Christian Corti wrote:
> The Anita electronic desktop calculators are a perfect example for the usage
> of
> selenium rectifiers in logic gates.
..and anyone who has restored one knows that the vast majority of the
back-to-back selenium diode packages have to be replaced with something else
as they no longer function properly. Ambient moisture kills Selenium as a
semiconductor, and even though these devices were packaged to avoid that to
some degree, after 60 years, stuff happens.
Many restorers resort to de-soldering the dual-diode packages from the circuit
boards, hollowing out the package (removing the Selenium rectifiers and the
potting material used) and installing back-to-back conventional Silicon diodes
that are rated for the appropriate voltages involved in these machines, potting
the diodes in place with some kind of material (epoxy?), and re-soldering the
package to the circuit board. These calculators used gas-discharge active
logic elements (e.g., thyratrons and dekatrons) and used (relatively speaking)
high voltages for their logic levels. Fortunately, these gas-discharge devices
seem to fare quite well with time, and though some do fail due to atomic-level
outgassing or simple breakage, the majority of them work just as well the day
the machine came off the assembly line.
Such practice with the Selenium rectifier modules makes the calculator look
original if done carefully, and allows it to function when operation was
impossible with the original devices. It is an extremely tedious and
time-consuming process, as there are a great many of these devices used in the
first-generation Sumlock/ANITA calculators.
I applaud anyone with the courage and patience to perform such surgery on these
unusual artifacts. Fortunately, the circuit boards are quite robustly made, and
the traces are large and well adhered to the base material of the circuit board
(unlike many later calculators), making such an operation feasible.
I am not brave enough to try this with the museum's ANITA Mk8. After 25+ years
of owning this artifact, I have not even tried to apply power to it in any
fashion, and probably never will. It is one of the very few calculators in the
museum that is probably not in operational condition, as I strive for all of
the exhibited machines to be operable and available for visitors to the
physical museum to play with if they desire. I'm content to leave it as it is
for a display machine, as it is in very nice original condition.
Interesting to note that many ANITA Mk8 machines have a single transistor in
them. It's in the power supply. The designers were comfortable enough using
these relatively fussy gas-discharge logic devices as digital devices(they had
developed machines like Colossus using this technology considerably before
transistors were a thing, so there was certainly historical precedent), but the
transistor was just fine for an analog purpose in the power supply.
Boy, did they ever get it backwards (in terms of the longevity of gas-discharge
logic elements in electronic calculators and what became the ubiquitous use to
transistors)!
Not intended at all to slight the accomplishment of Sumlock Comptometer in the
development of these calculators. They set the stage for the explosion of
what was to become a many hundreds of million dollar market by the end of the
decade, not to mention setting the electronic calculator up to be the driving
force behind integrated circuit development for a consumer-oriented device.
ICs before their development for use in calculators were only for big mainframe
computers, military weapons systems, the spooks at places like the NSA, and the
space program. For that matter, the ANITA Mk7/8 could be said to be the
progenitors for the development of the CPU on a chip, and by extension, the
personal computer.
Notice I didn't specify any machine, or say "first". Slippery slope there.
Rick Bensene
The Old Calculator Museum
https://oldcalculatormuseum.com
