Acorn BBC Master 128

For more information, see this article's corresponding Wikipedia page: BBC Master.

Acorn BBC Master 128

Preliminary Information
Release Year	1986
Leak Risk	Medium
Batteries	Removable Pack for clock battery
Mounting Technology	Through Hole
Capacitor Types	Aluminum Electrolytic, Tantalum, Ceramic, Polymer
Destructive Entry	No
Hardware Specifications
CPU	CMOS 6502 @ 2MHz
Memory Capacity	128kB RAM (32k main), 128kB ROM
Memory Type	4464 DIP
Storage	5.25in Floppy Disk (400kB for DFS, 640kB for ADFS). Network storage via Econet. SCSI hard disc by adapter.
Graphics	Custom Acorn VIDPROC
Display Interface	UHF, Composite, Digital RGB
Sound	Texas Instruments SN76489
Power Requirements
Input Voltage	240V AC (120V via solderable jumper in PSU)
Power Connector	Integrated BS1361 plug.

The Master 128 is a computer produced by Acorn for the BBC Computer Literacy Project. It is a very similar machine to the earlier BBC Micro, but includes many of the most common upgrades for that machine as standard, such as additional RAM, floppy disk support including the improved ADFS format. Other upgrades that required soldering on the Micro such as Econet can now be installed as a pluggable module.

It also added a real time clock, with many settings that were links in the BBC Micro now stored in NVRAM. The inclusion of batteries creates a leak hazard.

Known Issues

CMOS Battery Flat/Leaking

Causes

• In most BBC Masters, the CMOS battery is 3 AA batteries in series with a 1N4007 diode and 100 Ohm resistor. Primary alkaline cells are used, hence the diode to prevent charging. The official Acorn packs were 3 cells wrapped spot welded in an L shape, wrapped in heat shrink.
• Early Masters shipped with a rechargeable battery pack here. These were unreliable, leading to the shift to primary cells. Early issue motherboards may have charging circuits for this rechargeable pack fitted, hence the diode above.
• The battery packs are placed in the gap between the motherboard and the power supply, this mild leakage doesn't necessarily damage the motheboard. In severe leakage motherboard damage may occur, as can damage to the back of the keyboard PCB.

Solutions

• Replace with 3 AA batteries, a series diode and resistor, replicating the circuit described above.
• A standard 3-AA tray will fit in the space available so the circuit can be assembled once and cells replaced easier on subsequent servicing, or removed when not in use.
• Other batteries such as CR2032 may retain state initially but are unlikely to last long, the 146818 RTC chip used has a relatively large current draw.
• The battery rail does have a 10uF capacitor on it, which can retain settings for a few minutes across reboot so use without battery is possible if you are willing to reset the CMOS settings every time you use the computer.

CMOS Settings Lost

Causes

• CMOS battery flat/leaking as discussed above

Solutions

• Turn the machine off, hold Ctrl and R on the keyboard and turn back on with these keys held. You should get the message "CMOS RAM reset. Press Break to continue".
• This only resets some of the settings, you will likely need to set appropriate values for most settings with *CONFIGURE (*CO.)
• The master regards its ROMs as being 16 modules, each 16k in size. Slots 9-15 are in the internal ROM alongside the MOS. Slots 0-8 are mapped to cartridge ports, physical ROM slots or RAM depending on links. All 16 ROM modules can be enabled/disabled and this state is stored in the CMOS RAM. The enabled state is Use *ROMS to list the ROMs installed, and *INSERT (*IN.) or *UNPLUG to enable or disable ROMs. ROMs must be inserted to be used, including in configure. Note that while ROMs lists ROM numbers in hexadecimal, INSERT, UNPLUG and CONFIGURE expect them in decimal.
• To restore the factory settings, the commands would be :

*IN. 15
*IN. 14
*IN. 13
*IN. 12
*IN. 11
*IN. 10
*IN. 9
*CO. BAUD 4
*CO. NOBOOT
*CO. CAPS
*CO. DATA 4
*CO. DELAY 50
*CO. NODIR
*CO. INTUBE
*CO. FDRIVE 3
*CO. FILE 9
*CO. FLOPPY
*CO. IGNORE 10
*CO. LANG 12
*CO. MODE 7
*CO. TUBE
*CO. LOUD
*CO. PRINT 4
*CO. REPEAT 10
*CO. SCROLL
*CO. TV 0,1

• Press Ctrl-Break to fully reset the machine so the configuration is re-read.

Cause

• Machine displays the boot text (3 lines delcaring Acorn MOS, filesystem and BASIC). but doesn't give a ">" or"*" prompt.
• Usually the machine is attempting to do a floppy disc access before the prompt, and if there is no disc drive or no disc in the drive it will wait forever.
• This can either be an attempted auto-boot or ADFS trying to load the root directory from the disc.

Solution

• Press Ctrl+D+Break, Ctrl+Break will fully reset the machine aborting the existing access, D will cause DFS to be selected on the new boot which does not pre-load the root directory.
• If the machine is still attempting a floppy access, try Ctrl-Shift-Break, if auto-boot is configured holding Shift will skip auto-boot.
• Once you get a terminal, do the configure NOBOOT and NODIR from above to select settings that wont try floppy until explicitly commanded. configuring FILE 9 also selects DFS by default which never automatically reads a directory (but may autoboot).

Individual Keyboard Keys Intermittent

Cause

• Usually caused by dirty contacts inside the keyswitches

Solution

• If the keyboard hasn't been used in a while, often the key can be cleaned sufficiently well by repeatedly pressing the key.
• If this doesn't fix the key, usually the surface that needs to be cleaned is the interior part of the solder pin.
  • Unsolder the key from the keyboard PCB. Some care should be taken as this is a single sided board.
  • The key should now lift out the front of the keyboard. While the metal bracket supports the keys it does not retain them.
  • Using a pair of pliers, gently turn the pin you unsoldered anti-clockwise to unscrew it from the internal mechanism of the key.
  • Clean the top of the pin, especially around the screw thread on the top.
  • Reinstall the first pin, and clean the second similarly.

Disassembly Notes

The bottom side of the power supply is only covered by the main case of the computer, once reviewed the bottom of the PCB and thus rectified mains on filter capacitors will be exposed.

Motherboard is retained by 4 screws through the cartridge ports as well as the more obvious screw tying the I/O plate to the power supply.

Original Parts

Basic PCB Name

Note model numbers and board revisions here

Capacitor Specifications and Footprint Information

RefDes	Qty	Capacitance	Voltage	Mount	Diameter/Size	Height	Lead Spacing	Temp	Type	Brand	Series
		0 µF						0 °C

Replacement Parts

Basic PCB Name

Capacitors

RefDes	Qty	Compatible Part Number	Order Links
			Digikey

If parts are not available or different selection is preferred, you can use the values in the Original Parts section to perform a parametric search.

Kits

• Digikey BOM: https://www.digikey.com/

Related Links

• Videos or other things that might be helpful to check out