now I know on most wah circuits there's a capacitor that you can lift to turn it into a volume pedal - maybe this went open on my unit?  However, this is far from a conventional wah with envelope follower - there's no inductor on the board.  Any clue which cap this would be?

[Hal]

looking more closely at your web page, I see that my board is _almost_ identical to yours - the only differance I can see is that C10 is rotated 90 degrees.  However, my part values are drastically different from those you listed...
my C11 is 15 uf, yours is 1 uf, and they're in the same place on the picture...and same number.  

All my solder joints look origional, though, except for the ones around the power supply input - filter cap and polarity diode look like they were screwed wtih.

Too bad I'm useless at understanding that schematic

[Hal]

it magically fixed itself :-D!!!

I really like this little thing now.  I wonder if it'll stay fixed, or if it'll break again...