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, but the problem is that the picture is not centered 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.

 

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, but the problem is that the picture is not centered 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.

There should be a sweep synchronization signal on the horizontal signal . Is it missing (or it is delayed) since you changed L703 coil.

 

The problem is that the monitor has one "Sync" input and the video card has one horizontal and one vertical outputs, which i just tied together and fed them to the Sync input , on pin 4 of the din video input connector. Should i connect the horizontal sync output from the card direct to the R711 resistor? Will this help?

 

Well i did that "connect the horizontal sync output from the card direct to the R711 resistor" and solved the problem. Now i must figure out how to narrow the picture. Is to wide, than the CRT tube....

 

Well i did that "connect the horizontal sync output from the card direct to the R711 resistor" and solved the problem. Now i must figure out how to narrow the picture. Is to wide, than the CRT tube....

That is going to be a tough one. You are going to have to either compress the angle or, possibly, feed one line into a buffer on a microcontroller and then push it out in faster bursts. I am not that familiar with how to do this but good luck.

 

I must be blind. I can't find the horizontal deflection coil on your schematic.

The problem with changing the width is that it will probably also change the high voltage. This is bad in a number of ways. You may not be able to focus the beam, some of the parts may draw higher than intended currents/power, and the worst is that it can cause the CRT to emit xrays.

 

I can't find the horizontal deflection coil on your schematic.

Its on the upper right side of the schematic L701 on pcb 703.

 

This is picture a got so far.How can i change the angle, in order to narrow the picture?

 

Its on the upper right side of the schematic L701 on pcb 703.

Yup. I am blind. I thought that L701 was only the vertical coil.

 

This is picture a got so far.How can i change the angle, in order to narrow the picture?

I doubt that you can. The angle is mostly determined by the shape of the CRT and the horizontal deflection coil.

Here a a couple of ideas for you to try:
Change C712 to a different value. Be warned that this cap may have special characteristics such as high voltage or safety requirements. I would only do thisif nothing else works.

It looks like L702 has a magnet on it. You could try adding a magnet and varying its strength and orientation and see what happens.

Maybe pulling the yoke back a bit on the CRT neck will help. It could also make it worse. I am not sure.

I am also not sure how you can get rid of the fisheye lens effect. Magnets attached to the yoke with hot melt glue will effect the linearity but I suspect that you will not be able to get the right amount of change without having other distortions in the image. Use ceramic magnets and cut them to different sizes to adjust their strength.

 

Change C712 to a different value

This is a 10uF/25V electrolitic capacitor with no polarity. I thought playing with the RC network R723/C725 would change the saw tooth angle.
What do you think?

 

This is a 10uF/25V electrolitic capacitor with no polarity. I thought playing with the RC network R723/C725 would change the saw tooth angle.
What do you think?

I think that would be a very bad idea. It would significantly effect the operation of the high voltage power supply. See my post #6.

 

The operation of the power supply is already affected, by the fact that is pulsing at 18.432Khz, stead of 15.625Khz, and i believe also that that is the reason, why the picture is wider, than it should.

 

I changed the C712 from 10uF to 4,7uF 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 all for the ideas and the support.