Found and corrected reason that magic smoke was released

PUFF OF SMOKE FROM V2315CF MAIN UNIT WHEN FIRST CONNECTED TO IBM 1130

The Virtual 2315 Cartridge Facility (V2315CF) main unit was the second one I built. I had swapped it onto this 1130 system to check an unrelated issue. However, as I powered up the computer, white smoke escaped from a chip inside. 

ERROR ON 2310 INTERFACE BOARD HOOKED +12V TO AN OTPUT OF THE V2315CF

The V2315CF outputs a signal for when it is in virtual mode - when the 2310 (13SD) disk drive in the 1130 will not be spinning - to control the Unlock lamp on the 1130 console. This output is wired to the gate of a MOSFET which can light the lamp. 

Unlock is a signal generated by the 2310 disk when the platter is not rotating, allowing the operator to open a handle for sliding the 2315 disk cartridge into or out of the disk drive. It goes off once the drive begins spinning up. 

When we are in the more usual real mode of operation, spinning the 2310 disk as part of the operation of V2315CF, the disk drive itself is controlling unlock by grounding a wire coming from the drive to the 2310 Interface Board. A relay on the board just connects this wire to the outgoing wire from the board to the Unlock lamp itself on the 1130 console. 

If in virtual mode, the V2315CF lights Unlock when a virtual cartridge is not loaded, but extinguishes it upon loading. The relay connects the Unlock lamp to the MOSFET on the 2310 Interface Board, which is controlled by the signal coming from the V2315CF. 

Somehow I had the +12V input to the 2310 Interface Board connected to the output signal wire from the V2315CF. The output is driven by a 75451 open collector buffer chip, grounding the line to the MOSFET gate or letting it float up to the 3.3V that the terminator resistors supply. However, this chip was not designed to have +12V fed directly through the open collector output to ground. The result was the puff of smoke. 

The first V2315CF unit I had been using also had been subjected to +12V through the chip. It failed more quietly so I wasn't aware. I would not have tested the Unlock lamp function in virtual mode until later, so I didn't catch this.

IMMEDIATE FIX APPLIED

I cut traces on the current 2310 Interface Boards to sever the MOSFET gate and V2315CF output line from the +12V supply. A bodge wire was added to complete the connectivity as designed. This allows me to use the current version of the PCB until a new version arrives from the fab. 

I also removed the dead chips from both V2315CF units. These are only driving the Unlock lamp operation in virtual mode, so they aren't needed just yet. 

ORDERED REPLACEMENT 75451 CHIPS

I placed an order with DigiKey to have a stock of the 75451 chips used for outputs, as well as some 74LVC244 level shifters that are used for inputs. I will use a couple to repair the damaged V2315CF units.

REDESIGNED THE 2310 INTERFACE BOARD AND SENT TO FAB

I corrected the error on the 2310 Interface Board. In addition, I beefed up the traces that will drive the Unlock lamp just to minimize heating on the PCB for long periods when the real or virtual drive is unlocked.