[HAM] volume pedal mod for chopped D

[Dave Bishop]

>I don't have a schematic for therheostat box  . . . .

I'll send you a schematic privately.

>Also, I am not sure aboutthe shielded cable being grounded on one side. Do 
>I need a 3 conductorjack/cable( XLR type) or 2 conductor guitar type 
>jack/cable? Do I tap the hotlead off of the .25 mfd cap side that is 
>connected on the top terminal strip orbottom terminal strip? Where exactly 
>do I tap for the shield and where does itterminate? Do I plug a 2 conductor 
>guitar type cord into the output of the volumepedal? Also would a volume 
>pedal with on one jack work for this application?

A mic cable was my suggestion.  "XLR" cable has two conductors and a shield 
that sometimes serves as a ground wire.  The challenge you face is that the 
signal from the matching transformer is weak and high impedance, so it needs 
strict shielding against EMI/RFI.  In my imagination the signal will now 
come from the matching transformer through the cap, down conductor A, 
through the pedal's pot, and back up through conductor B to the pre-amp 
chassis ground.  The shield is soldered to the chassis ground at the organ 
end, but left floating on the pedal side so that it doesn't form a ground 
loop.  I would hard wire into the pedal so that you can maintain the shield 
as close to the pot lugs as possible.  Its easy to wrap the cable around the 
pedal when transporting.  An XLR connector at the organ side makes sense, 
but keep the shielding all the way back to the cap.  I'm suggesting that you 
trace the signal from the matching transformer to the junction of (1) the 
wire going to the tremulant control and (2) the .25 mfd cap.  You can see 
that if the rheostat was completely removed, signal would still go unimpeded 
through the tremulant control to the preamp.  For a remote volume control, 
I'm proposing that you let the signal go through the cap but then divert it 
at the other side.  I would also locate the other connection(s) from the 
rheostat to chassis ground and clip it so that the array doesn't become an 
unwelcome antenna.

>So basically I can put a 250K resistor( or one of the sameresistance of the 
>pedal I use) in parallel with the pedal in parallel with the terminals of 
>the .25mfd capacitor in the rheostat box?

This part I haven't really looked at.  Obviously, Hammond created a 32 
resistor array for a purpose (smooth expression with no crackling being one 
reason).  You'll have to do some measuring and experimenting to find the 
right combination of resistance values to perform the same function with a 
single pot. Perhaps you'll need to add some fixed resistance across the  pot 
to get the pedal throw to match up with the volume change correctly.

If you have a chorus/vibrato on your model D isn't it actually a "DV"?  With 
that mod the connection to the rheostat may have changed, but it won't be on 
the schematic.  In the BV series the mixing of vibrato and straight signal 
is done in the pre-amp past the rheostat tap.

Regards,

Dave Bishop