Re: Tech: Cinematronics Monitor

On Tue, 3 Nov 1998, Keith, Brendan wrote:

> Is anyone familiar with common failure modes in a Cinematronics monitor?
> I have an Armor Attack (and a few other boards to test) and this recently
> traded monitor will not stay active. It constantly trips the +25V breaker
> in
> the power supply.
>
> I have disconnected the Keltron HV unit, so it's not that. I measure good
> voltage on either side of the 7815 before the breaker trips (about 1-2
> seconds).
> The digital cable is disconnected so it's not trying to deflect.

        In fact, with the input cable disconnected, its deflecting the
beam off the screen -- a sure way to blow the breakers 100% of the time.

        Most ICs have internal pull-ups of all their inputs to improve
their noise immunity, if an input is left floating. With the input cable
disconnected, the inputs to the DACs are all "1's", so you're driving the
monitor to max. deflection on both axes.

> After closing up shop last night and thinking about it some more I figured
> maybe one of the deflection transistors could be shorted but the manual says
> not to run the monitor with them disconnected. Is that because the
> deflection
> driver transistors would then try to drive the deflection yoke directly? I
> have the schematics
> in front of me, but I'm a little more familiar with the digital domain
> rather than analog.
>
> Is it safe to disconnect the deflection yoke alone? Then the deflection
> transistor pairs?

        Why not just test the deflection transistors?

        And, yes, the deflection transistors are the most common failure
with that monitor.

Joe
Received on Tue Nov 3 09:56:51 1998