Diagnosing an issue w. 540 Video Board - UPDATE

[lowrybt1]

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

[dave]

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

[lowrybt1]

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.

[dave]

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