LGP-30 Memory Drum Update

Cory Heisterkamp coryheisterkamp at gmail.com
Thu Jan 5 07:35:36 CST 2017

On Thu, Jan 5, 2017 at 3:44 AM, Klemens Krause <
krause at informatik.uni-stuttgart.de> wrote:

>> http://www.radar58.com/temp/drum.jpg
>> http://www.radar58.com/temp/drum2.jpg
> From the fotos your drum looks better than our working one, with
> the exception of the large engraving on the right side. Especially
> the left part, were the registers are located looks very good.
> On our drum are also some dark traces without showing the Al of
> the drum.

That is encouraging news. The engraving on the right appears to be machined
into the drum as it's much deeper than the oxide finish and completely
uniform. I'm not sure what purpose it serves, if any. I've adjusted the 7
heads along the left side as one was just making contact.

> I can't see the 1" long spots you're speaking from. How much is
> "a couple"?

There's a total of 3 such marks on the drum not visible in the photo, 2
caused at some point in the past as the heads that caused them were
relocated from those tracks, and one short scratch caused by me after
having removed all but the last head bar assembly and checking for drag. It
seems each bar had at least one, if not more, heads in contact with the
drum, which I was not expecting.

> In your place I would try to revive the drum track by track:
> Looking for the registers, find out if the timing tracks S1 to S3
> are ok, then looking for tracks 0 to 3 in which the loader,
> program 10.4 sits, then track 63, which is used for booting the
> loader. Even if you have only some more tracks you can run simple
> hand coded programs on your machine. Just think at a KIM-1 single
> board computer with 256 bytes of RAM: People had fun with it! :-)

I think this is a good idea. I hadn't planned on running anything other
than hand coded programs, and having at least several tracks functional
will help me in troubleshooting the rest of the machine. I can then
transition to a solid state mem later.

> Cleaning the originally coated drum? I'm not sure how to do this.
> We clean our RK05 disks in a very robust way: with cheap burning
> spirit and paper towels. They have similar technology: Al-base
> with iron oxide coating. We rubbed away thick black traces from
> occasional head crashes and we never removed the oxide coating
> with this torture. In every case the disks were 100% error free
> after this. But in this case the risk ist low: the disks had errors
> and if we ever would have washed away the oxide coating, we have
> enough other disks to experiment with other solvents.
> The LGP-30 drum is much more singular, so we never tried to clean
> the surface to avoid the risk of cleaning away the coating.

I used a clean, soft, paper towel without any solvent or liquid and gently
ran it from left to right while rotating the drum. This picked up a little
fine oxide, but whoever serviced the drum last did an excellent job of
sealing it. After they replaced the plastic cover they ran strips of
electrical tape (which was still firmly stuck) down both sides. The air
filter is also still present on the machine which is a good sign.  -C

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