[cctalk] Re: ANITA ((was: Experience using an Altair 8800 ("Personal computer" from 70s)

Christian Corti wrote: ..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