Hugo Holden
Veteran Member
I thought I would post this observation in case anyone else had the same issue.
I noticed recently trying some different video monitors on my SOL-20 (Monitor from the IBM5155 computer) that there was a horizontal timing disturbance at the start of scan after the vertical interval, causing a horizontal timing irregularity in the position of text characters at the top of the screen. So I investigated it. See attached images.
It turned out that the sync separator in the video monitor was malfunctioning, because the SOL-20 was delivering a vertical sync pulse of around 760 to 800 uS long (when it should be around 190us or about three H periods). It was easy to modify the monitor's sync separator by changing a resistor value so it could work with the wider vertical sync pulse. However, this monitor when fed by a standard signal (as it receives say in the 5155 computer from its CGA card) the sync separator of course behaves perfectly normally as one would expect.
On inspecting the SOL's circuitry, I was surprised to find that the vertical sync pulse width is not generated digitally, but by a pseudo one shot composed of a cmos gate, capacitor , resistor and diode. By lowering the resistor value there below its current value of 100k, this shortens the vertical sync pulse width and it can be made to be close to standard.
However, this problem will only show in up practice if the particular TV or video monitor connected to the SOL has a sync separator design where it is upset by the abnormally long vertical sync pulse from the SOL. Probably all SOL's are like this, the timing components (100k resistor and 0.01uF capacitor) appear to be within spec. But it would be good if this was checked on another SOL just in case the problem is unique to mine.
The easiest fix is to simply reduce the 100k resistor value in the SOL, but in my case I left the SOL as it was, and changed a resistor in the monitor instead.
I noticed recently trying some different video monitors on my SOL-20 (Monitor from the IBM5155 computer) that there was a horizontal timing disturbance at the start of scan after the vertical interval, causing a horizontal timing irregularity in the position of text characters at the top of the screen. So I investigated it. See attached images.
It turned out that the sync separator in the video monitor was malfunctioning, because the SOL-20 was delivering a vertical sync pulse of around 760 to 800 uS long (when it should be around 190us or about three H periods). It was easy to modify the monitor's sync separator by changing a resistor value so it could work with the wider vertical sync pulse. However, this monitor when fed by a standard signal (as it receives say in the 5155 computer from its CGA card) the sync separator of course behaves perfectly normally as one would expect.
On inspecting the SOL's circuitry, I was surprised to find that the vertical sync pulse width is not generated digitally, but by a pseudo one shot composed of a cmos gate, capacitor , resistor and diode. By lowering the resistor value there below its current value of 100k, this shortens the vertical sync pulse width and it can be made to be close to standard.
However, this problem will only show in up practice if the particular TV or video monitor connected to the SOL has a sync separator design where it is upset by the abnormally long vertical sync pulse from the SOL. Probably all SOL's are like this, the timing components (100k resistor and 0.01uF capacitor) appear to be within spec. But it would be good if this was checked on another SOL just in case the problem is unique to mine.
The easiest fix is to simply reduce the 100k resistor value in the SOL, but in my case I left the SOL as it was, and changed a resistor in the monitor instead.