7854 front panel locking up

Martin Whybrow

I'm slowly going through the issues with my 7854, at the moemnt I'm chasing a problem where, after being on for some time, the mode selection switches stop working, all I get is a beep from the speaker and the readout flickers whenever I press a button. From cold it seems to work OK, although I also have a fault with the readout where all the characters are squashed vertically aalmost to a line. The CPU is still running as the bus is active, I'm struggling to figure out how the mode select works as it appears to drive the data bus directly, and the beep seems to indicate that the processor is seeing the button presses. Any suggestions where to start faultfinding this?


Martin -
I'm no 7854 expert, but I've been poking around in a few lately, and the manual has been my bedtime reading material, so I know just enough to be dangerous. That being said, a few thoughts:

From experience, the CPU can look like it's running, but be running through garbage, a problem I'm still trying to troubleshoot on a comatose 7854.

Do the button lights change when you press them?

Does it start working again after powering down & powering back up, or does it need time to cool down?

Being able to send a debounced _RESET is a handy trouble-shooting tool. I think I just brought TP1200-1 to ground.

The mode select buttons are read to the data bus through buffer U61 whenever the _FPR signal is active. This is triggered by the CPU's DBIN line OR'd with some address decoding logic.

Ya got a hard-copy of the manual? I can't imagine trying to troubleshoot these without one (or two!). There's a few folks on here that /really/ know their way around the 7854 - hopefully you can tease out enough details to pique their interest!