Repaired 509 card

509 CARD IS A 10 INVERTER CARD FOR WHICH I HAVE DOCUMENTATION

The specific circuit that I knew had failed had input at D07 and output at D11 but this particular card slot makes use of eight of the ten possible inverters on the card. An option rather than repairing the card is to modify the backplane to move the circuit to one of the spare inverters. 

Repairing the card can either involve replacing the failed SLT module on the card using a spare on my card stockpile, or rerouting the signals on the card to make those pins connect to a different but working inverter. Each SLT module implements two inverters. 

TESTING REVEALED WHAT HAD FAILED AND GUIDED ME TO THE REPAIR

The only bad gate was the one that I had found through debugging, the nine other inverters worked well. I had many spare cards with the 361479 SLT modules on them, some of which are specific to 3420 tape drives and not useful in computers. I swapped in the module from one of those tape drive oriented cards and tested to verify that all ten inverters now work properly. 

Failed SLT module - twin inverters

Underside of the module

The card went back into the machine, freeing up the other 509 card for use in the peripheral controller logic to be debugged later. The KB Restore key actuates the restore solenoids now.