Osborne 1 & Kaypro II Machine Hub
CP/M 2.2 sewing-machine luggable workbench reference — Astec PSU RIFA alerts, 5″ & 9″ CRT servicing, Osborne video shunt jumper pinouts, 5.25″ floppy & Gotek guides, and Z80/WD1793 diagnostics.
Astec / Boschert PSU RIFA Mains Filter Alert — REPLACE BEFORE POWERING ON
Both the Osborne 1 (fitted with an Astec AA11190 open-frame switching PSU) and the Kaypro II / 4 (fitted with an Astec AC8151, Boschert, or California DC supply) contain three notorious amber epoxy RIFA PME 271 M mains filter capacitors across the AC line input. It is practically a rite of passage in vintage computing that powering on an un-serviced Osborne 1 results in all three RIFA caps exploding within 5 minutes, filling the workshop with thick white smoke! Always replace the RIFA caps with 275 V–310 V AC X2/Y2 polypropylene safety capacitors before applying mains voltage.
1 RIFA Replacement Values (Astec PSU)
Locate the three rectangular golden-yellow RIFA PME 271 M / Y capacitors near the AC input header and voltage selector jumpers:
| Quantity | Value & Safety Class | Modern Replacement Spec |
|---|---|---|
| 1× Line-to-Line | 0.1 μF (100 nF) 250 V AC X2 | Kemet R46 / Vishay MKP 0.1 μF 275–310 V AC X2 (15 mm or 20 mm pitch) |
| 2× Line-to-Earth | 0.0022 μF (2.2 nF) or 0.0047 μF (4.7 nF) Y2 | 2.2 nF / 4.7 nF 250–300 V AC Y2 safety film or ceramic disc caps |
115V vs 230V Mains Selector Strap: On the Osborne 1 Astec AA11190, check the wire link on the PSU PCB (`LINK IN` = 115V US voltage doubler; `LINK OUT` = 230V UK/EU)! On later Blue/Grey Osbornes, check the rear voltage fuse cartridge orientation.
2 Multi-Rail Voltage Verification
Verify all DC output rails under load (remember: Astec switching supplies require at least one 5.25″ drive or logic board connected to regulate properly):
| Rail | Osborne 1 | Kaypro II (1982) | Feeds |
|---|---|---|---|
| +5.0 V DC | 4.90–5.15 V | 4.90–5.15 V | Z80A, 4164/4116 RAM, FDC, ROMs & TTL |
| +12.0 V DC | 11.6–12.4 V | 11.6–12.4 V | CRT Monitor B+, 5.25″ Spindle Motors, 4116 VDD & RS-232 |
| −12.0 V DC | −11.4 to −12.6 V (via 7912) | −11.4 to −12.6 V | RS-232 negative swing (MC1488) |
| −5.0 V DC | Not used (uses 4164s!) | −4.75 to −5.25 V | Early Kaypro II (81-110) 4116 DRAM VBB (Pin 1) & FD1793! |
3 Tantalum & Electrolytic Short Checks
- Early Kaypro II `81-110` 4116 DRAM Alert: The original 1982 Kaypro II motherboard (`81-110`) uses eight triple-rail 4116 DRAMs (`+5V`, `+12V`, `−5V`). Measure `−5V` on Pin 1 before powering on with RAM installed! (Later `81-240` Kaypro II/2X/4 boards switched to single-rail `4164` DRAMs).
- Shorted 12V Tantalum Caps on 5.25″ Drives: If the PSU chirps/ticks and refuses to start, unplug the two 4-pin Molex power connectors from Drive A and Drive B — the radial teardrop tantalum caps on the MPI/Tandon drive logic boards frequently fail dead-short across the +12 V rail!
High-Voltage CRT Safety & Video Geometry Alignment
Safely discharging and servicing the Osborne 1's 5″ monochrome CRT (Motorola/Audiotronics) and the Kaypro II's 9″ green-phosphor CRT (Elston/Motorola).
1 CRT Anode Cap Discharge Procedure (~10–12 kV)
- Unplug mains power and remove the plastic case shell (Osborne 1: undo the 4 long underside chassis bolts; Kaypro II: undo the top hood screws).
- Clip one end of a heavy-gauge insulated jumper lead firmly to the bare metal CRT mounting frame / chassis ground braid (never ground through the digital logic PCB!).
- Clip the other end of the jumper lead to the metal shaft of a long, flat-bladed screwdriver with a thick insulated handle.
- Keeping one hand in your pocket, slide the screwdriver blade underneath the rubber suction-cup anode cap on the side of the glass CRT bulb until you hear/feel it tap the metal spring clip. Hold for 5 seconds, wait 1 minute, and touch again to prevent dielectric rebound before unhooking the clip.
2 Video Geometry, Focus & Raster Controls
When the display is squashed vertically, rolling, off-centre, or blurry, use a non-metallic hex/flat plastic CRT alignment tool (a metal Allen key inside a ferrite linearity/width coil will crack the brittle ferrite core instantly!):
| Control | Location | Symptom Fixed |
|---|---|---|
| BRIGHTNESS & CONTRAST | Front panel knobs | Too dark, or visible diagonal retrace lines when Brightness is set too high |
| V-HOLD & H-HOLD | CRT PCB trimpots (or rear holes on Kaypro) | Screen rolling vertically or tearing diagonally into slanted bars |
| V-SIZE (Height) & V-LIN | CRT driver PCB pots | Top of text stretched while bottom is compressed (also check dried-out vertical electrolytic caps!) |
| H-WIDTH Coil | Ferrite slug coil on CRT PCB | 52-column (Osborne) or 80-column (Kaypro) text cut off at edges (use plastic hex tool only!) |
| FOCUS | Pot on Flyback / CRT Neck | Fuzzy character dots in the center/corners of the phosphor screen |
System Architecture & Model Selector
Compare Adam Osborne's 1981 Osborne 1 (Tan vs Blue/Grey cases + Double-Density & Screen-Pac upgrades) against Andrew Kay's rugged 1982 Kaypro II / 4 / 10.
Osborne 1 1981–1983 • Lee Felsenstein Design • 64 KB RAM
The machine that launched the portable computing revolution — designed by Homebrew Computer Club legend Lee Felsenstein to fit under an airline seat and bundled with $1,500 of CP/M 2.2 software (WordStar, SuperCalc, MBASIC, CBASIC, dBASE II). To keep the 5-inch CRT legible without high-bandwidth electronics, the Osborne 1 stores a full 128×32 virtual screen in RAM and displays a movable 52-column × 24-line window scrolled horizontally using CTRL + Arrow keys!
Key ICs & Memory-Mapped Architecture
- CPU: Zilog / NEC Z80A @ 4.00 MHz
- Main RAM: 64 KB using eight 4164 (64K×1, +5 V single-rail!) DRAMs — plus a separate 9th RAM bank (2× `2114` SRAM) holding the 9th Attribute Bit (dim/underline) at `$F000`!
- PIA 0 (Keyboard & IEEE-488): Motorola MC6821P — scans the 8×8 keyboard matrix and drives the front IEEE-488 / Centronics bus
- PIA 1 (Memory Banking & Control): Motorola MC6821P — switches the lower 16 KB between ROM/I-O (Bank 1) and full 64 KB RAM (Bank 2)
- Serial ACIA: Motorola MC6850P — RS-232 serial port & modem pocket
- Floppy Controller: Western Digital FD1793-B (Single-Density 92 KB/102 KB FM) or FD1797 (with Double-Density 184 KB/204 KB daughterboard)
- System BIOS ROM: 4 KB 2732 EPROM (Rev 1.2, 1.3, 1.4, or 1.44) + 2 KB 2716 Character Generator ROM
Early "Tan Case" (OCC1) vs Late "Blue/Grey Case" (OCC1A) & Upgrades
| Variant / Upgrade | Identification & Hardware Notes |
|---|---|
| Early "Tan Case" (1981) | Macro-grained beige ABS plastic case with two heavy toggle latches and no Carry-handle bezel shroud. Front panel holds all I/O connectors (`RS-232`, `MODEM`, `IEEE-488`, `EXT VIDEO`, `12V BATTERY`). |
| Late "Blue/Grey Case" (1982–83) | Restyled light-grey inner bezel inside a dark blue weather-sealed outer shell with improved airflow vents and rear-mounted mains switch/fuse block. |
| Double-Density Upgrade Board | Small piggyback PCB plugging directly into the 40-pin FD1793/1797 socket on the mainboard — doubles floppy disk capacity from 92 KB (FM) to 184 KB / 202 KB (MFM) and enables reading Kaypro/Morrow CP/M disks! |
| Osborne Screen-Pac (80-Column) | Daughterboard plugging into the Z80A + Character ROM sockets with a coax tap to the CRT board — adds true switchable 52 / 80 / 104 columns on the internal 5″ CRT and outputs standard composite video on a BNC/RCA jack! |
Kaypro II, Kaypro 4 & Kaypro 10 1982–1985 • Non-Linear Systems • 9″ 80×24 Green CRT
Built like an oscilloscope in painted dark-blue/grey sheet aluminium by Non-Linear Systems in Solana Beach, California, the Kaypro II solved the Osborne 1's biggest complaint by fitting a full 9-inch 80×24 green-phosphor CRT and double-density 191 KB 5.25″ drives right from day one.
Key ICs & Motherboard Revisions (`81-110` vs `81-240`)
- CPU: Zilog Z80A @ 2.50 MHz (original 1982 Kaypro II) or 4.00 MHz (Kaypro 2X / 4 / 10)
- RAM (`81-110` Early Kaypro II): 8× 4116 (16K×1, triple-rail!) for Video RAM (`$3000` banked) + 8× 4164 (64K×1) for Main RAM
- RAM (`81-240` Universal Board): All single-rail 4164 DRAMs + Motorola 6845 CRTC video controller
- Z80 Peripherals: Two Z80A PIO chips (Centronics printer, system bank/drive control latches) + one Z80A SIO/0 (RS-232 serial + 300-baud keyboard serial interface) + SMC 8116 dual baud-rate generator
- Floppy Controller: Western Digital FD1793-B02 (Double-Density MFM, 191 KB SS/DD on Kaypro II; 390 KB DS/DD on Kaypro 4)
- Detached Keytronic Keyboard: Connects via a 4-pin / 6-pin RJ coiled telephone cord (`+5V`, `GND`, `300-baud Serial TX/RX`) with built-in keyclick buzzer — Note: Foam-and-foil capacitive pads inside Keytronic keyboards degrade and require new Mylar foil discs!
Osborne 1 vs Kaypro II Bench Comparison
| Feature | Osborne 1 (1981) | Kaypro II (1982) |
|---|---|---|
| Case Construction | Moulded ABS plastic ("sewing machine") | Riveted/welded painted steel & aluminium |
| Built-in CRT Display | 5″ B&W CRT (52×24 window on 128×32 RAM) | 9″ Green P31 CRT (Full 80×24 characters) |
| Stock Floppy Format | 5.25″ SS/SD 92 KB (or SS/DD 184 KB w/ upgrade) | 5.25″ SS/DD 191 KB (Kaypro 4 = DS/DD 390 KB) |
| Keyboard Interface | Parallel 8×8 matrix (ribbon cable to front door) | Serial 300-baud ASCII over RJ modular phone cord (Keytronic foam-and-foil) |
| Expansion / Printer | Memory-mapped IEEE-488 / Centronics edge card | Dedicated 36-pin Centronics + DB25 RS-232 |
Kaypro Keytronic Foam-and-Foil Restoration: If your Kaypro II boots to `Please place your CP/M diskette into drive A` and loads CP/M, but pressing keys only makes the keyboard click or does nothing, open the keyboard and replace the crumbled open-cell foam pads under the key stems with new Texelec / Laser-cut capacitive Mylar foil pads!
Dual 5.25″ Floppy Drive Servicing & Gotek Emulator Guide
Calibrating full-height MPI 51, Siemens FDD100-5, and Tandon TM100-1A/2A drives, plus Osborne 1 vs Kaypro II Gotek cabling.
1 Full-Height 5.25″ Drive Overhaul
Osborne 1s use MPI 51/52 or Siemens FDD100-5 40-track belt-driven full-height drives; Kaypro IIs use Tandon TM100-1A (SS/DD) or TM100-2A (DS/DD) drives.
- Osborne 1 Custom Drive Power Routing Warning: On the Osborne 1, the 5.25″ drives do NOT use standard 4-pin Molex power connectors! Instead, Osborne routed +5 V and +12 V power directly over unused even pins (Pins 2, 4, 6 & 10, 12, 14) of the 34-pin ribbon cable into a modified MPI/Siemens drive logic board! Never plug a standard PC/Tandon floppy drive straight into an Osborne 1 ribbon cable without isolating the power pins on the 34-pin header, or you will short +12 V directly to GND!
- Clean Head & Felt Load Pad: On single-sided drives (Osborne 1 & Kaypro II TM100-1A), inspect the round felt pressure pad on the upper arm and clean the lower glass-ferrite read/write head with 99% IPA.
- 300 RPM Strobe Calibration: Hold a mains-powered 50/60 Hz neon/fluorescent bulb (or 50/60 Hz smartphone strobe app) over the black-and-white timing ring under the drive pulley while the motor is spinning, and adjust the servo speed trimpot until the 50 Hz or 60 Hz bars stand completely still.
2 Tandon TM100 & MPI Common Faults
- Tandon TM100 Broken White Plastic Door Latch Pins: The two white nylon hinge pins inside the Tandon front door lever snap frequently. Drill out the broken nub and insert a 3/32″ (2.4 mm) brass rod or 3D-printed reinforcement block so the disk hub clamps squarely!
- MPI 51 / Siemens Stepper Leadscrew Grease: Old grease on the spiral leadscrew or steel guide rails turns to glue, causing `BDOS ERR ON A: BAD SECTOR` or failure to seek to Track 0. Degrease completely with IPA and lubricate with light PTFE oil.
- Drive Select Jumper (`DS0` vs `DS1`): Both Osborne and Kaypro use straight, un-twisted 34-pin Shugart cables: Drive A = `DS0` (Jumper 1), Drive B = `DS1` (Jumper 2), with the 150 Ω termination resistor pack installed ONLY in the drive at the very end of the ribbon cable.
3 Gotek (`FlashFloppy`) Installation Notes
- Kaypro II / 4 Gotek Setup: Straightforward 34-pin Shugart interface! Set the Gotek jumper to `S0` (Drive A) or `S1` (Drive B), set `interface = shugart` in `FF.CFG`, and load `.IMD` or `.TD0`/`.HFE` Kaypro CP/M disk images on your USB stick.
- Osborne 1 Gotek Adapter Warning: Because the Osborne 1 sends +12 V DC on Pins 2, 4, 6 and +5 V DC on Pins 10, 12, 14 of the 34-pin floppy ribbon (which are all tied to GND on a standard Gotek!), you must either:
- Use a dedicated Osborne-1-to-Gotek 34-pin adapter board that splits Pins 2/4/6 (+12 V) and Pins 10/12/14 (+5 V) off to the Gotek's 4-pin power plug while grounding only Pins 16–34, OR
- Cut lines 2, 4, 6, 10, 12, 14 on a spare ribbon cable and tap Pin 10/12 (+5 V) + Pin 18 (GND) to power the Gotek!
Osborne Video Shunt Plug, IEEE-488 & RS-232 Pinouts
How to wire a missing Osborne 1 external video shunt plug (so the internal 5″ CRT works!), build a composite video adapter, and wire RS-232 serial transfers.
Osborne 1 External Video & Shunt Plug (`P3`) 2×5 10-Pin Edge • CRITICAL!
Why is my Osborne 1's 5″ CRT completely black even though it boots from floppy? On stock Osborne 1s (without a Screen-Pac), the video and sync signals leave the motherboard on the front 10-pin EXT VIDEO edge connector and loop back to the internal 5″ CRT ONLY when the little black plastic jumper shunt plug is inserted! If that plug is lost, the internal CRT gets zero video or sync!
| Top Row (Odd) | Motherboard Signal | Bottom Row (Even) | Internal CRT / Return |
|---|---|---|---|
| Pin 1 | VSYNC OUT (from motherboard) | Pin 2 | VSYNC IN (to internal 5″ CRT) |
| Pin 3 | +12 V DC (External Monitor Power) | Pin 4 | GND (0 V) |
| Pin 5 | VIDEO OUT (TTL Dot Stream) | Pin 6 | GND (0 V) |
| Pin 7 | HSYNC OUT (from motherboard) | Pin 8 | VIDEO IN (to internal 5″ CRT) |
| Pin 9 | NC | Pin 10 | HSYNC IN (to internal 5″ CRT) |
How to Replace a Missing Shunt Plug (3 Jumper Wires!): To activate the internal 5″ CRT, simply bridge:
• Pin 1 to Pin 2 (`VSYNC OUT` → `VSYNC IN`)
• Pin 5 to Pin 8 (`VIDEO OUT` → `VIDEO IN`)
• Pin 7 to Pin 10 (`HSYNC OUT` → `HSYNC IN`)
(To build an external RCA Composite Video adapter, mix Pin 1 VSYNC + Pin 7 HSYNC + Pin 5 VIDEO through a simple 2-transistor / resistor combiner to an RCA jack!).
RS-232 Serial (`DB25`) & IEEE-488 / Centronics Bus
Both the Osborne 1 and Kaypro II feature a 25-pin `RS-232` socket ideal for bootstrapping CP/M software (`PIP`, `KERMIT`, `XMODEM`) from a modern PC USB-to-Serial adapter:
| DB25 Pin | Osborne 1 (`DCE` Wiring!) | Kaypro II (`DTE` Wiring) |
|---|---|---|
| Pin 2 | RXD (Data IN to Osborne!) | TXD (Data OUT from Kaypro) |
| Pin 3 | TXD (Data OUT from Osborne!) | RXD (Data IN to Kaypro) |
| Pin 4 / 5 | RTS / CTS Handshake | RTS / CTS Handshake |
| Pin 6 / 20 | DSR / DTR Handshake | DSR / DTR Handshake |
| Pin 7 | Signal Ground (GND) | Signal Ground (GND) |
Notice the Osborne vs Kaypro Serial Difference: The Osborne 1 is wired as DCE (Pin 2 = Input, Pin 3 = Output — connects to a 9-pin PC serial port using a straight-through 2-to-3, 3-to-2 adapter), whereas the Kaypro II is wired as standard DTE (requires a Null-Modem crossover cable to a PC)!
Osborne 1 Front IEEE-488 / Parallel Printer Edge Slot (`P2`)
The 26-pin front PCB edge connector (`Top Odd = 1..25`, `Bottom Even = 2..26`, driven by PIA0 `MC6821`) functions both as a standard IEEE-488 (GPIB) instrument bus AND as a Centronics parallel printer port when toggled in the Osborne `CONFIG` utility (`Pins 1–7, 13–19 = DIO1–DIO8 / Data`, `Pins 9, 11, 21, 23 = Handshake/Strobe/Busy`, `Pin 24 = GND`).
Chip & CP/M Boot ROM Reference
Interactive IC Pinout Finder cards plus Western Digital FD1793/1797 floppy controllers and CP/M 2.2 BIOS EPROMs.
Zilog Z80A CPU
4.0 MHz (Osborne 1 & Kaypro 4/2X) or 2.5 MHz (early Kaypro II) 8-bit processor running Digital Research CP/M 2.2.
View Pinout →MC6821 PIA (×2)
Osborne 1 — Two memory-mapped Motorola 6821 PIAs handle the keyboard matrix, IEEE-488/Printer bus, and RAM bank switching.
View Pinout →MC6850 ACIA
Osborne 1 — Asynchronous serial controller driving both the front DB25 RS-232 port and the acoustic modem pocket.
View Pinout →4164 DRAM (64K×1)
Osborne 1 (8× 4164) & Kaypro II/4 — Single-rail +5 V 64 KB main dynamic RAM bank.
View Pinout →4116 DRAM (16K×1)
Early Kaypro II (`81-110`) Video RAM bank — Triple-rail (−5 V, +12 V, +5 V) DRAM chips.
View Pinout →Floppy Controller & Z80 Peripheral IC Reference
| Chip | System | Function & Bench Notes |
|---|---|---|
| WD FD1793-B / MB8877A (DIP-40) | Osborne 1 (SD) & Kaypro II / 4 (DD) | Single/Double-Density floppy disk controller (+5 V & +12 V; note: very early `FD1793-02` also uses −5 V on Pin 1 or ties it to GND on `-B` revisions). Fujitsu `MB8877A` is a direct cool-running CMOS/NMOS equivalent. |
| WD FD1797-PL (DIP-40) | Osborne 1 (Double-Density Upgrade) | Used on the Osborne Double-Density daughterboard (supports IBM sector side-select flag). |
| Z80A PIO (`Z8420A`) ×2 (DIP-40) | Kaypro II (`U43` & `U47`) | Parallel Input/Output controllers driving the Kaypro's Centronics printer data/strobe and system control bits (drive select, bank select, RTC). |
| Z80A SIO/0 (`Z8440A`) (DIP-40) | Kaypro II (`U34`) | Dual-channel Serial I/O controller: Channel A = rear DB25 RS-232 serial port; Channel B = front 300-baud RJ keyboard interface! |
CP/M Boot BIOS & Character Generator EPROM Guide
| ROM Socket | Original EPROM | Version & Upgrade Notes |
|---|---|---|
| Osborne 1 System BIOS ROM (`UD4`) | 2732 / 2732A (4 KB, 24-pin) | ROM v1.44 is the final bug-fix release (supports both Single-Density and the Double-Density upgrade board). Standard 24-pin 2732A drops straight in! |
| Osborne 1 Character ROM (`UA18`) | 2716 (2 KB, 24-pin) | Generates the 8×10 character font and semigraphics glyphs. |
| Kaypro II Boot ROM (`U47` / `U34`) | 2716 (2K) / 2732A (4K) / 2764 (8K) | Early `81-110` boards use a 2 KB 2716 (`81-149C`); later boards accept 2732A or 2764. Popular upgrade: KayPLUS / TurboROM (adds hard drive support, terminal emulator, and multi-format disk reading!). |
Osborne 1 Keyboard Membrane Tip: Like the ZX Spectrum, the Osborne 1's fold-down mechanical-plunger keyboard uses a 3-layer flexible Mylar membrane inside! If keys fail on an Osborne 1, first clean the external 20-pin flat ribbon jumper between the keyboard and front bezel, and replace the internal membrane if silver tracks have oxidised.
Resources & Further Reading
Blog posts, interactive bench tools, and CP/M luggable documentation archives.
Smiggy's CP/M & Z80 Posts
Bench Tools & Machine Hubs
- IC Pinout Finder — Z80, 6821, 6850, 4164, 4116 & more
- Recap Planner — capacitor replacement guide
- Ohm's Law Calculator
- C64 Machine Hub — Commodore 64 reference
- C128 Machine Hub — C128, C128D & DCR reference
- SX-64 Machine Hub — Executive Portable C64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- Atari ST Hub — 520ST, 1040STFM & STE reference
- Apple IIe Hub — Unenhanced, Enhanced & Platinum reference
- BBC Micro Hub — Model B, B+ & Master 128 reference
- Acorn Atom & Electron Hub — Atom & Electron reference
- ZX Spectrum Hub — 16K/48K & +2/+3 reference
- Sinclair QL Hub — 68008, Microdrive & ULA reference
- Dragon 32/64 Hub — Dragon 32 & 64 reference
- Amstrad CPC 464 Hub — CPC 464 & Datacorder reference
- Oric-1 & Atmos Hub — Oric 6502A & ULA reference
- Amstrad CPC 6128 Hub — 128K & 3″ disc reference
- MSX Machine Hub — MSX1, MSX2, MSX2+ & Turbo R reference
- Big-Box Amiga Hub — A1000, A2000, A3000 & A4000 reference
- Atari 8-Bit Hub — 400/800, 800XL & 130XE reference
- VIC-20 Machine Hub — Commodore VIC-20 reference
External Osborne & Kaypro Archives
- Bitsavers Osborne 1 Technical Manual — complete factory schematics, theory of operation & BIOS listings
- Bitsavers Kaypro Service Manuals — Kaypro II, 4, and 10 schematics & alignment guides
- RetroCMP Kaypro Resource — ROM images, motherboard part numbers, and disk archives
- VCFed CP/M Forum — active Osborne & Kaypro hardware repair community