Last Resorts - RCA CDP18S711 Instruction Manual

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up with key C down. With RUN up, check bus and
address lines to see if any look different from the others.
They will, of course, be at different levels or bouncing
around but you might spot something suspicious that
would indicate a short or open for one of these lines.
Try operating with only a 1024-byte RAM (U16 and
U17). Try the other two RAM chips in these sockets.
Check U5 inputs and outputs to verify that all stages are
inverting properly.
If you don't get a pulse at pin 10 of U2 when you flip
RUN up with key C down, U12 may he bad. This pulse
is a difficult pulse to see and you might have to
breadboard a latch or use a latching logic probe to catch
it. If you get the display on pulse at pin 10 of U2, you
should then see U2 output pulses on pins 2, 3, and 9. If
you don't, try replacing U2.
RCA COSMAC VIP Instruction Manual

Last Resorts

As a last resort, try replacing Ul and the ROM. Check
the supply voltage at all chips. Examine the PC card for
hairline breaks in the printed conductors. Fill up
plated-through holes with solder to insure continuity.
Check all signals. They should swing between ground
and +4 or 5 volts. If you see a logic signal at some
intermediate voltage, like +1 or 2 volts, check the source
IC.
Once you get the operating system running, over 90%
of the hardware will be operating properly. There are no
critical adjustments to be made or maintained. All system
timing is controlled by the crystal clock. With reasonable
care your COSMAC VIP system should run for years
without any problems.