Atari 1040STE, Field Service Manual

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and R 1, R2 and R3 off = 1/16. R0 off, R1, R2 and R3 on = 15/16. This signal then
passes through a transistor amplifier, and from there to the video monitor connector.
3.9.1 Symptoms and Fixes
1. Missing primary color. Check the output of the transistor amplifier. X402 is
blue, 0401 is green, X400 is red. Look for a staircase pattern (eight levels of
intensity). If the signal is there, trace forward to the video connector, if not,
trace backward to the Video Shifter, until the faulty component is found.
2. Primary colors present, secondaries missing or incorrect. Replace the Video
Shifter. Coarse change in the intensity (not a smooth dark to light transition).
Replace Video Shifter or look for a short on the output of one of the three
color outputs for the appropriate color.
3. Specks or lines on the screen. This can be caused by bad RAM; if RAM has
been tested and is good, replace the Video Shifter.
4. Wavering display, horizontal lines not occurring in the same place every time.
The processor may be getting extra interrupts (if the processor is required to
handle additional interrupts it will not have time to change all 16 color re isters
during a horizontal scan time). Examine the MFP interrupt request (pin
32).There should be an interrupt every 126 microseconds (2 display lines)
from Display Enable (pin 20). If additional interrupts occur, locate the source:
the inputs at pins 22–29 should all be high. If no external (to the MFP) source
for the interrupts is found, replace the MFP.
Note: If the keyboard is not connected, the input to the 6850 will be low,
causing continual interrupts.
Screen Three: This tests the vertical scrolling function. The screen is divided
diagonally, white on top/right, and black on the bottomlleft. The screen is scrolled,
making more of the screen black. until the entire width of the screen is black. Then
the reverse takes place, so that the black portion becomes smaller again. Any break
in the border or gaps or sudden shifts indicates a problem in the MCU or RAM.
Screen Four: This tests horizontal scrolling. A series of triangles moves across the
screen from right to left. The border should be a straight line and there must be no
gaps or sudden shifts. EXCEPT THERE WILL BE SOME SHIFTING ON THE TOP
TWO LINES. Any other irregularity indicates a problem in the MCU or RAM.
3.10 KEYBOARD TEST (K)
Two types of tests are run. The keyboard self–test is done first, and if this passes, a
screen is displayed representing the keyboard. If multiple tests have been selected,
only the selftest is run. The operator presses keys and observes that the
corresponding character on the screen changes (reverses background color). The
key will also be displayed in the lower half of the screen. The mouse buttons and
four directions are also shown on the screen. Connect the mouse, move in any
direction, and that arrow will flicker.