Thorn -- On the Buffer schematic, the "To Other Channel" designation does in fact refer to connecting to the 1Meg resistor in the other channel that connects to pin 5 of the IC. I drew the schematic this way because only one channel of the buffer is shown, yet the biasing rail developed by the two 47K resistors and 10 uF cap serve both channels -- ergo, the To Other Channel designation. As Joe explained, for an op amp to operate linearly in this application, its output voltage need to be able to swing both positively and negatively relative to ground. Normally, that then requires a positive and negative power supply (relative to ground) to power the op amp. But in the Fisher receivers, there is only (conveniently) a negative low voltage source available. Therefore, to satisfy the power requirements for the op-amps, they are powered by only the negative source, but then a voltage divider (made up of the two resistors and cap mentioned) is used to bias the normal ground reference of the op amps to a point midway between ground, and the value of the negative supply. That reference voltage is the -7.5 volt designation you see on the schematic, being on half of the -15 volt level applied to the op amps at pin 4. Finally, because the input and output of the op amps both operate at a DC value below ground level, both input and output coupling caps are required so that the input bias voltage, nor the resulting DC output voltage is shorted to ground, and the DC levels are isolated from the input and output audio signals.
As for the missing 220K resistor at the output of the buffer circuit on Joe's schematic, that resistor should in fact appear in ALL installations of the buffer (and on any schematics the buffer circuit is applied to as well). Without it, there is no ground reference for the other side of the .22 uF output coupling cap from the buffer IC. When the new Preamp Output and Power Amp Input Jacks are strapped together as they were when they operated as Reverb connecting jacks, there is no problem with this resistor missing from the circuit, as the input 1Meg resistor in the revised power amp design will perform this function in this case -- assuming you have also done the power amplifier driver stage modifications as shown in the modified 400 power amplifier schematic! If you did not do these modifications and simply installed the buffer circuit in your 500C -- leaving the driver circuit of the stock Fisher design in place, then NO ground return exists for the input AF Amplifier stage of the power amplifier section. The result will be very significant distortion!
For those who DID implement the power amplifier driver stage modifications, the 220K is still required -- becoming important when the jack straps are removed to allow the receiver to operate as a separate tuner/preamp and power amplifier (or both) for use with other pieces of equipment. In this case, without the 220K resistor installed, there will be a significant "pop" in the speakers when a different power amp is connected to the preamp output jacks, created from the charging of the .22 uF output coupling cap when the external power amp is connected (assuming it is powered on). But also, even if the 220K was installed, if the power amplifier driver stage modifications were not performed, then the input AF Amplifier stage will loose its reference to ground when the jack straps are removed, again, potentially causing distortion in the power amplifier section (if a return is not provided in the preceding equipment), and harm to the 12AX7 driver tube as well.
I hope this helps!
Dave
Edited to reflect the comments in my following post.