[time-nuts] More ancient digital clock fun

[Mark Sims]

All this talk of old digital clock chips reminded me of my first digital clock...  built almost 40 (EGAD!) years ago with a Mostek MK50252 clock chip from Radio Shack (when those words weren't a travesty).   And it had utterly unbobtainium GREEN LED displays.  It stopped ticking in the last millennium.   I thought it was the clock chip (it is the only chip on the board),  but could not find one.  I did find a MK50250 and tried it,  but no dice...  oh well,  stuck it in a box and forgot about it.

I dug it out and tried it again today.  Still no dice.  Tried the other clock chip and it did the same thing...  only one digit lit.  Hmmm,  can't have two clock chips failing the same way...   A little scoping around showed the segment signals were segmenting and the digit signals were digiting...  but the last two digit signals were funky looking.   Traced the problem to a shorted LED digit drive transistor on the second digit.  Replaced that and IT LIVES!.   Complete with the funkiest alarm sound ever.  It looks like the chip has some internal connection from the output digit drivers where a fault causes the thing to stop scanning the digits.