[vcf-midatlantic] Wrap-up of System Source Repair Workshop 1/29 & 1/30, 2022

David Gesswein djg at pdp8online.com
Wed Feb 2 01:30:28 UTC 2022

On Sun, Jan 30, 2022 at 11:45:33PM -0500, Jeffrey Brace via vcf-midatlantic wrote:
> Maybe others want to chime in on what they were working on.
First I tried to read the RD53 from the 11/73 discussed on the list earlier.
The drive wasn't spinning up so we lifted the drive electronics board to
get at the spindle motor and got it to spin. Remarkably easy to do. Two
captive screws to undo one side of the board and it pivots up on some pins
at the other end.

The MFM reader mostly didn't see anything. One point it seemed to identify 
proper data for a short time.  With a scope it was outputting just very 
narrow pulses at improper interval.

We were going to try to swap the electronics board with a drive I brought
but when we pulled a flex cable one of the pins stayed with the connector.
Three more pins were about to break. Instead we jumpered the four lines
hoping the initial broken pin was the problem. No such luck. We decided
that further work was going to be time consuming. May still be possible to 
recover but not easy. I think he was going to try to get the tapes read 
and see if that goes better.

I moved on to the 8/I. At end of last workshop it would single step short 
programs toggled in but not run at speed. Much poking and found the memory 
voltage was collapsing when running. Two regulator cards were bad. Swapped in 
a spare and one from the PDP-12 and it could run programs. Replaced bad bulbs 
on front panel and one bad drive transistor. Was able to load diagnostics 
over the serial port. 

Instruction test 2A passed. 2B failed with a memory location looking like it
was incremented by one. The first 4k of memory passed it diagnostics.
Tried to run FOCAL language. Crashed after printing CONGRATULATIONS.
More for next visit. Left it with two light pattern programs that can be used
to show it does something.

Got to play with the front panel on the LINC for a little while. It was showing
a reasonable amount of life but looks like its going to need some attention.

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