LGP-30 Memory Drum Update

Jon Elson elson at pico-systems.com
Tue Jan 3 12:11:48 CST 2017

On 01/03/2017 10:58 AM, Cory Heisterkamp wrote:
> While waiting for the machine, I decided to investigate the stuck drum.
> This unit has 71 read/write heads plus what appears to be an inductive
> pickup for the system clock. Upon closer examination I discovered multiple
> heads in contact with the drum surface preventing rotation. And in the
> process of removing the mounting bars that secure the heads only then did
> damage become visible on a couple of tracks (scored oxide under the heads).
Most likely the same issue as the G-15 we had.  Dust was 
allowed to get into the drum area and pack under the heads.  
Probably if you pull the heads and clean them, it will 
restore clearance.  Of course, the bearings may be bad, or 
will have to be replaced anyway as the grease may have hardened.
> What I’m wondering is if anyone is familiar with the setup/adjustment
> procedure for getting the heads set correctly. There *might* be a couple of
> unused tracks I can relocate heads to, but my thought is that if half a
> dozen heads were already in contact, then the rest may be perilously close
> as well (swelled drum?). My odds of setting 71 heads perfectly on a 50 year
> old worn drum is…well…not great.
If the drum can be set up to run true again (may need 
attention to bearings) then I think setting the heads up 
won't be that tough.  I suspect it is done with a feeler 
gauge, this is low-resolution stuff with large gaps in the 
heads, so the heads probably run with a gap of at least 
.005" (~ 0.1mm) (I'd think, without actually knowing).
> For kicks, I tried to use a piece of cheap (=thin) (0.004”) notebook paper
> as a feeler gage to see if I could identify the offending heads prior to
> support removal. This was a no-go as clearance was too tight. So, is it
> true these ride 0.001” off the surface?
Well, it could be.  That sounds really close for the vintage 
involved.  So, maybe the drum or oxide has swelled.  Anyway, 
if there is much damage to the oxide, it may not make sense 
to try to repair it.  if the heads that jammed it left 
divots in the drum, or the surface is uneven (likely if 
swelling actually occurred) then it may require extreme 
efforts to repair.
> I suspect with temp and humidity changes, and given the age, I would be
> better off building a solid state drum emulator for the 4KW mem, but
> retaining the drum for the clock and possibly the 3 fast registers..if I
> can get those (7) heads set correctly.
Why not just replace the whole works?  If you are going to 
replace the long lines with electronic memory, doing the 
short lines and the clock track should be trivial.  I think 
a mid-sized FPGA could do it all quite easily.
My guess is that if the surface is uneven, it may not read 
back data reliably.  The high spots might be fine, the low 
spots will have dropouts.  This is all assuming swelling was 
the culprit.

It is also possible that machined parts suffered stress 
relief over the years.  Wrought metal has stress imparted to 
it when rolled, and then machining will partially relieve 
the stresses, causing warpage.  The warping continues over 
time.  To eliminate this, critical parts are machined close 
to size, heat treated to relieve the stress, and then finish 
ground to exact dimensions.  It is possible some of the 
stress wasn't relieved during manufacture.

And, nobody expected a 195x machine to be running in 2017, 
especially as anybody in the computer business knew those 
transistors were right around the corner, and would almost 
certainly replace tubes.


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