SX-28 Screen Voltage

[W7SOE]

The screen voltage in the SX-28 comes from a resistor divider, R30 and R31, that divide 280V.

The schematics shows the resistors in series, with R31 to ground.  The BOM lists R31 as 11k and R32 as 4k.  I see this as providing 205V, not the 100V the schematic notes.

What am I missing?

Thanks

Rich

[KA2DZT]

Have you taking into account the screen current that the screens draw??

[WBear2GCR]

Which tube?

Which model radio?
This is not so simple or straight forward.
Check the online sites that have detailed model info.
The schematic you have may or may not correspond.

You probably know this is one: http://www.radioblvd.com/SX28Notes.html

[DMOD]

The screen voltage in the SX-28 comes from a resistor divider, R30 and R31, that divide 280V.

The schematics shows the resistors in series, with R31 to ground.  The BOM lists R31 as 11k and R32 as 4k.  I see this as providing 205V, not the 100V the schematic notes.

What am I missing?

Thanks

Rich

Yep, as DZT stated, you are not considering the sum of the screen currents that this divider supplies.

An isolated divider would show 205 volts at the center, but the divider is not isolated. Extra current is flowing from the 280V supply through R32 to the screens. There is a total of 45 mA flowing through R32.

Phil - AC0OB

[W7SOE]

Thanks for the replies.

So I am guessing that the 100V is some sort of "nominal value".  I am measuring with the divider connected and all the tubes installed....

Rich

[KA2DZT]

Signal level will also affect screen current.  Screen voltage shouldn't be too far off from the 100 volts.  You may have to recheck everything you're doing including looking at the right schematic for your set.  Try measuring the screen voltage at the screen pin on the tubes.

Fred

[N8ETQ]

Posted by: W7SOE
Insert Quote
The screen voltage in the SX-28 comes from a resistor divider, R30 and R31, that divide 280V.

The schematics shows the resistors in series, with R31 to ground.  The BOM lists R31 as 11k and R32 as 4k.  I see this as providing 205V, not the 100V the schematic notes.

What am I missing?

Thanks

Rich


Posted by: KA2DZT
Insert Quote
Signal level will also affect screen current.  Screen voltage shouldn't be too far off from the 100 volts.  You may have to recheck everything you're doing including looking at the right schematic for your set.  Try measuring the screen voltage at the screen pin on the tubes.

Fred




  Yup,

   Both gud points...


     The SX-28 likes 110 VAC Mains.. Back in "the Day"
you were prolly lucky to have that. Now a day's 110
would be considered a "Brown Out".  My shop and shack
is regulated at 115 VAC by virtue of my GRC 1581A.

     With no further information I can only assume
you feel screen "E" is to high...  Could be, but we don't
know...


     I don't speculate about this stuff. Please provide
more detail about your concerns.

   GL Your SX-28 is a Sweet Piece... Well worth preserving.
Adjust your "Mains" voltage for a 6 to 6.3 voltage
as measured at the FIL connections at the tube sockets
and recheck "E scr"..

/Dan

    "I like any RX that weighs more than my dog"