Radio Shack TRS-80 Service Manual page 22

TROUBLESHOOTING
Problems with the MC-10 may be separated into
two major areas - I/O problems and dead com-
puter problems. If the computer will display the
sign-on message, then a fairly simple set of
testing procedures may be used to isolate the
problem. If, however, the computer displays
random characters on the screen or has no dis-
play, then a major failure has occurred.
In the case of a major failure, the first thing to
check is the +5-volt power supply. If the voltage
level is off by more than 10%, then the power
supply is the problem. Refer to the Power Sup-
ply section for hints on how to isolate the prob-
lem.
If the power supply has a proper +5 volts out-
put, then check the clock output from the mod-
ulator at U12 pin 13. If the clock is oscillating at
the proper frequency, and you have no display,
the problem is in the video area. Refer to the
section on the video interface for hints on fix-
ing this problem.
If the MC-10 displays garbage on the screen,
then some problem is causing a failure of the
CPU. At this point you will have to remove the
top and bottom shield. After you have removed
the two shields, test the operation of the MC-10
to be sure that the problem was not a short to
the shield.
With both of the shields removed, check the
address and data lines of the 6803. After the
reset switch has been pressed, check for indi-
cations that the CPU is operating. If the CPU
shows no signs of activity, then one of the
input signals is preventing the CPU from oper-
ating or the CPU is bad.
If the CPU is operating, then the most likely
problems are a memory or buffer chip failure or
a short on the address or data bus. In this case
you will have to methodically check the
address and data lines, while looking for a line
that shows a problem.
In the case of a simple I/O problem, please refer
to the appropriate interface section discussion.
VIDEO INTERFACE
Almost all of the work for the video interface is
performed by the VDG (U11 - MC6847). There-
fore, this chip is the most likely source of a
video problem. However, it is always a good
idea to isolate the source of a problem before
trying to desolder chips.
If the problem is no picture or a distorted pic-
ture, then a new switch box and cable should
be tried. If you still do not have a picture, then
use an oscilloscope to look at U11 pin 28. You
should have a video signal at this point. If you
do not have a video signal, check +5 volts (pin
17) and the 3.579 MHz clock (pin 33) , if you
have both of these inputs and no video out,
then U11 is bad.
If you have a video signal at pin 28, but no pic-
ture on your TV, then there could be a problem
with the modulator. Check the +5-volt input
before replacing the modulator.
The only other type of video failure which can
occur in the MC-10 is the loss of synchroniza-
tion with the CPU which will cause excess
glitches on the screen. If this occurs, then the
circuitry composed of U18 and U19 is failing.
KEYBOARD INTERFACE
The keyboard interface is composed of one
resistor pack and one IC (U14). A keyboard fail-
ure will usually be caused by a mechanical fail-
ure of the keyboard, or a short or open in the
cable. Mechanical failures will usually be only
one or two isolated keys failing. Cable failures
will cause an entire keyboard row or column to
fail.
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