[Central-Electronics] CE 20A 9 MHz Oscillator

[Ron VE8RT]

Hi Jim,

  I have my CE20A set aside for a winter project, but it needs a lot of
work and parts.  Everything you've brought up is of interest to me,
thanks for posting.

    Ron VE8RT

On Mon, 17 Jun 2019 15:27:21 +0000 (UTC)
Jim Candela <jcandela at prodigy.net> wrote:

>  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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-- 
Ron VE8RT <ve8rt at yknwt.ca>