On Sat, 25 May 2024, Rick Bensene 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.
We had replaced (or better, bypassed) several of the rectifiers with 1N4007. But most of them are still ok in our machines. I guess we have about five Mk8 here, some are in quite bad shape. The biggest problems within the Anita is caused by the high-value carbon resistors. Especially the 20 megohms go bad/high(er) resistance, so the thyratrons (cold-cathode relay tubes) don't ignite reliably.
Interesting to note that many ANITA Mk8 machines have a single transistor in them. It's in the power supply. The designers were
No, it is not in the power supply. The transistor is used to invert and drive a control signal (called HIGHWAY OUT in the schematics), going to the cathodes of the individual register state drivers (the ECC 81 at the back of the machine). All Mk8 should have this transistor. I guess that the designers discovered some issues and needed a quick and simple fix.
comfortable enough using these relatively fussy gas-discharge logic devices as digital devices(they had developed machines like Colossus
They are designed as digital devices, hence called cold-cathode relay tubes. All thyratrons act digitally, there is only an on and an off state, both very distinct.
Christian
