monitor question (problem diagnosis)

[Tony Duell]

> 
> I forget. What does a single vertical line in the
> middle of a display indicate? It might be something

The obvious answer is 'loss of horizontal deflection' ;-)

> entirely different from an image that collapses into a
> horizontal line, but I seem to remember one of them
> representing a definitive component failure. If it

In 99% of fixed-frequency (not multi-sync) monitors and TVs, the 
horizontal deflection system also generates the high voltages for the 
CRT. So that most failuers in the horisontal deflection circuit will 
remove the high voltages and you will get a totally dark screen. 

About all that will remove the deflection and not kill the high voltage 
is an open-circuit in the yoke or its coupling components. In fact in 
some monitors even this will mess up the circuit enough to seriously 
reduce the high voltage.

Note that in multi-sync monitors the horizontal deflection and high 
voltage circuits are nearly always seperate (this goes back even to 
things like the IBM5154 EGA monitor which could do 2 different scan 
rates). With those, many component failures can kill the deflection but 
not the HV.

And in almost all monitors the vertical deflection system is a seperate 
circuit. If that fails you get a horizontal line (the screen will not be 
blanked unless there's a protection circuit).

> makes a difference, the unit is a compact Mac, a
> Classic I think. Can any (and all) *new* display

Ah... In the Mac Plus there are very common causes of this. Either a 
burnt contact on the yoke connector or a defective coupling capacitor. 
IIRC that's a 3.9UF _non polarised_ component, near the yoke connector.

> components still be had for such a unit. Substitutes?

Again, all I've worked on is the Mac Plus. Almost all the components on 
the 'analogue board' (PSU and monitor) are standard. The transformers, of 
course, were custom made (although I think I've seen flybacks listed 
somewhere). The R's and C's are standard. So are the trnasistors from 
what I remember.

-tony