Radio Shack TRS-80 Service Manual page 77

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TRS-BO
®
Service Manual
coupling this signal onto Phase A. (Consult the earlier
theory of operation for a detailed description of how this
circuit works.) Oscilloscope pictures of the artifacting
circuit are shown in Figures 5-10 thru 5-12. One strange
symptom is that of the left side of the screen being shaded
a different color from the rest of the screen. This
indicates that the one-shot is taking too long to time out,
causing the level of Phase A to be shifted during the active
portion of the video display. This may be caused by a
defective U4, by leakage or improper value of capacitor C26,
or by increase in value of resistor RlO.
5.3 Keyboard
5.3.1 No Keyboard Entry
No keyboard entry may be caused by a variety of factors.
These include: broken flex cable which connects keyboard to
main PCB, defective PIA U7, +5 volt connection to pullup
resistor pack RPl, defective RPl, a short to ground on any
of the keyboard row inputs, or a defective keyboard. In
particular, note that the capacitors on the joystick fire
button leads CC15 and Cl8) might short and cause this
problem.
5.3.2 Some Keys I noperative
The most common cause of some keys not working when others
do is an open somewhere -- either on the keyboard itself, or
in the connecting flex cable, or on the PC board. If an
isolated key does not work, the problem is almost certainly
on the keyboard itself. However, if a whole row or column
of keys does not work (see the keyboard matrix on the
schematic diagram), then any of the above is a possibility.
Other possible causes are a de f ective PIA CU7) and defective
RPl.
5.3.3 Wrong Character for Key Pressed
If a different character is displayed other than the one
that should be displayed for a given key, the problem is
most likely a short. Again, the short could be on the
keyboard itself, in the connector, or on the PCB. All of
the aforementioned possibilities should also be checked.
One additional possibility is a defect in the processing
section, particularly on the data bus.
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