LED D1 Not Illuminating

[David Read]

Greetings!

I've been using my uKenbak-1 for about two years and it has been working great. However, a couple of weeks ago LED D1 (leftmost looking at the front) stopped lighting. I checked and it is no being powered. I also replaced the LED in case that was the issue to no avail.

Any suggestions what to check next? The corresponding button is working because I can still input values using the bit. Programs all run fine, other than the missing display bit.

Appreciate any guidance on debugging this.

Best Regards,

Dave

[Mark Wilson]

I think "D1" (Bit7) is connected to the 74HC595 shift register, so you could check the seating on that. Also continuity from its pin Q7 (7th from left, bottom row) to the LED leg.

[Vicente González]

Also remove the '595' from its base, check for any dirt or bent pins, reinsert it... and good luck.

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[David Read]

Thank you for the suggestions! I'll delve into it next week.

[David Read]

Thank you for the pointers to check the 74HC595. 

To close the loop on this, after some continuity testing, the issue was the IC, not a connection. The pin driving the LED never goes high. Surprised at an IC failure, but after replacing it, the issue is resolved.