SOL-20 Update from MARCH Workshop

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	by Bill Degnan - 07/05/2012 02:03
A photo of the rear power supply components taken through a magnifying glass. Click image for larger view. Here's a pic of the bottom of the board. Note the repaired trace in the corner. Re-seating the chips and/or using the system for a few hours boosted the overall motherboard power reading closer to +5V (4.86). These readings are almost exactly the same as a working SOL-20 I used to compare with. I decided it was time to probe into the logic components next. A photo of the display after halting the processor by using the in-circuit emulator. I don't have a picture of the display while the system is running, but the system sends a constant stream of the ASCII character FF to the screen. Herb Johnson set up his oscilloscope so I could probe the circuits, plus none other than Lee Felsenstein was there for a few hours to observe our progress towards restoring Herb and my SOL-20. Not bad company. Bill S also brought his SOL and made a few OS disks for us. Fortunately the SOL-20 Herb Johnson brought along worked (he had never tried it before, what luck!) I was able to use his working system to compare and contrast with mine. We found little differences in performance after a number of hours of testing. The problem with my system is not obvious and more work is required. Where things stand at the moment is that the system cannot write to RAM. It gets as far as reading (or attempting to read) the SOLOS ROM, but whatever the next step is after that fails. We spent many hours working through the checkout procedure from the SOL-20 manual. We were not able to find the cause of the problem yet, I will post our findings and update this post asap. If you look at my earlier post the in-circuit emulator showed that I did see something in C000 two years ago, I need to re-test that. Maybe I simply have some bad RAM. If the first video RAM chip were bad that could possibly explain why the system cannot get past this point. It could also be a TTL chip issue I have not id'd yet. Herb started working on his keyboard pads. I have not yet started restoring the keyboard, but I am able to use the keyboard without the cover and keys, by pressing my fingers on the bare board connectors. In the earlier post, I mentioned how the two edge connector lines on the personality board were cut...this is done because at some point PT changed the type of ROM they use in their personality modules. The newer version does not need the signals so they cut the traces. To be sure, my personality module was tested in Herb's working system. So we know that's OK. NEXT: Test RAM, post more details from the workshop (I have more notes). Then set up my oscilloscope and run more tests by following the check-out procedures from the manual. Also, there are online resources found on http://www.sol20.org New Photos posted from June 30 2012 workshop About Lee Felsenstein: http://en.wikipedia.org/wiki/Lee_Felsenstein Reply