ADM 3a with no horizontal scan

Seth Morabito lists at
Fri Apr 8 13:59:58 CDT 2016

* On Fri, Apr 08, 2016 at 02:33:25PM -0400, Mouse <mouse at Rodents-Montreal.ORG> wrote:
> > Today, for whatever reason, my Variac was turned down to 110V, and I
> > didn't notice. I turned on the ADM 3a, and... it's fine.  [...]
> Fascinating!
> I have trouble imagining what sort of fault could cause those symptoms.
> I would have agreed with whoever (tony, I think) said that if you have
> HV then the horizontal scan is almost certainly operating in some
> sense, but this makes me doubt that.  I have trouble coming up with a
> failure mode in that design that could kill the horizontal scan but not
> the HV based on the power supply input voltage, so there is probably
> something very unusual in there.

It IS odd.

One more observation: Two of the three +5V regulators cut out
when their input voltage exceeds about 11.5V. The input voltage
comes from a 9.4VAC winding on the primary transformer. Input ripple
is VERY low, maybe 200mV. So, I suspect those regulators need to be

I'm sure it needs to be recapped, too, so I'll do that before I
investigate too much furtther.

Seth Morabito
seth at

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