To pick this thread back up I am looking at early color Zenith sets and they also use VDRs and thermistors. The degaussing circuit uses a pair and some sets have a VDR in the yoke.
To start somewhere I am interested to understand the the function of the VDR in the AGC Keying circuit and how to test.
- 29JC20 chassis AGC Keying control grid voltage used a 3meg fringe lock pot feeding from the low voltage source
- 26KC20 through 25MC36 chassis used VDR #63-4906 in place of a pot
- 24MC32 and several later chassis used VDR #63-5494, while the part number changed the circuit looks the same
Looking at the snippet from the 25MC30 chassis the 105v source should be stable making it easy to measure the voltage drop and current through the VDR (assuming I have a good VDR). This info could then be used to set up a test circuit for trying substitutions. I will assume with voltage applied it comes in somewhere around the same value as the middle of the pot used in the earlier circuit. How much can this low voltage supply vary, enough to warrant using a VDR I guess.
