Friday, August 18, 2017

Xerox Alto work, donated Hawley mouse, and work on some other components


We received a donation of a Hawley (mechanical) mouse for use with our Alto system, provided by Dave Redell. It was very clean cosmetically, but had a few small internal issues. This mouse turns mechanical encoder wheels with multiple 'fingers' riding on the wheels, but one of the fingers was bent out of contact. 

The finger was easily corrected. We then tested the mouse operation and found it failed to work well with horizontal movement, but was great when moved up and down. Some investigation inside the mouse turned up some stuck bearings. They responded well to a light machine oil and some manipulation.

The mouse is a delight, smooth moving and very precise. The major downside of these mice is lint and dirt, which clogs the inside after it is used awhile. Regular cleaning will address this problem.

We turned our attention to the other Alto, belonging to Bruce Damer and his Digibarn collection. Having finally repaired the power supplies, we did a full load test on all of them. Some of them had capacities of 12 to 15 amperes, but the main 5V supply we tested to the 60A capacity of Marc's HP Electronic Load.

Next up was the CRT. The display was actually wired to hook to a more modern Xerox system, such as a Dandelion, which uses a remote peripheral bus that is completely incompatible with the Alto. Ken Shirriff investigated to find that the new protocol is implemented by a board that hooks to the original vertical, horizontal and video lines which are what our Alto wants.

Ken built a cable that interfaced the new display to the original Alto display connector. We hooked up the CRT test tool loaned to us by the Living Computer Museum. The screen stayed black, but when the power switch is flipped off the pattern from the test tool is visible, bright and clear, as the monitor image collapses.

We moved the monitor to the workbench and began tracing signals to find the fault. First up, we monitored the display sweeping and doing flyback. Next we traced the video input signal through the amplifier stages all the way to the connector that leads to the CRT itself. The signal is fine, which means we have a more subtle problem that is blocking the electron stream from hitting the phosphors on the tube face.

We ran out of time, but the next time we get together we will trace through the voltages and signals on the various grids and other CRT elements. Likely we will find one of the grids is markedly more negative than the prior one, due to some component issue. We expect that when the power switch is flipped off, some voltages decay faster than others, unblocking the stream for a brief time so that we can see the pattern on the screen.

We looked over three disk drives we have on hand (other than the drive in the Alto). All three have crumbling foam at the point where clean air is forced through the cartridge. Crumbling bits of foam will lead to head crashes, as we have already experienced. Next session, we have to clean and replace these foam gaskets. 

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