SA-800/801 saga continues / schematic request

When I connect up drive "X", the +24V series pass transistor gets very warm and the +24V drops to around +8.5V (regardless of whether or not the stepper motor leads are connected). On the circuit board, a Shugart ASIC 16270-1 gets quite hot, and is clearly dead. This chip is labeled WR/CHNL in an SA-850 schematic, and is listed in the Shugart spares catalog as "IC, Write Channel". It is indeed fed from the +24V line on pin 15, and connects to the erase and center tap terminals on the head. [I have not measured the head to see if it was damaged - nope not]. I expect that if I pull that chip, the board will not overload the +24V supply. When I connect up drive "Y", the +24V series pass transistor on the power supply gets very hot, but nothing on the circuit board gets hot at all, and the +24V drops to 0. The board measures as a dead short with an ohmmeter (even "X" shows *some* resistance). The stepper driver IC, a standard part UNL2074 (quad darlington power transistors in a 14-pin DIP no less) has a suspicious bump on its head, so I will probably start by pulling it and see what changes (I hope that is it), and if that is the issue, I will also check the voltage clamping diodes in the output circuit and the stepper windings and the 7404 that drives it to make sure it wasn't collateral damage. I expect regardless of the cause, the dead short will be not *too* difficult to find and correct. So, now the saga of the Altos power supply is fully understood. It was the second drive from the DSD-440 that actually killed it. Fortunately, the DSD-440 power supply was apparently robust enough to put up with the abuse. With a little luck, I can make one whole SA-801 out of the pair (focusing on "Y" since I have no source for that 16270 ASIC), and then use the mechanism of "X" with the board from the Altos SA-800 to make a working SA-800 for the Altos. JRJ

from drive "Y". I expect it will be fine as well. Then I can swap out