Running 15.75kHz VDU's at 21.85Khz

[Argus25]

This article might be of some interest to vintage computer enthusiast members.

It explains how the common vintage IBM 5153 colour computer monitor from the 1980's can be modified to auto detect EGA and increase its horizontal scan rate and display 16 colour EGA images:

http://worldphaco.com/uploads/INCREA...OR_TO_AUTO.pdf

[dragonser]

Hi,
yes this is useful. I did manage to damage a cub microvitec monitor trying to get it to work at a higher line frequency. Some arcade game boards have a non standard video output ( was medium resolution 24 khz ? )
regards Peter B

[Argus25]

Originally Posted by dragonser

I did manage to damage a cub microvitec monitor trying to get it to work at a higher line frequency.

This is why in the modification to the IBM 5153 monitor the zener array is added to prevent failure of the horizontal output transistor. It is an interesting balance of factors. If the tuning capacitance on the line output transformer is dropped in value to allow the shorter flyback time for EGA, the peak voltage on the H output transistor collector increases.This is offset to some extent with reduced width which occurs when the H rate is increased anyway, but it still needs the additional zener array as outlined in the article. Also as noted there, IBM got away with it well in the 5154 monitor, because they separated the EHT generation from the H scanning system.