SUPPLY Unit ZA/CAN 4772
Slow and steady still with this project, focusing primarily on ensuring the Supply Unit is in good order before going back to the Sender.
I have now confirmed the oddball reading of just over 70 Ohms I got on one of the Supply Unit Resistance tests relates to a pair of 300 Ohm 1/2-Watt resistors set up in two parallel feeds in the Vibrator circuit for the Receiver Supply. This parallel arrangement of the two 300 Ohm resistors should produce a resistance value of exactly one half the rated value of either resistor, or 150 Ohms. So one, or both, of those two resistors has dropped significantly off specifications. Confirming test yet to be done.
I spent a chunk of yesterday afternoon searching the web for a supply of 300 Ohm, 1/2-Watt Carbon Composite resistors, which proved a bit of a challenge. Current standards now bracket that Ohms value by about 20 Ohms either side. I did, however, eventually find a supplier with 300 Ohm 1-Watt carbon composites which will work nicely, so will be ordering 10 for the project. That will give me a chance to match a pair for this fix and leave me spares for the rest of the project, if needed. The two in the Supply Unit are R13D and E, so at least three others are in the 52-Set in other locations.
When I had reinstalled the Supply Unit in the Carriers No. 4 to allow for the Isolation Relay to be tested in the Sender, I had the meter set up to monitor the Sender data. Last evening, I fired up the Receiver to see how the signal from WWV at 10.0 MC was behaving, so I had switched the meter back to monitor the Receiver. I was very surprised, and pleased, to see the LT was reading a rock steady 11.25 Volts DC and the HT a rock steady 155 Volts DC. Checked again this morning and same results. They are no longer wandering all over the dial, as low as 10 Volts LT and 120 Volts HT. What I find odd is I have yet to do any electronic adjustments or repairs to the Supply Unit that may have caused this to happen. I still intend to more closely examine the Connector assembly between all five 8-Pin Connectors across the set, but another related possibility is oxide buildup on all those contacts, so I shall add cleaning all the related sockets and plugs with Deoxit just to be sure.
I have also noted that when the time comes for hot testing the Sender, it needs to be done at High Power. I am glad I picked up the extra CPP-2 Power Supply from Brian Asbury last year. It is soon to come in real handy.
David
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