Can A Broken LED Short FX?

[dumbassbob]

I have a DOD FX17 Wha/Volume pedal that has a broken LED.I want to use the pedal to control the VCO on a rack mounted Digital Delay It shorts to ground i think.Can haveing the broken wires inside can they short out.You cant get to anything without ripping it apart.I just bought the pedal on ebay,should I send it back.Any help??

[Don Moose]

IF your delay wants a 0-5V control voltage, this thing may work. If it wants a potentiometer, you need to return it - you need something that is strictly a volume pedal - one that requires NO power (a DeArmond or ShoBud volume pedal)

The broken LED may be an indication of some bigger problem. If you're not up for ripping into it to fix, send it back.

[dumbassbob]

Broken LED

IF your delay wants a 0-5V control voltage, this thing may work. If it wants a potentiometer, you need to return it - you need something that is strictly a volume pedal - one that requires NO power (a DeArmond or ShoBud volume pedal)

The broken LED may be an indication of some bigger problem. If you're not up for ripping into it to fix, send it back.

I only want the 0-5v.control voltage.But can 9 volt battery cause a short to DDly,If the wires are broken There is No LED.and you cant see into FX-17 to see if the broken LED wires are exposed

[Mark Hammer]

In DOD FX, the LED is generally operated/driven/illuminated by the same flip-flop circuit that handles the switching. Different specific circuit than Boss, but same general principle. There are all kinds of reasons why the unit might work just fine even though the LED doesn't (or alternatively, all kinds of ways you can fry or otherwise screw up the LED without necessarily affecting the rest of the circuit). CV "expression" pedals, such as what your delay seems to want, are essentially 9v batteries connected to one terminal of a pot, with the other terminal connected to ground in some manner, and the to-be-controlled effect seeing the output from the wiper.

If the delay wants 0-5V DC, then you will want to constrict the maximum voltage at the wiper to 5v. The simplest way to do this is to regulate the 9v battery down to 5v with a 78x05 3-pin regulator. The regulators come in various form factors, power ratings, etc. You'll probably want the one that looks like a TO-92 small signal transistor. Those are good for up to 100ma, which is way more than you'll need to control the delay. The nice thing about them is that they will continue to provide a stable 5v until the DC fed to them hits around 7v. That means you can use the 9v battery and expect predictable control until the battery has aged quite a bit.

Of course, you can also use a trimpot at the "input" side of the foot-controlled pot to adjust maximum output voltage in a passive manner. The problem with that is that the amount you have to adjust will keep changing as the battery ages and its voltage starts to drop. The regulator approach is a little more complicated in terms of the build, but ay less complicated in terms of use and usability.

Just out of curiosity, was it your intent to still be able to use the pedal as a wah-volume pedal, or were you interested in it purely for cannibalistic purposes to control your delay?

[52 Bill]

The DOD FX-17 has no control pot. It works on a variable capacitance principle. It also has a output jack that is designed to supply 0-5v just for the purpose of controlling other things. It will only supply the full voltage when it is powered by an ac adaptor. I don't remember it having an led indicator.

Bill

[Mark Hammer]

I guess you're right ( http://www.diyguitarist.com/DIYStompboxes/FX-17.htm ). Boy, was I way off.

With this bit of insight, my advice is to fix the pedal to do what it does, simply because it's unique, and don't try and adapt it. Paul Marossy has a schematic that he drew for it - http://www.diyguitarist.com/Schematics/FX17-Schem.pdf - indicating that the 0-5v is an output rather than input jack, so if it works it should be able to supple a 0-5v output.

Ofr course, what we still don't know at this point is what the delay unit needs to see. As Don correctly notes, sometimes all they want is a variable resistance to ground. My old blue MXR rackmount delay just wants a 50k variable resistance to ground.