Radio Shack TRS-80 Service Manual page 84

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TRS-BO
®
Service Manual
The top two numbers on the screen represent the right
joystick. By rotating the joystick, you should be able to
get each number to change from 0 to 63 and include all
numbers in between. The next two numbers represent the left
joystick, which should be tested in the same manner. The
bottom two numbers represent the two fire buttons, and will
be 1 when the fire button is not depressed and 0 when the
fire button is depressed. (The left number is the left fire
button and the right number is the right fire button.)
If any of the above testing does not yield the desired
results, replace the components discussed above. Remember,
if it is necessary to replace CR7 or CR8, use exact Radio
Shack replacement diodes of the proper part number as these
are special diodes with a low forward-voltage drop.
5.9 cartridge Problems
In checking any kind of symptom dealing with an external
cartridge, the obvious first step is to be sure that you are
using a known good cartridge. If a known good cartridge
causes the computer to lock up or execute random garbage,
the most likely cause would be open address or data lines to
the edge connector. To check this, use a dual trace
oscilloscope, and check each address and data line, with one
probe at the processor and the other at the connector. You
should, of course, see identical signals. One other
possibility that could cause this symptom would be a "dual
addressing" type of problem, which would allow more than one
device at a time to be activated. This would most likely be
caused by the address decoding circuit consisting of SAM
(U22) and UlO and Ull.
If the cartridge does not auto start, the lines Q and CART*
should be checked for opens · to the connector.
5.10 Power Supply
The Color Computer 2 power supply must function properly for
the unit to work. Following is a flow chart for complete
testing of the power supply. The power supply should be
checked without any I/O devices or cartridges attached, as a
defective external load could confuse the findings. In
addition to tight (+/- 5%) regulation of the DC voltage, the
power supply should be relatively free of AC ripple or high
frequency noise. Refer to Figure 5-3 as well as the
flowchart for information on AC content. If the power
supply is "almost" functioning correctly, but not quite, the
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