SA-800/801 saga continues / schematic request

Jay Jaeger cube1 at
Mon Aug 24 20:21:11 CDT 2015

On 8/23/2015 9:39 PM, Jay Jaeger wrote:

> 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.

Some GOOD NEWS for a change. I pulled the UNL2074 from drive "Y", and
the short remained. The next most obvious component for a dead short was
actually a little 4.7uf filter capacitor - and indeed that was the
culprit. The board from drive "X" donated its corresponding capacitor,
as the odds of me fixing that board are not very good, owing to the
16270-1 Shugart ASIC. Returned the UNL2074 back into the board on "Y".

Erased (with a tape eraser for open reel tapes back in the 60's),
formatted a floppy and tested, including interchange with the original
second drive in the Altos, and all is well.

Tomorrow I get to test the 2nd 801 (drive "X") mechanism with the board
from drive "Y". I expect it will be fine as well. Then I can swap out
the electronics from the original first Altos drive onto drive "Y" (they
are plug compatible that way) to get another working drive.

BTW, the replacement FD-1791B-02 (I actually tested to) did help with
the hung floppy controller after errors syndrome - it happened
significantly less often that with a FD-1791B-01 .


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