The saga continues!
This weekend I had some time to do some more poking around and found that R3 (the pot that controls the heater voltage to the RF osc tube) was burned to a crisp from one of the end terminals to roughly about halfway to the other terminal. I also found that the wire going from point 11 to the heater of the RF oscillator tube had been disconnected completely. The new wire (which I originally and erroneously thought was just replacing this disconnected wire) actually turned out not to originate from point 11 at all. The wires in the piece of equipment are all gathered and tied in a bundle making it very difficult to trace where each wire goes. I had to remove the entire IF oscillator section to try and trace out the various wires. Anyway, the new wire going to the heater of the RF oscillator tube actually was coming from the heater supply to the 6J6 IF oscillator tube (terminal number 3 on the schematic). So, it looks like whoever worked on it in the past completely bypassed the selenium rectified, regulated DC supply to the RF osc heater and instead used one of the 6.3 V windings on the power transformer to supply 6.3 V AC to the RF oscillator heater. I'm sure this was prompted by the burned out pot R3.
The one thing that I am still confused about is how this modification even works (I know it works because the RF, IF and audio oscillators all work). Looking at the schematic, one end of the heater of the RF osc tube is grounded. The 6.3 V windings on the power transformer are not. So, where is the return path for the RF tube heater current via chassis ground? The only possible places it could return is via the grounded part of the Selenium rectifier and the negative terminals of the filter caps C3A and C3B. Is that possible?
Also, what would cause R3 to burn out like that? I am trying to restore this instrument to original specifications, but would like to avoid the replacement R3 suffering the same fate.
Here is the link to the schematic again
http://bama.edebris.com/manuals/motorola/t1034a
