Fix the Screen Resistor

Initial Conditions

With a test lead connecting the center tap of the screen resistor to a sort of burnt looking wire that obviously used to be connected, measured screen voltage of around 40 volts.

Disconnected the test lead and measured no load voltage on the screen divider tap of around 200 volts (should be 100 under load). The DC resistance to ground on the burnt looking wire that used to be connected to the tap was about 870 ohms.

Initial Debug

Something is drawing down the voltage so resistance checking is in order.

Prior to doing anything else, I decided to re-do the entire 15 tube resistance-to-ground testing since my breakout box had a short and the output transformer was missing the last time I did it. Made sure the radio is configured like this.

Tube location picture.

I reviewed the screen distribution circuit on the schematics for both old and new radios.

I copied the expected resistances to ground from the SX-28A docs.

I copied the relevant values from the tube measurements.

'Guess' is based on schematic resistors

			TUBE/PIN    EXPECTED    R TO GND    GUESS    R TO SCREEN     V
			V7 pin 6     10,000       974        0          .2

			V6 pin 6     12,000      2,110     1,000       1,140
			
			V5 pin 4     12,000       166      1,000        810
			
			V3 pin 4     400,000     2,150 

			V2 pin 6     12,000      2,140

First guess is C27 (.02 400 V) on V5 is partially shorted:

Clipped it on one end and measured: 200ish ohms! Don't recall ever seeing a capacitor turn into a resistor. Must be Gremlins. After cap replacement:

			TUBE/PIN    EXPECTED    R TO GND    GUESS    R TO SCREEN     V

			V5 pin 4     12,000      10,780     1,000       ??

Page: /5th_radio_resurrection/Screen_voltage/Screen_voltage.shtml

Last modified: Thursday, 23 Apr 2026