By Nermeen Nabil Khear 
There’s a Blue Bendix in Texas, and thanks to [Usagi Electric] it’s the oldest operating computer in North America. The Bendix G-15, a vacuum tube computer originally released in 1956, is now booting, and running code from paper tape. [David, aka Usagi] received the G-15 about a year ago from The System Source museum. The goal was to get the computer running so museum patrons could interact with a real tube computer. We’ve been following along since the project began.
[Usagi’s] latest G-15 video covers the last few problems on the road to running code. The biggest hurdle was the fact that the system wasn’t responding properly to the GO button on the typewriter. [Usagi] was able to isolate the issue down to a flip flop and then to a particular signal on an AND gate — the RC signal. The gate appeared to be bad, but swapping the entire circuit card multiple times had no effect. Something else had to be going on.
After hours of troubleshooting and a bit of hair-pulling, [Usagi] changed a diode circuit card downstream of the suspect card. This miraculously fixed the problem. It turned out the diode card had a tiny solder bridge since it was built in the 1950’s. This bridge put a heavy load on a buffer, causing grid leakage. For those of us who aren’t old [TubeTimers], grid leakage is a tiny current from the grid of a tube into the drive circuitry. Leakage is present on all triodes, and tube testers would often misdiagnose good tubes as bad for this reason.
Once the bridge and a few other problems were fixed, the machine sprang to life, not with a roar, but with a solid thunk as it slammed the incredibly wide typewriter carriage into a nearby shelf. If you do nothing else this year, watch the video from the 20-minute mark. You get to see the pure joy a hacker gets when their project starts to work.
The Bendix was executing DIAPER — Diagnostic Program for Easy Repair. DIAPER runs a series of tests on the machine and rings a bell every time a test passes. Not a little bell in the typewriter, but a big 120 V beast hiding inside the computer itself. Ding, fries are done indeed!
[Usagi] did have some help this time around — thanks to a tip from [Avery] he contacted HP Agilent Keysight to inquire about a basic scope. Apparently, they know his videos and are huge fans of the Bendix because they sent him a really nice 4-channel digital oscilloscope. It definitely helped push the Bendix over the finish line! We love seeing companies give back to the community this way — and hope to see more in the future.
Now, this isn’t the last Bendix G-15 video from [Usagi]. There are several more tapes to run a full DIAPER test. The typewriter itself needs quite a lot of work before it will accept keystrokes, and we’re sure [Usagi] has a few more surprises up his sleeves.
You can still find a few tube computer projects floating around. You can even replace your 555 with some.
This articles is written by : Nermeen Nabil Khear Abdelmalak
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