Processor Technology
SOL 20 Restoration


Because of our recent success with reviving two MITS Altairs we decided to try another S-100 system. We moved a Processor Technology SOL 20 from the warehouse to the Lab space, cleaned and inspected the inside of the case. We disconnected the power supply from the motherboard and the S-100 expansion chassis, and reformed the electrolytic capacitors in the power supply. We plugged it, turned it on, and measured the power supply voltages. The +5 VDC and the +/-12 VDC for the motherboard were OK. The +/-16.5 VDC for the S-100 bus was OK, but the +7 VDC for the S-100 bus was dead. (It looks like the +7 VDC output has a separate ground wire, so we likely didn't have the DVM connected correctly.) Since it didn't have any S-100 boards installed we reconnected the power supply harnesses and powered it on. We didn't see any smoke, flames, or sparks, and it didn't blow the fuse, so we called that a success.

We don't have the video cable so at this point we don't know how much of the system is working. (We just bought a PL-259 to RCA video cable on eBay)

We tried a few keys on the keyboard and noticed that shift-lock didn't work. We disassembled the keyboard and found that all of the foam pads for the keys were really degraded. We will order a replacement set of keypads from


We connected the SOL to a monitor and powered it on. The screen was blank except for the bottom two rows that contain alternating number 9 and a box if the display control characters switch is turned on. We looked at the signals on the video RAM. Both have reasonable signals so it looks like the video circuitry is working.

The legs on the ICs are covered with black corrosion. This will prevent a good electrical connection between the ICs and the sockets on the motherboard. We started removing the ICs, cleaning the legs with with DeoxIT and a brass brush, put a little DeoxIT on the socket contacts, and reinstalled the ICs. We are about half way through this process and should finish it next week.

All of the old degraded conductive pads in the keyboard have been removed. We pulled a Wang keyboard from the warehouse so we can harvest the pads and put them in the SOL keyboard. That should be completed by next Saturday.

After we finished our work for the day we found a description of a test on another WWW site that said to remove the personality module (the board that contains the monitor EPROMs) and power the SOL on. The 8080 CPU will execute a sequence of RST 7 instructions because with no EPROMS installed the CPU will just get 0xFF for data. This will fill all of the system and video RAM with and alternating 0X00 and 0X39 pattern which will will display lines of alternating 9 and 0.