GT65 AMSTRAD to 18.432KHz upscale

Hello everyone.
I need to feed to an GT65 Amstrad monitor, a video signal from an hercoules type video card. The video signal has a 18.432Khz horizontal synchronization frequency, and 50Hz vertical.
I managed to synchronize the picture, horizontal by changing the inductance of the L703 coil, and vertical.
The only problem is that the picture is oversized on CRT, horizontal. I try to adjust the frame via the magnets on the back of the neck, but it did not bring much. I think by changing some components on the pcb of the monitor will fix the problem, but i don't know which? Any help will be appreciated. Attached is the electronic schematic of the monitor, and a picture showing the problem.

The geometry magnets are not the solution. The underlying issue is the video display period has no horizontal borders so the 'proper' fix is to adjust the settings on the Hercules card.

You could try dropping the supply voltage by adjusting VR602 but it may have other consequences. Monitor the voltage at TP501 and see if dropping it below 12V helps. Under no circumstances increase it above 12V, even if the picture shrinks slightly.

Brian.

betwixt said:

The geometry magnets are not the solution. The underlying issue is the video display period has no horizontal borders so the 'proper' fix is to adjust the settings on the Hercules card.

You could try dropping the supply voltage by adjusting VR602 but it may have other consequences. Monitor the voltage at TP501 and see if dropping it below 12V helps. Under no circumstances increase it above 12V, even if the picture shrinks slightly.

Brian.

Click to expand...

It is not possible to adjust the hercules car,it has no jumpers on it. I already has lower the 12V voltage to 10.2V measured at TP501, the minimum allowed by the pot VR501.
Im thinking of changing the RC time constand of network R723 and C725, in order to change the angle of the sawtooth. What do you think?

The problem isn't just the width of the picture. If you look at the right side the edge of the picture is missing even though the raster doesn't reach the screen edge. You can adjust the Hercules card but it is done with a software patch, I'm not sure if that can be done on the system you have there though. What you need to do is either adjust the horizontal margin registers or the dot clock speed in the Hercules controller. The margin may allow the remainder of the lines to be shown but the dot clock would be better as it controls the width of the image within the raster. You may find it more productive to leave the monitor alone and change the crystal on the video card. There were many Hercules variations but I think most use a crystal at 16.257MHz, if you change it to 13.78MHz it will drop the horizontal scan rate to 15.625KHz but unfortunately also drop the vertical rate from 60Hz to about 51Hz which may then become a problem.

I would be very wary of changing the values around the horizontal driver stage. It may work but Q703 is driven with pulses and if you adjust their width it will also control the time it passes high current and that in turn may lead to overheating.

Also be aware that the whole horizontal output stage is normally tuned to the fifth harmonic of the horizontal frequency. It makes best use of the available energy to generate the HV for the CRT. Altering capacitor values may have other side effects such as excessive or insufficient HV.

If I had to make a guess at the best components to try, it would be the linearity coil L702 and the S-correction capacitor C712 but I make no guarantee it will work.

Brian.

betwixt said:

If I had to make a guess at the best components to try, it would be the linearity coil L702 and the S-correction capacitor C712 but I make no guarantee it will work.

Brian.

Click to expand...

I changed the C712 to 470uF but with not affect to the width at all. Finally the changing of the C729 from 68nF to 33nF make a significant change:thumbsup:.
Thank you for the ideas and the support.

- - - Updated - - -

betwixt said:

If I had to make a guess at the best components to try, it would be the linearity coil L702 and the S-correction capacitor C712 but I make no guarantee it will work.

Brian.

Click to expand...

I changed the C712 to 470uF but with not affect to the width at all. Finally the changing of the C729 from 68nF to 33nF make a significant change.
Thank you betwixt for the ideas and the support.
View attachment 143673

The edge of the picture is still missing but at least you can see what it says now.

Brian.