[GreenKeys] adventures with a M28 compact KSR

[Nick England]

Things I have found out while resurrecting a M28 compact KSR. Here's
some notes for the archives in case someone else has the Need To Know
some day.

The M28 compact has an electrical keyboard much like the M32 - but not
quite the same. The signal contacts are somewhat different, the reset
mechanism is different, etc.

My 28 had a broken plastic contact block (where the codebar contact
wires space/mark against a metal signal contact strip). I couldn't
ever get any superglue or epoxy to work on this plastic, but found a
32 keyboard and said aha, I'll just rob the contact block. Well not so
fast - although the plastic frames of the 32, 33, and 28-compact
contact blocks appear identical, the metal contact strips are
different and are secured to the plastic contact block by melted
plastic studs (in this 32 keyboard at least). So it took some
remelting and reshaping to remove the 32-style metal strip and
associated phenolic insulators and replace them with the 28-style
metal strips and associated phenolic insulators. We'll see how this
holds up.

The 5-wire electrical signal from the keyboard goes to a distributor
driven off the gearshift box. This distributor is much like that in
the LD or in the two-shaft, multiple-contact TD (LAXD, LBXD, LCXD) -
but not exactly the same. The contacts and springs are the same but
the phenolic contact block is a bit different. Luckily my distributor
block was OK and I just needed to swipe a few contacts from an LBXD
parts unit.

With the keyboard and base appearing to work OK, it was time to add a
typing unit to the base. I found one that fit (it needs to be the
narrow kind as used in an ASR, not the early wide type). I made sure
it turned freely and put into place. I turned on the juice and the
motor motored, the gearbox geared, the keyboard keyed, and the
distributor distributed, but the typing unit just sat there doing
absolutely nothing. Huh? Some inspection revealed that the gearbox's
driving gear was about 1.5" away from the typing unit's driven gear -
no wonder the typing unit wasn't getting any motive power!

It turns out that the 28-compact's typing unit has a (unique?) main
shaft with the driven gear located in the center of the shaft instead
of toward the left end as in other 28 typing units. There was an
untapped hole drilled in what looked like a good location on my main
shaft so I tried sliding the driven gear down to a new location - but
it wasn't quite the right location and the gears only partially
meshed.

SO....I hauled out a previously stalled project - a Tempest
28-compact, which has a photoelectric keyboard and photoelectric
distributor. The typing unit has shielded selector magnets hooked up
to a military-style connector. I figured I could just make a
connection from the military connector to pins 0 and 3 of the regular
19-pin typing unit connector (0 and 3 are where a normal 28 has the
selector magnet loop) and I'd be ready to "borrow" that Tempest typing
unit for my regular 28-compact. Well, not so fast....
First the Tempest unit wouldn't fit because of a low-voltage
transformer bolted to left rear side frame. Why is it there? The
regular 28-compact has a low-voltage transformer bolted to the cabinet
to power the light bulbs. But not the Tempest unit...maybe because
this transformer also feeds the bulbs in the photoelectric
keyboard/distributor - I dunno, but it had to come off.
Unsoldering the transformer leads from the 19-pin connector revealed
that pins 0 and 3 were carrying the transformer's low voltage output
and pin 2 was used as a tie-point for the 120vac bell/stuntbox
circuit. The moral of this is don't try mixing a Tempest typing unit
with a non-Tempest base unless you do some checking and rewiring as
necessary. So I did - here's hoping I made enough notes to restore the
typing unit to Tempest duty some day (whenever I get info on
converting the photoelectric signals to something useful).

But I finally got there - the quick brown fox is jumping merrily from
keyboard to typing unit and to another KSR in the 60ma loop. I still
need to pull the typing unit off and clean and lube it because it
almost always prints the right characters, but not 100% - here's
hoping I don't have to figure out how to adjust the selector magnet
armature buried somewhere in that double-shielded Tempest box.

Damn, but these machines are engineering marvels!!

73 & Have Fun
Nick England K4NYW
www.navy-radio.com