Screen Supply Overhaul
Much (much!) earlier, when I removed the Relay Shelf, I caught a glimpse of a dark smudge in the shadows near the front-fuses. But I didn't dwell on that - I was focussed on the BIAS area... Now, I decided to look deeper. I was suspicious when I looked closely; I was aghast when I removed the screen rectifier board - there were obvious signs of combustion, and weird melted-looking goop on the rectifier board. The rectifier diodes had been replaced at some point in the past, and the equalizing-capacitors C251-C258 were entirely gone. I rolled up my sleeves...
- Screen-Supply rectifier, Before and After
Burned Chassis and Screen rectifier board, as it came to me
New Bridge-Diode module for screen-rectifier, rated 1.2kV 35A
I discarded Collins' rectifier board, then cleaned the chassis. I salvaged the 0.01uF / 1.6kV C221 ceramic cap, and re-used it with... a modern rectifier module. This module is rated at 1.2kV 35A which ought to do the job nicely! What's more, it's mechanically robust, and compact. If there is a screen-fault, I believe the 1A fuse F205 will NOW do it's job, and save all the downstream components (rather than the components blowing up to save the fuse :-O ).
While I was working on the screen-supply, I looked at the filter-capacitor C204. The original was rated at 40uF / 350V, so a modern 47uF / 400V @85degC seemed like a gentle improvement. When I looked closely at C204, my decision to replace it was confirmed: The original can was dimpled out-of-round by the mounting clip; time and heat had also taken it's toll and the mounting-clip had dug through the insulating sleeve.
- Screen-Supply Filter Capacitor C204
Possibly compromised, original C204. Date-code of (19)63 (week)21
Replaced C204
While I was in the neighbourhood (the back panel), I felt it would be a good idea to check the Screen bleeder-resistor R203, the High-Volage bleeders (R205, R206 and R207), along with the meter's HV divider-resistors (R229, R210-R214. All check "OK", and there appears no HV or Screen shorts.
Later, raising the Screen Supply meant my new C204 voltage-rating was marginal... Here, I did a mod to a mod ;-) 500V / 105C rating are improvments over the 450V/85C new capacitor I recently installed.
