3819 card repaired, tested and ready to reinstall; another card found with a failing tantalum capacitor

SWAPPED CIRCUIT FIVE WITH UNUSED CIRCUIT TWO ON CARD

I rerouted the input and output pins for the circuit (D13 and B13) to the spare circuit, which had been working properly in tests but would not be used in the D5 slot where this card will sit. This was a messy task but finally it was done. 

FUNCTION TESTING ON TEST BENCH

I wired the card in my jig to +3 and +6V, hooked up the grounds, ran the incandescent bulb through +12V to the output pin of each circuit and alternated connecting the input pin to ground (low) or leaving it open (high). I ensured that every circuit had a dark bulb when the input was open and a bright glow when the input is taken low. 

VERIFY +48V CONNECTION DOESN'T CAUSE FAILURES

The only use this card has for +48V is for quenching diodes to return any back EMF that might be produced when solenoids switch off, but the diodes are in a reverse position such that nothing should flow from the 48V input pin to anything else on the board. I checked that the +3, +6, output and input pins were unaffected when +48V was present. 

CHECKING GROUND TRACES AROUND SOCKET D5

Since another team had encountered a similar failure on this card and found their ground traces destroyed on the board (backplane) around this socket, I used my VOM to test connectivity for the normal ground pin D08 as well as the extra ground pins that handle the extra current if all the solenoids/lamps are being drive simultaneously. 

In theory a 3819 card could be pulling 2.4A if all eight circuits are sinking 300ma apiece. That is why the card has three ground pins instead of the usual single ground. A dead short on a tantalum capacitor across the 48V rail drew quite a bit more current than this, vaporizing traces on the card and potentially on the board as well. 

The board has power connectors to pin D04 on both card slots B4 and B5, containing 3819 cards driving the console printer solenoids, with internal traces on the board over to pins D04 and D05 of card slot D5 where the 3819 mainly controls the 12V lamps. There is no connectivity between D04, D05 and ground, either to the same pins on card slots B4 and B5 nor to the standard ground pin D08. This needs to be repaired with some wirewrap to bridge the grounds of the three 3819 cards. 

TESTED MY OTHER 3819 CARDS WHILE I HAD THE TEST JIG SET UP

I discovered that one of the cards was showing only 10 ohms resistance across the tantalum capacitor that covers the 48V supply. This is a failed capacitor and at that resistance would drive almost five amps until it further damaged the card and board. I quickly removed the bad capacitor. 

I ordered a couple of higher voltage tantalum capacitors - not an exact match in size but should be able to cram into the space on the card since it is only 3.5 x 7.6 mm but will verify that when they arrive. I picked 75V parts to give more margin. 

On the 1130, the 48V supply is only regulated (partially) by the ferroresonant transformer and tends to run high, 53 or 54 volts is common to see when lightly loaded. As the IBM capacitor is only rated at 60V, it won't take that much of an excursion to exceed the rating; tantalums are very sensitive to overvoltage and tend to fail in a short circuit, sometimes dramatically.