Update: The community consensus on this mod can be found below.
I have a fairly nice looking Drake TR-4 transceiver on which, at some point in its life, someone hung a mystery circuit off the PTO. I don't know what the mystery circuit is or what it's supposed to do, but it doesn't seem to interfere with operation. I'm putting this page up in the hopes that someone else will know what it does.
Figure 1. The mystery circuit. Speculated values for trimmer capacitors and ? symbols are in the text.
Figure 1 shows the schematic of the mystery circuit as I understand it. The two variable capacitors are labeled "302M", which I believe means they are 15-120pF. The two ? symbols are glass packages banded Black-Orange-Yellow-(gap)-Brown, black band toward chassis. I believe this makes them 2N34A diodes, cathode to ground. The two "halves" of the circuit are labeled "20" on top of the trimmer on the left, and "80" on top of the trimmer on the right. Four to five inches of wire separate the left and right halves on the chassis, with the left half being crammed up in front of the filter can and the right half mounted just forward of T11. Figure 2 shows placement on the chassis.
The two halves of the circuit are shown in more detail in Figure 3. The top image in this figure is the circuit on the left half of the schematic, located at the front of the radio. The bottom image is the circuit in the right half of the schematic, located just in front of T11 toward the middle of the radio chassis front-to-back.
The final piece of this puzzle is connection to the PTO. I removed the shield around the PTO, and the blue wire connecting to the PTO connects to a jumper inside the PTO on the left side (viewed from the rear) connecting the vertical phenol with the PCB underneath the PTO. Figure 4 details this connection.
So ... any suggestions?
A Plausible Explanation
Community consensus on this mod is that it is a RTTY modification, either incorrectly assembled or incorrectly captured by me. It bears close resemblance to a circuit documented by Drake in some of their manuals (although it does not appear in my TR-4 manual), and essentially uses diode conduction to switch in or out (relatively) small capacitance values in the PTO circuit to shift the oscillator by the RTTY mark-to-space difference.
There are two theories on the two halves of the circuit: One is that they represent the two popular RTTY mark-to-space shifts, 170 Hz and 850 Hz. The other is that they correct for the TR-4 (and TR-3) peculiarity that 20 meters tunes "backward" -- allowing a reverse-shift with the same keying logic so that your RTTY signal isn't upside-down on 20m. Without carefully computing or measuring the circuit, I am inclined to side with the latter viewpoint, simply because of the "80" and "20" markings on the trimmer capacitors (see Figure 1).
I think we can call this mystery solved! Thanks to everyone who offered insight on this topic, and in particular the members of the drakelist mailing list.