HP-86A screen calibration problem

[Paul Berger (Canada)]

Unfortunately there is no adjustment. The horizontal and vertical sync signal are generated on the video controller chip, likely derived from the system clock. I did measure the sync frequencies on time and found that they are similar to NTSC timing but far enough off to cause problems with a monitor that expected NTSC signal timing. I even found that some NTSC CRT monitors would not even sync up. I think your best bet would be to get a composite CRT monitor that was designed for computers as they usually allow for a wider range of sync signals. You will likely have to adjust the the image size as well which most CRT monitors have controls for. I am using a Zenith ZVM-123-A green screen monitor and I know of one other person who used the amber version of this CRT successfully with an 86A. Apparently some people have also had success using composite-to-VGA convector boxes, but I had no luck with the one I tried.

Update took the top off my 86B and measured it with my frequency counter, the main oscillator is 12.259259 MHz almost exactly the 12.26 MHz marked on it the horizontal frequency is 15.636935 KHz which is a little slower than nominal NTSC the vertical rate is 59.911632 Hz which is very slightly slower than NTSC the signals being slightly slower would mean that the line interval would be longer which would account for it scanning past the edge of the screen.