Re: 2467B focus/astigmatism anomaly...

Chuck Harris
 

Hi Raymond,

I agree that C4403 needs to be investigated. It will be
a week, or so, before I have any parts in hand to try.

C4403's intended purpose is to make moving changes to the
focus as the trace moves about on the screen.

Lifting one leg should stabilize the focus issues, if C4403,
or the dynamic focus circuitry, is part of the problem.

Something killed Q4402, be it old age, physical abuse, or
a random zap from C4403. Ordinarily, I would think that the
backwards diodes CR4410 and CR4411 should have protected against
all but the largest insults... Maybe they are compromised
too?

-Chuck Harris

Raymond Domp Frank wrote:

On Sat, Aug 17, 2019 at 03:14 PM, Chuck Harris wrote:


My thought is the only way a new astigmatism voltage for optimal focus can be
required is if the beam current has changed between the gun's anode #1 and the
astigmatism lens. If you can believe the stacking of the electrodes shown in
the schematic, the focus lens comes before the astigmatism, and after the
anode #1.

And, it is the toughest to meter, as it is 1.4KV roughly, and about 15M
resistance.

I didn't give Q4432 much thought... perhaps I should, as it would shift the
focus
electrode voltage quite nicely... It is an MPSA94, which is about as you
surmised,
VCBO and VCEO = 400V
The beam current definitely changes each time between turning intensity down and up again, as the case describes. Although I can understand a jittery/jumping Q4432 role, is there a reason *not* to suspect C4403 leakage? You *did* find a broken Q4402 earlier, didn't you? And where/how does the intensity down/then up scenario cause Q4432 to make its jump, since normally there's (virtually) no (change in) current in the focus electrode circuit? Still sitting on my earlier post...

Raymond

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