Hi Steve: Can you get the +3V from any of the remaining conductors on the rear of the RP4000? That would make it easy to make a single-ended external test cable.
I have installed a 1/8" jack on the front of the RP4000 and used a 100k isolation resistor on the disc out and it works great here for measurements. Greg N6LDJ > You can key the TX by applying > +3.0 VDC to the TXE lead (disconnect controller FIRST or diode OR).
