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My C4P-DF has become jinky in the following way.... Powers up fine (initial screen, press reset to get to H/D/M). But will not recognize\respond to keyboard presses for some (variable) period of time (15mins to 4 hours). After swapping out spare boards, I narrowed the problem to the 540 video board. Apparently, some component on that board needs to warm-up before the system will acknowledge\process key-presses. Any thoughts on the sequence I should follow for testing various chips on the 540 to narrow down the source?

Thanks, TB

It's good you were able to narrow it down to the 540 board. I'm assuming, for a c4P, that you have a 540B (color) board.

Sounds like a flaky trace or pin. Perhaps a hairline crack (or bad solder joint) somewhere that retracts just enough when cold to break a contact. Since you can write the H/D/M to the screen, I'd focus on the keyboard decoding and interface logic.

You will benefit from a can of freeze spray to cool down the chips in question to try to reproduce the issues. With a magnifier, look for any hairline cracks in the IC legs or the traces, or bad solder joints. If the chips are socketed, try re-seating the chips.

Do you have a scope or logic probe?

The problem could be anywhere along the address decoding for the keyboard, the 8T26 latches, or the R/*W signal. Clearly, The R/W line must be able to go low, in order to write the H/D/M to the screen. But check U4K, pins 6 and 8 to make sure they are both pulsing. If you can check the waveforms, make sure they are periodic pulses, and the signals on pin 8 is the inverted version of the signal on pin 6.

Try freezing and inspecting U6H, U6I, U6J, U6K, U4H, and the 8T26's at U3H,U3I. That would be a start.

Good luck--Keep us updated

Dave

Thanks as always. It is a 540B. I have a logic probe and know how to use it. Is freeze spray basically a compressed air can held upside-down?

I purchased a used scope but have not had time (or found the right resource) to get up to speed in its use. Will let you know how it goes.

Freeze spray cans don't need to be upside down. Otherwise they are similar. I don't know which is more economical.

The best way to learn how to use a scope is to attach the probes and hook them up to whatever signal you are curious about, then start twisting dials. The behavior will become evident.

Dave