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+--- Thread: HP-86A screen calibration problem (/thread-4095.html)
HP-86A screen calibration problem - gferluga - 06-06-2015 10:27 AM
Dear all:
When I plug my 86A to any composite screen or TV monitor, I don't get the screen picture calibrated.
The edge of the screen is out of the borders.
I try both NTSC and PAL signals.
All displays are LCD or LED.
I don't have any CRT.
I there any trimmer inside the 86A to calibrate picture size?
I attached two examples.
Any idea?
Thx
RE: HP-86A screen calibration problem - Paul Berger (Canada) - 06-07-2015 01:28 PM
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.
RE: HP-86A screen calibration problem - gferluga - 06-07-2015 04:30 PM
Thank you very much for your recommendations Paul.
Have a nice day.
---Giancarlo
RE: HP-86A screen calibration problem - Dave Britten - 06-07-2015 04:36 PM
Looks like the typical results of a monitor trying to auto adjust but not being able to figure out where the edges of the image are. Either manually adjust the image size and position using the monitor's controls, or write a small program that fills the entire screen with a black/white checkerboard pattern, alternating every pixel. That usually helps the auto-adjust routine figure out the size and clock (even with modern computers using an analog VGA connection).
RE: HP-86A screen calibration problem - gferluga - 06-18-2015 07:40 PM
I try today with a HP 82913A (12 inch) monitor and works fine!
I try with an Apple II composite monitor and doesn't work!
As Paul said probably it's a frequency behaviour.
I wonder to know if a Commodore 64/128 or Philips composite monitor will work...