It lives! Imlac PDS-1, serial #2 is running again.

From: Tom Uban <uban_at_ubanproductions.com>
Date: Wed Sep 4 07:28:00 2002

Congratulations Bob, good job! I know the feeling of seeing that display
come up
very well.

Jameco still carries the straight 7473 part.

Maybe next time that there is a VCF East, we can link our two Imlacs and play
maze war.

--tom


At 01:49 AM 9/4/2002 -0400, you wrote:
>Hey,
>
>After a small battle, and several hundred hand-soldered decoupling caps
>(the original engineers figured they didnt need any), and a few dead bugs,
>I've gotten an Imlac PDS-1 running once again.
>
>The original Imlac hardware design divided the main CPU and display
>processor up onto about 65 small cards with a handful of TTL chips on
>each. No ground planes at all, and the 'decoupling' consists of a 15 uf
>bulk cap on each board along with a ferite bead inductor. No high speed
>decoupling caps, with the exception of 3 chips on the clock generator board.
>
>With a few decades of age on the linear power supply filter caps, the
>logic supply noise was amazing, often over 4 volts peak to peak on the +5
>rails. Small wonder the machine would crash and hang, its kind of amazing
>it ever ran this way at all.
>
>So a good deal of the work I had to do to get this machine running again
>was to hand-solder several hundred decoupling caps, one for each chip in
>the machine, more or less. I could have re-capped the main supply, but given
>the inductance between that filter cap and all those un-decoupled TTL
>chips, adding the decoupling caps at the end of those long power supply
>leads made more sense. After all, the nose at the power supply filter
>caps was nearly zero, while the ground bounch on each small logic board
>was quite large. Without a ground plane, the TTL chips had to seek a
>common ground through long thin runs of etch, so I added the caps as close
>to the power and ground pins of each package.
>
>While this is hardly original to the machine's original design, I know of
>no other practial solution to getting this rare old machine running
>properly, so out came the soldering iron.
>
>I still have a massive ammount of cosmetic restoration to do, but getting
>to see it run its "HELLO" test program with green vector-generated
>characters was well worth a few days with the iron and scope in hand.
>
>By the way, during my debugging, I came across a bad 7473. Its a odd
>little JK flip flop, and I'd like to find a replacement. Note that the
>74LS73A is NOT compatible, its got a different truth table! Should anyone
>have one of these kicking around, please let me know.
>
>And now, its time to sleep....
>
>
>
Received on Wed Sep 04 2002 - 07:28:00 BST