SA-800/801 saga continues / schematic request
cube1 at charter.net
Sun Aug 23 21:39:58 CDT 2015
Anyone out there have:
1) The Schematic for the newer SA-800/SA-701 board part number 25229.1 ?
(It might also go by 25228 or 27121). The SA-800 maintenance manual on
bitsavers from Feb 78 describes the original discrete board (such as I
have on my Altos in the SA-800s) and an LSI-based board which is
apparently earlier than the one in my SA-801 drives - the parts layout
does not match the layout on these newer boards.
2) A source for Shugart ASICs, particularly 16270-1 ?
And the saga to date:
As you may recall, one SA-800 on my Altos was misbehaving. I decided to
swap in an SA-801 I had from a Data Systems DSD-440 (a two drive cabinet
with 2xSA-801, power supply, and controller intended to talk, I think to
a Q-Bus controller...). Anyway, that blew the fuse on my Altos on the
second track of a floppy test, but (IIRC), the Altos was not damaged.
Call this drive "X".
I then decided to swap in the second SA-801 from the DSD-440. That also
blew the fuse on the Altos, and this time the power supply was damaged.
Call this drive "Y" (The Altos power supply has since been repaired -
that saga was recorded earlier. ;) ).
Replacing the bearings on the problem SA-800 did not help appreciably
with its "wobble" problem. (The spindle shaft is still a possibility, I
suppose, along with a damaged clamping frame).
Having gotten my 20+ year-old home-built floppy supply all fixed up and
fused and load tested, I was still a little hesitant to apply it to
either of these drives X and Y. So, I disconnected X and Y, and checked
out the power supply on the Data Systems DSD-440 Except that the +5V is
low, at 4.5V, the supplies seemed OK, in particular, the +24V was fine.
So I tested the +24V under load, and it held 24V to 3A (and more, I am
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
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.
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