Monitor refresh rate query

[Chuck Guzis]

On 06/17/2016 08:36 AM, Noel Chiappa wrote:

> (Parenthetically, what exactly is the mechanism that causes damage if
> you run an old CRT monitor at too high a refresh rate? I assume the
> excessive speed generates too much heat somewhere, and causes
> transistors to fail, or something like that?)


It's not the refresh rate that will kill things, but the horizontal
frequency.  The high voltage in most CRT monitors (and TVs) is developed
from the scanning signal via a high-voltage "flyboack" transformer that
serves dual purposes--it matches the the horizontal deflection coils to
the output transistor (or tube), may develop some intermediate voltages
for driving the various CRT elements and also develops the high voltage
the CRT anode requires (after rectification).

FBTs are customarily designed for a specific range of scanning
frequencies, give or take.  So it's usually safe to operate a FBT
circuit that's been designed for 15.575 KHz (standard NTSC frequency) at
18 KHz or so (difference between IBM CGA and MDA frequencies), though it
may take some tweaking of components.

One gets into trouble with monitors where design shortcuts have been
taken.  For example, the IBM 5151 monochrome monitor for use with the
MDA has no horizontal oscillator--it uses the pulse train generated by
the 6845 CRTC to develop its horizontal scan.  While this eliminates the
need for a "synchroguide" type of setup, where an oscillator "locks
onto" a sync signal, it's also dangerous.   Normally, a too-high sync
rate would simply cause the monitor's horizontal oscillator to fall out
of sync (that's when you see the diagonal bars forming across the
screen).   But the 5151 monochrome display has no oscillator and simply
follows what's given.  Ultimately, if taken too far, the voltage in the
FBT secondary exceeds the ratings of the winding insulation; an arc
develops between windings and the FBT self-destructs, sometimes taking
the horizontal output transistor with it.

I smoked one or two FBTs while developing SIMCGA a long time ago.  I
think I may even still have a nice 14" monitor with a defunct FBT
kicking around from those days.

--Chuck