LGP-30 Memory Drum Update

[Jon Elson]

On 01/03/2017 01:47 PM, Brian L. Stuart wrote:
> On Tue, 1/3/17, Cory Heisterkamp <coryheisterkamp at gmail.com> 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
>> 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.
> A while back I read a procedure (probably in reference to the G-15).
> Quite frankly, it scared me a little, but I'll pass it on.  The idea is to
> use sound.  The tech would use a screwdriver as a sounding bar
> between the casing and his ear.  Then the head was tightened down
> until you could just hear it start to brush.  I don't remember for sure,
> but I'd have to think that you would then back off just enough for
> the brushing sound to stop.  I don't recall whether the article said
> that this was done with the motor running or the drum was being turned
> by hand, but if it were my machine, I'd set the heads turning the drum
> slowly by hand and then check for any brushing sound when the motor
> comes up.
YIKES!!!  Well, I think the first order of business is to 
check and possibly replace the drum bearings.
Then, get a sensitive dial test indicator  (I have an old 
Federal with .0001" markings, and a Federal
electronic indicator that reads down to 50 uInch.)  Make up 
a fixture to mount it on the drum frame and see if the 
surface of the drum is still true.  It is possible that the 
bearing inner races might need to be turned on the shaft for 
minimal runout.  If the surface has several wobbles per rev, 
then I think the drum will not be serviceable without 
turning the oxide true again.  I have NO IDEA how thick the 
oxide might be on these.  It did seem like the G-15 oxide 
was pretty thick, maybe .010" or so.  If the above procedure 
is for real, then it pretty much had to be, to survive such 
abuse.

Doing some totally off the wall calculations in my head, I 
figured out the data density would have been in the range of 
100 bits/inch on the drum circumference.  Calculating from 
the circumference and the number of bits per track, you 
should be able to calculate the bits/inch.  Assuming pretty 
wide gaps between the head pole pieces, and the bit density, 
it should not require insanely small head flying height.

You can fairly easily get brass shim stock down to .001" or 
so (kitchen aluminum foil is about .0015") to use for a head 
height setter.

So, does the LGP-30 have permanent storage on the drum, or 
is it erased by a magnet at every revolution?

The G-15 modeled the drum just like a mercury delay line, it 
had a write head, a read head and a bar erase magnet on 
every line, so there was no gap for the write head to turn 
on and off between words.  The FF that drove the write amp 
normally recirculated the data from the read head, except 
when a word was being written.  Each drum track had a long 
line (for program and data) and a short line. Most of the 
short lines held 4 words, so they were quickly accessible.  
One short line only held one word, that was the accumulator.

Jon