[Central-Electronics] CE 20A 9 MHz Oscillator

[Jim Candela]

 Bob,
   I did not go into my 20a's to look, but since the screen resistor is 47K, the screen current without plate voltage likely won't be catastrophic. 

It is good to mention here that the 6MU8 is a good pin for pin substitute for the 6U8A. The glass is a little taller to support the higher plate dissipation rating to the pentode section.
Two more assembly inconsistencies I've noticed:
There is a 15 ohm resistor on the Mode switch used to support PM. I have twice seen a 1 meg in there instead. The three colors are the same, but the order is different!
Then the two  9Mhz coils are close together for mutual coupling. There are three holes for two coils...pick the wrong two holes, and it just cannot be setup proper..
Jim



    On Sunday, June 16, 2019, 7:42:42 AM CDT, Paul Christensen <w9ac at arrl.net> wrote:  
 
 I own two CE 20A transmitters.  While repairing a 458 VFO, I noticed that
the 9 MHz oscillator was gradually failing on the older 20A.  I traced it to
a cracked 560 ohm B+ resistor, R20.  Replacing it restored the 9 MHz master
oscillator.  

While making the repair, I noted that the 47K screen resistor (R72) is fed
direct from the 300V B+ supply and is not tied to the cold end of the R20.
The schematic shows what seems to me as being the right way to configure the
screen where if R20 opens to the plate, then voltage to the screen also
opens.  In the current configuration, if R20 opens, voltage is still
supplied to the screen grid and in effect becomes the plate.  

Both 20A transmitters are wired the same way.  Possibly a factory wiring
error?  Before I make the change to reflect the schematic, can anyone tell
me if I'm mistaken in my belief?

Paul, W9AC

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