LGP-30 Memory Drum Update

[Cory Heisterkamp]

On Jan 3, 2017, at 2:22 PM, Klemens Krause wrote:

> 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
> tracks.
>> 
> 
>> 
>> 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.
> 
> Klemens


Lots of great ideas guys! After work today I cleaned the 64 track heads and made temporary 0.004 spacers using paper strips and a hole punch. This puts all the head assemblies in the clear for now so nothing can get damaged. Using Klemens suggestion of tinfoil, I made up the perfect 0.001 feeler gauge and set the gaps for the short registers. Adjusting these is a little tricky since you first have to break loose the head securing screw, then adjust the height screw. However, the securing screw can take what is an acceptable gap down to a drum contact once torqued. A little 'back and forth' while stationary and that's resolved but perhaps there's a better way to adjust these while the machine is running (as has been suggested while watching the waveform on a scope). I don't think I'm that brave.

Anyhow, my next move is to spin the drum up on the machine (when it arrives) with just the clock and short register heads and then see how much expansion we're looking at. I can then bring in the other tracks heads assembly by assembly once I'm comfortable with the runout and expansion. Spinning it by hand reveals quiet bearings and a nice tight assembly.

In the interim I'll start work on the Flexowriter. Something I'm much more comfortable with.  -C