29 Nov
2007
29 Nov
'07
12:55 p.m.
On 28 Nov 2007 at 12:53, mbbrutman-cctalk at brutman.com wrote:
The design of the horizontal drive stages is very different between the
MDA nad CGA monitors.
The CGA monitor is conventional, in that there's the normal
horizontal oscillaotr/PLL circuit. I doubt that it would run far enough
off-frequency to damage the output stage no matter what frequency of sync
pulse you gave it (althoguh it will obviously not lock).
The MDA monitor is simple. There is no horizontal oscillator circuit _at
all_. The incoming sync pulses (or rather, drive pulses) go to the base
of the horizontal drive transistor, which is transformer-coupled to the
output transistor. It's possible to drive that way off frequncy, where
the output stange is no longer resonant, and then the output transistor
(HOT to you) fails.
I suspect (although I am not trying it), that you could damage an MDA
monitor by plugging it into a CGA card.
Note that this applies to the IBM monitors. Some clone MDA monitors had
the conventional oscillator/PLL circuit, I assume they're a lot harder to
damage.
With regards to the 'killer poke' somebofy mentioned, It's impossible to
bring the drive frequency down to 0, the signal is capcitor and
transofrmer coupled. But it's certainly possible to reprogram the 6845 on
the MDA board to drive the monitor way off-frequency, at which point the
HOT is likely to fail.
-tony
(I won't be able to test it until I dig out a
CGA card. And now of
course I'm just paranoid and hoping that I didn't hurt it.)
Shouldn't bother a CGA, other than not syncing. For that matter, an
MDA plugged into a CGA port, while not giving an in-sync display,
should not emit magic smoke.