12V Safety and Control

My BIAS transformer was freshly replaced - in addition to the negative tube-bias voltage, this transformer also provides 12VAC for safety, and control. To ensure trouble-free operation in this 12V safety and control, I did not initally hook up the BIAS transformer's 12VAC secondary, but rather I selectively applied a current-limited bench-supply.

My test strategy:

• Use the bench-supply to inject 12V upwind of CR206, to verify wiring K203 through K101 Antenna Change-Over Relay, to the rear-panel T/R RCA-jack.
• Inject 12V downwind of CR209, and selectively bypass S205 Lower Door Safety Switch, and the 3-min timer K202, and see if pressing S203 "ON" switch will engage relay K203
• Inject 12V where the transformer will connect, bringing up all lights, allowing HV Relay to be enaged with front-panel "ON", and allowing the rear T/R jack to switch the antenna once the HV Relay has switched. The whole 12V enchilada :-)

• 

  Good! Injecting 12V onto CR206 anode allows K101 Antenna Change Over Relay to be controlled by the rear panel. Maximum current-draw is 0.31A with K101 and K205 engaged.

• 

  More Good! Injecting 12V onto C211b let me try the "ON" button, and watch K203 engage! Maximum current-draw with K203 engaged was 0.21A.

Then Badness :-( Applying 12V to the terminal-strip at the back of the Relay Shelf, where T203 12V winding will eventually connect, caused over-current (3 amps!) trip. Clearly, something was unhappy. At this point, I just introduced about 24" of wire, and the panel-lights. I hadn't planned to pull the front-panel yet, but now I had to. I found panel-light connections shorting - solidly.

APPARENTLY my 30S-1 is missing some black sleeving on the dial-sockets, which led to the full-shorting of the 12V transformer. Looking also at the broken/repaired LOAD dial, I wonder if a previous-owner tried to sneak the bulb out from behind the dial, breaking it in the process and also losing the essential sleeve insulator :-O Did abundant grief follow a simple bulb-change?!?!?!

• 

  Behind the front panel, I found shorted socket-connections, and (behind the dials) contact between the clip-socket-contacts and the chassis! I must say, I never expected a deadly-serious problem in such a simple area as a light-bulb :-O

• 

  I re-soldered all light-connections, adding heatshrink sleeves over every terminal. I added large tubing over the entire socket for the two dial-bulbs. I sleeved the interconnecting wires to prevent wire-chafing. I added judicious bits of electrical-tape to prevent bracket-contact.

SUCCESS! Everything 12V-related now works with the bench-supply. The Safety switches are all doing their thing (un-bypassing opens K203 nicely), the red "Amp Ready" light works, the front-panel "ON" and "OFF" push-buttons do their thing, the rear T/R works. Even the new lighting mod is nice: just the outer meter-lights turn on until the Safety interlock + timer is completed, then the dial-lamps turn on to let you know you're good-to-go!

I was interested to see that the TOTAL 12V current-draw, with all lights and relays energized, is 0.75A. My new Peter Dahl transformer is rated 0.800A Continuous Commercial Service - a nice fit.

It is entirely possible that the light-bulb short I discovered was the cause of my T203 smoke-show.... I'll never know, for sure, because there seems to be several plausible causes :-O

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CR216 and BIAS Multi-Metering