DOD FX25: questions about the circuit layout & theory

[ovnifx]

I'd like to make a specific mod to an FX25, but honestly I don't quite understand how the circuit works yet.  There are some schematics at this page:
http://topopiccione.atspace.com/PJ11DODfx25.html
but none of them seem to match the pedal I have exactly.  If I understand those schemas and commentary correctly, it should have two IC's: an op amp and a dual OTA.  Mine has four IC's: two single OTA's, an op amp, and a HEF4007UBP "dual complimentary pair and invertor".



So: Could anybody point me to the schematic that most closely resembles this board, and let me know any obvious substitutions?  Aside from the very helpful commentary at topopiccione, are there any other good explanations of the working of this circuit that you can point me to?

And, most particularly- what I want to do is add a jack for an expression pedal to sweep the filter open and closed, in addition to the envelope sensor.  My expression pedal has a 100K sweep, FWIW.  What part of the circuit should I be looking at?  I figure it may not be as simple as just adding on a couple of wires and a jack- what trouble can I expect?

Thanks!

[George Giblet]

Your circuit is the older model which has two CA3094 OTA chips instead of the Single LM13600 dual OTA chip (which I believe is at least the F revision of the FX25 board).  The translation of one to the other is fairly straight forward, at least in principle.  Take a look at the Small Stone Phaser on the GGG site,

http://www.generalguitargadgets.com/index.php?option=content&task=view&id=109&Itemid=139

Here you will find a CA3094 version and a LM13600 version you should be able to see how the circuit maps over to the LM13600 - if you dig up a LM13600 data sheet and a CA3094 data sheet you can see inside the chips and get a better understanding of the equivalent pins in each case.

There could be a few other small changes - I can't comment on those without tracing the circuit.

As far as enpression pedal goes you would want to drive a control voltage into the junction of R7 and R9, or set-up a variable resistance from that point to Vcc.  To do that you will need to disconnect the envelope circuit from D5, R30 and C11.  Whether you keep R30 or C11 depends on how you do it, any experiments would be best done without C11.  So,
- method 1: break R7+R9 from C11 etc, connect a 1MEG pot to VCC.
- method 2: break R7+R9 from C11 etc, connect a pot (something like 10k) with wiper to R7+R9, one terminal to gnd and the other to Vcc. You might need a small resistance from the gnd side to the pot to prevent dead spots in the control.
Also might will need to play around with the law of the pot (Lin, Log, Reverse log etc) to get the expression pedal to have the right feel so the effect isn't bunched-up at one end.

[Mark Hammer]

Not an especially noteworthy insight, but the CD4007 is there as part of the solid-state switching circuitry.  You can see the entire circuitry here: http://hammer.ampage.org/files/dodswitch.gif

[ovnifx]

Awesome, both very helpful replies.  Thanks!!!