More failures found and repaired on card; almost ready to put back into the machine

FAILED TRANSISTORS IN SLT MODULES FOUND, MODULES REPLACED

The circuit in the 3819 card requires a first transistor to conduct in order to block the second power transistor from powering the lamp. These transistors are provided as SLT modules, hosting four transistors per 361497 module. IBM's informal name for this module is FTX meaning four transistors. 

At least one transistor on each module was damaged by the power incident when the tantalum filter capacitor shorted the 48V to ground. This is a very common SLT module, which you can find on many SLT cards. I extracted two from a donor card in my stockpile, I think it was a card from a communications controller but it may have been a tape drive part; all that matters is that it had working FTX modules.

I pulled out the bad modules from the defective 3819 card and installed the spare parts. I put the card in my testing jig but I still had some circuits that were illuminated in the absence of a control input signal. At that point I began checking the continuity of traces on the card to see why my transistors weren't conducting. 

Defective modules removed

BURNED/BROKEN GROUND TRACES INSIDE CARD

I found that the emitters of the transistors, which were supposed to be connected to ground (pin D08), were floating on many of the circuits. The traces inside the card had acted as a fuse to protect the power supplies by evaporating so there was no ground for the shorted tantalum to dump voltage into. 

There aren't that many pins on the card that are connected to the main (D08) ground, so I just added some wires on the back of the card to restore connectivity. This mostly worked, as I discovered in the subsequent bench testing. 

Jumper wires to restore ground connectivity

TESTING STATUS AS OF THE END OF THE WORKDAY 

My test jig had multiple power supplies, giving me +3, +6, +12 and +48 so that I could put the card through its paces. The 12V supply runs through an incandescent lamp whose other lead is connected to a card output pin. The lamp should not be lit unless the input pin associated with that circuit is pulled down to ground. When the input is grounded, the bulb should glow brightly. 

I had borrowed a good 284 transistor from one of the circuits to replace the shorted transistor I had discovered earlier. Thus, we have one of the eight circuits which will not drive a lamp. The expectation is that the other seven should work properly.

All did except for one circuit where the lamp was lit regardless of the input pin state. This fifth circuit uses input pin D13 to control a lamp or solenoid on pin B13. This is wired to the File Ready lamp and will indicate that a disk is spinning and ready to use in the internal disk drive. 

I will dig into this failure when I get back to the shop and correct it. I do have enough parts to make six circuits work, unfortunately, the circuits working are 2-4 and 6-8 but I need 3-8 to match the wiring to the lamps and solenoids. I don't want to rewire the backplane of the 1130, so any changes I make will still be on this 3819 card.

Since the machine has a total of three of these cards to control various solenoids and lamps, I don't want this card to migrate to the other card's slots since it is repaired to work as necessary in its current location. The other two slots (B4 and B5) make use of the first circuit which is the one I cannibalized to replace the burned out 284 transistor on another circuit. The repaired card will not work properly in B4 or B5).