I purchased this Sol-20, together with some S-100 expansion boards, documentation, an 8″ floppy drive, and a bunch of floppies, as surplus from the Systems Design department at the University of Waterloo. Probably got it somewhere around the year 2000.

• Serial Number: 900010
• Main board ASSY 110200 rev B
• Expansion boards:
  • Central Data Corp 64K RAM
  • Thinker Toys Disc Jockey I
  • Godbout EconoROM 2708
  • Digital Research Computers 16K EPROM
  • Central Data Corp 64K RAM (another one)
• Floppy Drive: Discus 2+2

It worked when I got it, circa 2000, but there was very little information about it available at that time, and I didn’t know how to get it to boot from the floppy drive.

But, as of now (2023), it’s not fully-functional. There were two shorted capacitors in the power-supply, which I’ve fixed, so it boots now. But the keyboard, one of the old “foam-and-foil” capacitive keyboards, has deteriorated and is unusable.

Power Supply

I believe my Sol-20 is probably of later manufacture than most that I see online. The power-supply, in particular, appears to be a more integrated/simplified design than those I see in most photos and documentation (ie, schematic diagrams) that are available online.

My power-supply assembly is marked “ASSY 110120 rev A”, and the PCB part is “ASSY 110210 rev A”.

Differences from the more common power-supply are:

• The really large +5V electrolytic ripple-filter capacitor is mounted directly to the PCB, rather than to the chassis.
• +5V supply is implemented using a simple 78H05 regulator chip, rather than discrete components.
• There is no provision for different line voltages. 120V only.
• The transformer has only two secondary windings: an 8VAC winding and a center-tapped 27VAC winding. The 8VAC winding supplies both the regulated +5V to the CPU and the unregulated +8V to the card cage. )Older Sol-20s used a separate winding to supply unregulated +8V to the card cage.)

Here are some close-ups of the PCB itself:

I reverse-engineered the schematic of the ASSY 110120 rev A power-supply, which I am sharing here (click for PDF version):

The shorted capacitors I replaced were the 2.2uF tantalum bypass caps at the inputs of the 7812 and 7912 regulators, C7 and C8. Judging by some flux residue around them, I suspect this was not the first time they had been replaced.

Personality Modules

When I received the Sol-20, it had two personality modules. The EPROM module was installed in the usual slot. The PROM module was floating around loose inside the power-supply! Good thing I found that before it shorted something out.

Both modules seem to have some variant of SOLOS on them. I have not yet determined what differences may be present.

S-100 Expansion Boards

When I received the Sol-20, it had five S-100 expansion boards installed in it. I think some of these had just been shoved in there for no reason, because there was way more memory installed than was actually addressable, and there were address conflicts. But here’s what I have:

Central Data Corp 64K RAM

• Four banks of 16K
• “4116” DRAM chips
• Only three banks populated with RAM chips (48K)
• Current configuration:
  • Bank 0: 0000-3FFF
  • Bank 1: 4000-7FFF
  • Bank 2: unpopulated
  • Bank 3: C000-FFFF
  • Deselection jumpers are set to poke holes in Bank 3 for:
    • C000-C7FF (SOLOS personality module)
    • C800-CFFF (Sol-20 2K onboard RAM)
    • E000-FFFF (Disk Jockey is at the start of this range)

Thinker Toys Disk Jockey I

• Kind of a cheap-and-dirty design. All the address decoding is done by a “6301” 256 x 4 PROM, so the memory address range is basically hard-coded. Can only be changed by ordering a custom PROM from the factory.
• The standard PROM gives the address range E000-E3FF, broken down as follows:
  • E000-E1FF: ROM routines
  • E200-E2FF: RAM
  • E300-E3FF: Memory-mapped IO registers

Godbout ECONOROM 2708

This board is configured really goofy.

• 16K of EPROM, organized in four independent blocks of 4K
• Current configuration:
  • Bank A:
    • E000-EFFF
    • Installed chips: “ALS8 1” through “ALS8 4”
    • Disabled
  • Bank B:
    • F000-FFFF
    • Installed chips: “ALS8 5” through “ALS8 8”
    • Disabled? The switch is in the “down” position, which is undocumented.
  • Bank C:
    • B000-BFFF
    • Enabled, but no chips installed
  • Bank D:
    • C000-CFFF
    • Disabled
    • No chips installed

Digital Research Computers 16K EPROM Board

• 2708 (1K x 8) EPROMs
• Addressable on 16K boundaries
• Specific EPROMs can be disabled by adding/removing diodes.
• Current configuration:
  • Address range: C000-FFFF
  • 0 wait states
  • C000-DCFF are unpopulated and disabled by diodes
  • DD00-FFFF contain “ALS-8” EPROMs dated “12/6/80”.

Central Data Corp 64K RAM

• Four banks of 16K, each independantly addressable
• “4116” DRAM chips
• Full 64K populated
• Current configuration:
  • Only the third bank of 16K appears to be enabled, at address 8000-BFFF