IMSAI 8080 front panel intermittently has stuck bit.

[menkelis]

There’s a intermittent problem with A13/A6/D6 (third bit from left) wanting to be a one on deposit or examine or sense switch read
and then on another try (or two) being correct.
Anyone have a starting place for me to have the bit not be flaky?

 

[menkelis]

I have tracked down the A13/A6/D6 issue I believe. Now A11 only is stuck high.
I can have AlLL address switches down, but toggling Examine brings up A11.
Also read from sense switches A11 is always set.

 

[menkelis]

Turns out that switch A11 went bad. I removed it and found that the in circuit part was open. Used Deoxit to get switch working then soldered it back but flipped so that the bad section is not in circuit.

 

[Dwight Elvey]

Oops, I see you found a bad switch. It is not the most common failure but it looks like it solved your problem.

There are open collector drivers that drive the bus. When these fail they often fail to pull the address line low. I forget which ones but a quick peak at the schematic will tell you which chip it is. These are the most common failure that matches your symptoms.
Dwight