• Please review our updated Terms and Rules here
  • Exhibitor application for VCF West 2022 is now open! If you are interested in exhibiting, please fill out the form here.
  • Here are the results of the VCF East 2022 Post Event Survey: Survey Results

Model 1's screen output is too low

zoggins

New Member
Joined
Aug 29, 2021
Messages
5
The output starts about 2 or 3 inches down on the screen and the cursor is somewhere below the screen. I verified its the computer not the monitor, but does anyone have any pointers on where to start looking?
 

ldkraemer

Veteran Member
Joined
Mar 14, 2013
Messages
2,095
Location
Chaffee, MO
I'd suggest having a look at the Vertical and Horizontal Pulses and compare those to the waveforms of the
SAMS Photofacts for the Model 1.

How about a Photo of what you are seeing.

Larry
 

Eudimorphodon

Veteran Member
Joined
May 9, 2011
Messages
4,403
Location
Upper Triassic
A photo would be very useful, yes. As described the issue could be an "Analog" problem, or it could be something messed up with video memory addressing, and the angles of attack for those things are very different.

The quick "try this" is there are two small variable resistors which can be turned with a small screwdriver on the side of the logic board in front of the power/cassette/video ports. (Only accessible with the case taken off, of course.) They affect horizontal and vertical pulse timing. I forget which is which but fiddling with them should make it obvious. It's possible adjusting the vsync pulse timing will fix the positioning issue, but it's not "usual" for it to drift drastically.
 

zoggins

New Member
Joined
Aug 29, 2021
Messages
5
I have attached a picture. The top row is the list of numbers and the prompt is off the bottom of the screen. This happens on both my LCD and an original Model 1 monitor.
 

Attachments

  • 20220126_132957.jpg
    20220126_132957.jpg
    260.5 KB · Views: 11

Eudimorphodon

Veteran Member
Joined
May 9, 2011
Messages
4,403
Location
Upper Triassic
Skimming through the service manual I think the pot you want to try twiddling to adjust vsync timing is labeled "R44", I would probably try that first.
 

zoggins

New Member
Joined
Aug 29, 2021
Messages
5
R44 appears to be a plain old 10k resistor. R22 appears to be adjustable and affects the vertical sync. However, turning it to the point that the prompt line appears on the screen the screen starts vertically scrolling.
 

Eudimorphodon

Veteran Member
Joined
May 9, 2011
Messages
4,403
Location
Upper Triassic
R44 appears to be a plain old 10k resistor. R22 appears to be adjustable and affects the vertical sync. However, turning it to the point that the prompt line appears on the screen the screen starts vertically scrolling.

Sorry for the bum steer about the designation, I was trying to not too obviously dig though a PDF of the schematic in the middle of a really boring meeting. (Pulling out my printed copy would have been too obvious.)

If I get a chance I'll re-read the section of the manual that talks about the video divider chain and see if any light bulbs come on. It could still be an "analog" issue in that delay circuitry, like a resistor or capacitor value has drifted enough that you can't correct for it with the pot, but I'm also wondering if it's possible you there's a "stuck bit" in the circuitry that watches the counters for the intervals for outputting characters or the sync pulse itself that's causing the active area to be misplaced.
 

zoggins

New Member
Joined
Aug 29, 2021
Messages
5
No worries. I did discover some really crappy bodge work involving Z57, which is part of the sync circuit. I just pulled Z5, Z6 and Z57 and put in sockets. Waiting for new chips to arrive. I am checking the analog now.
 
Top