Re: OT: Classic Macs group?

["Michael J. Mahon" <mjmahon@aol.com>]

jsnospam@cimmeri.com wrote:
> Michael wrote:
>
> > Although Paul's suggestion to use the centering magnets would certainly
> > move the raster, the fact that it's fine with another analog board
> > indicates that it's actually the sweep timing that's off, not the
> > raster centering.
> >
> > The control you're looking for is the "horizontal hold" or "horizontal
> > lock" or "horizontal frequency" control.  It is often an adjustable
> > slug in the horizontal oscillator coil (not the horizontal output
> > transformer).
>
>
> Michael,
> I agree that the sweep timing is off.
>
> The circuit appears simple.  One of the outputs of the flyback
> transformer is paralleled to gnd via a rectifier, and parallel to gnd
> via a  .033uf cap; then it goes in series to a {coil + resistor
> paralleled to each other}; then through the "width" adjustable coil;
> then to a unipolar electrolytic cap; then straight to yolk.
>
> The only coil adjustable here is the width one.   I do know that the
> previous {coil + resistor} combo have some effect on "horizontal
> linearity" whatever that is.. but they are not adjustable.

I expected a phase-locked loop for horizontal sweep (so it keeps
running even if deprived of horizontal sync pulses), but apparently
it's not.  If the Mac's horizontal sweep is pulse-driven, and has no
"horizontal frequency" adjustment, that reduces the possibilities.

> It's possible that the sweep circuitry prior to the flyback is off
> somewhere, but I suspect more likely *after* the flyback.   I checked
> the value of that final unipolar cap -- and the capacity is good...
> but if the ESR of it were too high, could that influence the timing?

It's possible that the capacitor is leaking in only one polarity,
causing a net DC current to flow through the yoke, producing the
horizontal displacement.  That capacitor is the DC blocking capacitor,
so replacing it would be a very good thing to try.  The capacitance
value can be larger, but the voltage rating must be equal or higher.

-michael

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