ADM 3a with no horizontal scan

Peter Coghlan cctalk at
Fri Apr 8 03:49:18 CDT 2016

> I've been on a tear trying to resurrect old projects here, and next on
> the list is an ADM 3a.
> 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.
> Before I go diving into the schematics, I figured I'd ask here: Is
> this a common failure mode? Has anyone else experienced no horizontal
> scan on a 3a? Any tips on where to look first?

I have an ADM 5 (which might have the same monitor circuit) which did something
like this but the vertical line faded out after a while.  There was a bad joint
on the little driver transformer just ahead of the horizontal output transistor.
I guess the joint was managing to make intermittent contact allowing EHT to be
generated and the CRT capacitance to be charged up for a while.

If your vertical line is not fading out, EHT is being generated which rules out
pretty much everthing (unless it's an unusual monitor circuit) except the scan
coils (unlikely), connections to the scan coils and any correction components
in series with the scan coils.

Peter Coghlan.

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