RCA CDP18S711 manual Last Resorts

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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.