LGP-30 Memory Drum Update

Klemens Krause krause at informatik.uni-stuttgart.de
Tue Jan 3 14:22:00 CST 2017

On Tue, 3 Jan 2017, Cory Heisterkamp wrote:
> 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
We relocated some of the heads on our second LGP-30. This is not 
difficult. We put 3 or 4 layers of 3 micro-meter kitchen aluminium
between head and drum. If I remember right, the distance is 12 micro-
meter. Maybe, that we made a fine adjustment 3 or 4 layers with the
oscilloscope, to get the same output voltage at the head, than the
other heads. Maybe that 3 layers of 3 micro-meter Al have something
more than 9 micro-meters.

> 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 I counted right on every head bar are 3 unused positions as spare

> 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?
The (german) maintenance manual whose scan is on our website speaks
about 12 micro.meters.

There are 64 data tracks, 4 timing tracks (including the main clock
at the rightmost position) and 3 tracks for the registers. Most 
problematic are the timing tracks. If they are faulty, there is no
simple solution to change the heads, because there is no provision
to write them in the machine. 
If there are only few tracks defective, and not exchangeable, I
would try to get most of them in the lower adress-room of the
machine, and at least the last track. The first 3 tracks are
used by the "operating-system", program 10.4, and the last track
is used by the initial loader, that loads 10.4 itself.
But timing tracks S1, S2, S3 and the registers themselves must
be functional.
If this is not, a semiconductor drum-emulator ist indeed the
only solution.
If you lift all the head bars 1 or 2 mm, then the drum can
rotate even with this fake.


> Any input is welcome.  -C


klemens krause
Stuttgarter KompetenzZentrum fyr Minimal- & Retrocomputing.

More information about the cctalk mailing list