SX-28 R4

[W7SOE]

Another SX-28 question.

R4, is the screen dropping resistor for V1.  Mine is an early model so I am going by the early schematic, but it shows R4 as a 1K resistor to 100V.  I find that the resistor actually goes to 280V like R5, as shown in later schematics.  (Lucky me, I get a hybrid!)

I don't remember what was originally there...

In later schematics that resistor is 25K.  I am guessing that I also should use 25K but wanted to check here that this makes sense.

Rich

[KB1VWC]

If the first Rf is a 6SK7 then use the 1K. The circuit was modified quite early in production to a 6AB7 and the screen resistor change to the 25K. Supposedly, only the original build of 150 or so receivers had the 6SK7 in the front end and that was quickly changed to the 6AB7. If your serial number dates from August or Sept of 1940, then you have an original SX28. By October, the 6AB7 was the norm. I think they silkscreened the tube types on the sockets, so it's pretty easy to tell. Have fun with the project. Great receiver.

 Steve

KB1VWC