[vcf-midatlantic] Tales from the Repair Workshop 6/12 & 6/13, 2021

Herb Johnson hjohnson at retrotechnology.info
Tue Jun 15 03:13:21 UTC 2021

Heath H89 (actually an H88 upgraded)

At the May workshop on the museum H89, I replaced some 20V caps on a 20V 
DC line on the CPU board. This is a common H89 problem where the caps 
short out on a DC line that's over-voltage.

Jeff Saltsman also at the May workshop, repaired his own H89 floppy 
drive which had gummy disk-hub bearings. I did the same repair on the 
museum H89 drive; we both succeeded. So the museum H89 would boot HDOS 
but the keyboard returned odd codes on several keys. Jeff later 
"repaired" the floppy controller board. There was also an H89 in pieces 
in a box (not Heath box) which looked ripe for parts.

At the June workshop, I found on the H89 a damaged keyboard encoder chip 
on the terminal board. I replaced keyboard and chip from the box o' H89 
items. A terminal board from the box failed with smoke and flame. Again 
a capacitor short which I fixed but to no good result; but it sourced 
the keyboard encoder chip. That fixed the funny-keys problem.

A surprising amount of rust was found at the bottom of the museum H89 
which I partly cleaned. Also I repaired by replacement, the 
recently-repaired floppy controller board, with a swap from the boxed 
H89. Thus it's useful to have multiple and even damaged duplicate 
computers in the Warehouse.

Jeff Saltzman provided assistance and test disks, as he did in May. 
He'll provide some disks for the Museum's use. I suggested he pull the 
floppy drive from the boxed H89 and restore that drive. It likely needs 
the same repair as previous drives; these are all the same model.

The H89 now boots and operates CP/M and HDOS from its internal floppy 
drive. It's useful for demonstration but may need more testing.  I"ve 
brought home the museum's external H89 drive in cabinet, for 
restoration. Of course I'll return it when I can, say within a month. 
I'll have a Web page to show the various repairs, among my other H89 
repair pages at retrotechnology.com

DSD floppy drive/controller

Work for myself was on a PDP-8 compatible floppy drive system by DSD. I 
had duplicate hardware and assistance from David Gesswein. His extra 
drive cabinet completed my drive controller and Shugart drives. I built 
flat-cables and assembled and cross-tested both DSD units. David and I 
got mine working! (The controller has a local-test mode, no PDP-8 
needed.) I may have a floppy-drive system for my PDP-8/F! I'm grateful 
for David's help and equipment and his time.


I only wish I could have helped David with the Straight 8's problem; 
it's tech on the trailing edge of my experiences. 1960's transistor 
logic is not 1970's TTL logic! One card is equivalent to a few TTL chips 
if that. We are so lucky that TTL has endured half a century or so. 
David provides an account. You won't find that detail anywhere else!

And I was able to help Dan FitzGerald with some Apple Mac equipment, as 
Dan had requested on the Google registration form. I provided some moral 
support for others at the workshop. But I likely missed other workshop 
activities as they were in two other rooms. Busy weekend.

regards, Herb

Herbert R. Johnson, New Jersey in the USA
http://www.retrotechnology.com OR .net
preserve, recover, restore 1970's computing
email: hjohnson AT retrotechnology DOT com
or try later herbjohnson AT comcast DOT net

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