Screen Supply Overhaul - Revision history

Gordonp at 21:55, 13 August 2020

2020-08-13T21:55:33Z

← Older revision		Revision as of 21:55, 13 August 2020
Line 28:		Line 28:

	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.		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.
	[[File:upratedC204.jpeg\|500px\|thumb\|center]]		[[File:upratedC204.jpeg\|500px\|thumb\|center\|Up-rated replacement for C204]]

Gordonp at 21:53, 13 August 2020

2020-08-13T21:53:29Z

← Older revision		Revision as of 21:53, 13 August 2020
Line 25:		Line 25:
	</p>		</p>
	<br clear=all>		<br clear=all>


			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.
			[[File:upratedC204.jpeg\|500px\|thumb\|center]]

Gordonp at 20:49, 23 July 2020

2020-07-23T20:49:07Z

← Older revision		Revision as of 20:49, 23 July 2020
Line 25:		Line 25:
	</p>		</p>
	<br clear=all>		<br clear=all>


			[[Repair\|Back to Repair]]

			[[12V Safety and Control\|On to 12V Safety and Control]]

Gordonp at 20:46, 23 July 2020

2020-07-23T20:46:46Z

← Older revision		Revision as of 20:46, 23 July 2020
Line 18:		Line 18:
	<p>		<p>
	<span>		<span>
	[[File:Screen_HV_resistor_check.jpg\|500px\|thumb\|~~right~~]]		[[File:Screen_HV_resistor_check.jpg\|500px\|thumb\|left\|Measured resistance values noted]]
	Measured resistance values noted

	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.		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.

Gordonp at 20:46, 23 July 2020

2020-07-23T20:46:04Z

← Older revision		Revision as of 20:46, 23 July 2020
Line 15:		Line 15:
	File:Screen_c204_repaired.jpg\|Replaced C204		File:Screen_c204_repaired.jpg\|Replaced C204
	</gallery>		</gallery>

			<p>
			<span>
			[[File:Screen_HV_resistor_check.jpg\|500px\|thumb\|right]]
			Measured resistance values noted

			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.
			</span>
			</p>
			<br clear=all>

Gordonp: Created page with "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 t..."

2020-07-23T20:42:18Z

Created page with "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 t..."

New page

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...

<gallery caption="Screen-Supply rectifier, Before and After" widths="400px" heights="400px" perrow="2">
File:Burned_screen_rect.jpg|Burned Chassis and Screen rectifier board, as it came to me
File:Repaired_screen_rect.jpg|New Bridge-Diode module for screen-rectifier, rated 1.2kV 35A
</gallery>

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.

<gallery caption="Screen-Supply Filter Capacitor C204" widths="400px" heights="400px" perrow="2">
File:Screen_c204_orig.jpg|Possibly compromised, original C204. Date-code of (19)63 (week)21
File:Screen_c204_repaired.jpg|Replaced C204
</gallery>