Minimal boot configuration
When attempting to diagnose a Model A or Model B that doesn't boot, it can be useful to remove all non-essential ICs, to help identify the cause of the fault.
In this minimal configuration, the machine should boot the Machine OS ROM, and display a Language? prompt on the screen (due to the absence of the BASIC ROM).
Disconnect any peripherals (such as floppy drives) and all external cables other than the power and video cables. Remove any expansion cards fitted to the motherboard, and disconnect the keyboard and speaker.
Then remove all of the following (if fitted and if socketed), keeping them in a safe place:
- IC3 - System 6522 VIA
- IC69 - User 6522 VIA
- IC78 - 8271 chip or 1770/2 disk interface module
- IC52 - ROM/EPROM
- IC88- ROM/EPROM
- IC100- ROM/EPROM
- IC101- ROM/EPROM
- IC89 -68B64 Econet
- IC7 - 2C199 Serial ULA
- IC4 - 6850 serial ACIA
- IC74 - uPD7002 analogue to digital converter
- IC98 - TMS6100 speech system chip
- IC99 - TMS5220 speech system chip
Leave IC51 (MOS ROM) in place.
Ensure the links (jumpers) are in the following positions:
- S9 -assuming IC78 is not fitted/removed, fit a link/jumper shunt or resolder the bridging wire to S9.
- S21 - fit two links as follows (if not already fitted): NW to NE and SW to SE (two each East-West).
- S25 - note it’s position, then try the machine with it fitted in the South position. If the machine still fails to start, try removing link S25.
If the machine now boots, replace the ICs one at a time, and check if the machine still boots with each addition. The following order is suggested:
- IC52 - BASIC ROM. At this point, the BBC BASIC prompt should be seen on boot instead of Language?.
- IC3 - System 6522 VIA. IF you have two 6522s, try either one in this position. IC3 controls the keyboard and audio circuits. If IC3 is working, reconnecting the speaker should give the usual two beeps on boot, and reconnecting the keyboard should light the Caps and Shift Lock LEDs on boot, as well as allow you to type in commands.
The exact order of replacement for other ICs will depend on the configuration of your machine. Note that you will need to remove S9 again after replacing the 8271/177x floppy controller, to allow its non-maskable interrupts (NMI) to reach the CPU.