In 2010 I reformed the power supply caps and did some general cleaning. Look back at it then and the other items I obtained. Then time passed ... now it's 2025, what happened? In 2023-24, my friend Neil Cherry got me interested in 6800 again. I bought some SS-50 cards and looked at my accumulated SWTPC items over that period. Now in Jan-Feb-Mar 2025 I'm getting Bruce's system back on line.

general strategy in 2025

The first set challenges are to clean up the rust, replace components. also means reforming the power supply to get it back to proper voltages. Then figure out Bruce's modifications and probably undo some of them.

I'll have to determine the operation of the TTL SS-30 card, because several mods of the 6800 system are related to its use. More likely I'll extract the TTL SS-30 card from my restoration and reverse some or all of the modifications. Then I need to test a minimal 6800 board set to establish ROM monitor operation. Then I can test the other SS-50 and SS-30 cards, finally the floppy controller, and then with a restore disk drive bring up a FLEX operating system.

My particular strategies are to examine the various cards and see what their condition is and what modifications were made. Also general clean-up strategies of components and connectors. I worked on this out-of order of my general strategy, but I'll report results by-card rather than chronologically.

The 6800 system as recieved, had three 8K memory boards addressed to the lower RAM 0000-5FFF, and a 4K board at 7000-7FFF. With a working ROM monitor, I can add and test that memory. I can also test the serial-S board. Ultimately I'll test the disk controller, a floppy drive box, and bring up FLEX 1.0.

What software will you run when it's restored? The ROM monitor is SWTPBUG, a simple monitor to do simple testing. After restoring the floppy controller and drives, I'll run FLEX, a common 6800 floppy operating system. There's many disk-images of FLEX software archived on the Web. There's some additional FLEX disks that came with the computer. I'll see if they are single or double sided drives and thus disks. It uses a 1771 FDC so they ought to be "readable" (and images writable) on my ancient 486 system using Dunfield's imagedsk.

Restoration by the board:

restoration of the chassis and power supply are discussed below.
Mods of the MP-B motherboard are discussed below.
Mods of the MP-A CPU board are discussed below.
Functions of the TTL SS-30 board are discussed below.
Repairs of the SS-30 MP-C serial board is discussed below.
Net result of all Bruce's mods
Repairs of the SS-30 MP-S serial board is discussed below.
MP-M 4K memory is below
MP-8 M2 8K memory is below
another MP-A card is below

2025: 6800 chassis, power supply

I pulled the SWTP 6800 chassis out of storage, pulled out the SS-50 cards, and looked hard at the chassis and mobo. There's some rust here and there on the IC's and who knows how the mobo is underneath.

I popped off the MP-B motherboard and the underside looks a little dusty but electrically OK. I worried about the Molex connectors and damage from all the force needed to insert and extract the boards. There's a number of standoffs glued to the bottom which provide insertion relief. Note a bit of rosin crud near this standoff. I may add wood under the other SS-50 connectors.

Here's a view of the keyswitch area of the chassis. The keyswitch (far left) switches the AC line side of the power supply. There's also a reset button and power LED. In the power circuits, there is a 10A fuse, seems high.

Reforming the caps and monitoring the power supply voltages was uneventful. I took notes which essentially said the DC voltages came up to their unregulated values; I ran 130V AC eventually to be sure the caps had a voltage margin. My AC line voltage generally runs 120VAC. Unregulated DC no-loads at 120VAC was 10.1V (nominal 8V, on-board regulated to 5V), 21.2V (regulated to +12.2V) -21.2V (regulated to -11.9V).. At 87VAC, voltages were 6.7V, 14.9V, -15.0V no loads which gave 5V -12V +12V after regulation. the MP-B has 1V and -12V regulators TO-220 good to under an amp; and a local T0-220 +5V regulator for the ICs which decode the SS-30 selects (memory mapped I/O slots, like the Apple II).

2025, MP-B motherboard & mods

Some of the mobo IC pins were rusty, typical TI 1970's black pins. I replaced the ICs and cleaned the sockets with brush and added DeOxit using a copper wire as a brush. The sockets did not accumulate obvious damage. Here's the cleaned up ICs on the mobo.

modifications: I took time while the mobo was clear, to determine what mods were made. Here's a sketch of the mobo board and mods, also the location of key SS-30 cards.

Later, I refined the sketch to include features of the SS-30 bus. Note the wire from the back of the SS-50 bus to the SS-30 bus. UD-1 on the SS-50 is user-defined, open. It's wired to UD4 on the SS-30, again user-defined. A jumper on slot 5 is from UD-3 to slot 5 /select. That's for the DC2 floppy controller, gives it more address space.

There's also a baud rate switch, stop-bit switch, and a "data" LED on the front panel. A multi-poll switch is wired to the SS-50 baud-rate lines, at the front of the chassis. The slected baud rate is wired back to the to the top of the wirewrap card on the SS-30 slot 0. The stop-bit two pole switch and "data" LED are also wired to that card. I'll detail that card later. Essentially it proves to be a cassette interface, but also a new baud-rate generator and 6800 clock, replacing both on the MP-A CPU. I'll reverse the baud and clock mods, run without the custom ss-30 card.

2025, MP-A board de/repopulated

I found a number of mods on the MP-A 6800 CPU board. The 2010 photo shows some wiring and missing chips in the sockets, lower left. Additionally these IC's and wire show some mods, and here's additional mods of the ICs Note the small jumper in the IC socket. These seem crude but they were cheap, effective and reversable: this is how things were done in the mid-1970's. Here's my schematic of the mods. Later I'll add a CP-A schematic from SWTPc.

The MP-A related mods serve two purposes. One is, to route a clock to the CPU board to drive the CPU clock circuits. The MP-A has arcane transistor clock drivers, a design right from early Motorola 6800 data books. The other mod, is to disable baud rate generation to the SS-50 bus. By removing the 14411 baud rate generator and 74L04 driver IC, those signals are disabled. By design, the TTL card drives the baud-rate SS-30 lines, which are directly connected to the SS-50 lines.

Additional MP-B wire and MP-A mods, route a clock signal from the SS-30 TTL card on a user bus lines to the MP-A's clock-driver circuit. This sketch of the motherboard shows added wires from SS-50 line UD-1, to SS-30 line UD4 - that connects a UD4 pin on the custom TTL card to a UD1 pin on the MP-A which feeds a 6800 clock signal. The switch wiring from the front-panel baud and stop-bit switches, is bundled along the mobo edge, to connectors atop the custom TTL board.

There's also a SWTPC mod for the SWTBUG ROM, an added address line for a larger ROM, which I will retain. The small RAM on the CPU board is still addressed at A000H for the 'bug ROMs (SWTBUG). So that tells me, once I restore clock and baud rate to local CPU board generation, I should be able to terminal with the ROM monitor with the CPU board and probably the serial 6820 board. That's good, only two boards and the motherboard to debug.

TTL SS-50 wirewrapped counter card

Additional MP-B wire and MP-A mods, route a clock signal from the SS-30 TTL card on a user bus lines to the MP-A's clock-driver circuit. This sketch of the motherboard shows added wires from SS-50 line UD-1, to SS-30 line UD4 - that connects a UD4 pin on the custom TTL card to a UD1 pin on the MP-A which feeds a 6800 clock signal. The switch wiring from the front-panel baud and stop-bit switches, is bundled along the mobo edge, to connectors atop the custom TTL board. There's wiring from the top of the TTL card to a DB-25 connector on the back chassis.

I'm not discussing the baud rate generator or cassette features of the TTL card at this time. My immediate plan is to operate and test the system without the TTL card, out of caution.

By designer's schematic and tracing those chassis & mobo modifications, I determined the following about the TTL card. The SS-30 TTL wirewrapped board is a baud rate generator , 6800 clock source, and decode/encode for cassette audio. Baud rate and cassette and CPU clocks, are wire-wrapped around a TTL hardware divider based on a 1MHz crystal clock. There's also analog circuitry for the cassette endoding/decoding to/from serial I/O. The board drives the baudrate lines on the SS-30 (and the -50).

There was no TTL card chip/IC layout drawing. So I made one by observation of the card's ICs. It correlates with the schematic.

MP-C serial 6820 SS-30 card

Photo later of the MP-C. This view includes the MP-C with the MP-S and TTL cards. The MP-C card uses a 6820 to bit-bang serial. It's something that Motorola did early in the 6800's distribution before the 6850 UART and supported in their ROM monitor. SWTPC replicated the capability and I think supported it in SWTBUG, the ROM on this CP-A CPU board.

There's wiring from the top of the MP-C card to a DIN connector on the back chassis.

SWTPC docs show the general RS-232 serial hookup for either the MP-S or MP-C card.

Net result of all Bruce's mods

Here's a schematic of all the mods made on Bruce's SWTPC 6800 system. Here's a drawing of the wiring on the MP-B motherboard.

The net effect is 1) a TTL card for SS-30 baud rates and MPU-A CPU clock. 2) wiring from the TTL card to the MPU-A for the CPU clock. A mix of wires, PC board cuts, bent-pin or removed ICs, and use of the non-dedicated "user lines". Reference the other sections on cards mentioned for specifics and sketches and photos of the mods and wiring. I'm not discussing the baud rate generator or cassette features of the TTL card at this point.

SWTPC 6800 boards not yet in play

The following are cards I've not tested or worked on yet. As I work on them I'll add content. They are likely standard as available from SWTPC.

MP-S serial SS-30 card

The MP-S card had some damaged IC chips and sockets, which were replaced. The 6850 UART socket was old Molex pins; I replaced them with a proper socket. The ceramic/gold 6850 had damaged pins, I replaced it with a plastic cased part. Here's the result and the removed chips shown on the side.

Here's a view of the critical SS-30 boards in place. Here's another view. These roughly show the cabling associated with those cards. There's wiring from the top of the MP-S card to a connector on the back chassis.

SWTPC docs show the general RS-232 serial hookup for either the MP-S or MP-C card.

MP-M 4K memory

The MP-M 4K memory card, addressed at "7" or 7000H-7FFFH.

MP-8 M2 8K memory

The MP-8 M2 4K memory cards, addressed at 0000H, 2000H, 4000H. They cover memory from 0000H to 5FFFH.

another MP-A card

In June 2024 I had an opportunity to buy another MP-A board at a low price. It needed many chips and had old-school sockets, even Molex-pins. But the price was good and I obtained it. I've repopulated it with all the odd TTL and a 6830 ROM. If I test it on this SWTPC 6800 system I'll report it here.

Copyright © 2025 Herb Johnson