LGP-30 Memory Drum Update

COURYHOUSE at aol.com COURYHOUSE at aol.com
Tue Jan 3 13:14:23 CST 2017

In a message dated 1/3/2017 11:11:54 A.M. US Mountain Standard Time,  
elson at pico-systems.com writes:

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 
> 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  
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 
> 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 
> as well (swelled drum?). My odds of setting 71 heads  perfectly on a 50 
> 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 
> 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.

Cory - then  what holds the oxide to the  drum?  Horrible  thoughts of what 
happens to binder layers on mag  tape... flaking, sticky  shed....    
I deal with this  problem all the  time  on  some of  the historic  video 
tape we have  done conversion on out of our media  lab.....

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