ADM 3a with no horizontal scan

tony duell ard at
Fri Apr 8 09:41:44 CDT 2016

> This particular 3a has no horizontal scan. None at all. I just get a
> vertical line down the center of the screen. First thing I checked was
> the horizontal deflector on the yoke, which seems secure.

I am going to assume this is a conventional design, I would be very
surprised if not. In which case, to get anything on the screen the 
horizontal output stage must be working, as the CRT electrode
voltages come from the flyback transformer.

Normally the horizontal deflection yoke is capacitively coupled to the 
horizontal output stage (often straight off the collector of the transistor). 
There may be a width control inductor and a horizontal linearity inductor 
in series too. So it's about 4 components (yoke winding, the 2 inductors, the
coupling capacitor) and the interconnections to check. 

If the coils (yoke and inductors) show DC continuity then most likely they 
are OK and in any case you would get some deflection. Most likely, actually,
is the coupling capacitor. It will be non-polarised (maybe plastic film, maybe a
reversable electrolytic) and a fair voltage rating. They do fail...


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