BBC Micro B+ Troubleshooting Guide

This guide documents fault diagnosis for the BBC Micro Model B+ and B+128. Many faults are shared with the Model B, with B+-specific failure modes — shadow RAM, sideways workspace RAM, the WD1770 floppy controller, and the relocated sideways ROM sockets — called out separately.

Reference Documents

• Acorn BBC Microcomputer Service Manual (October 1985), Sections 1 and 2.
• Advanced User Guide for the BBC Microcomputer (Bray, Dickens, Holmes, 1982).
• BBC Microcomputer Advanced Reference Manual (Acorn, 1984).

Initial Diagnosis Workflow

A healthy B+ on first power-on should:

1. Beep (1 kHz, 0.1 s) within 50 ms of mains application.
2. Display the Acorn-style banner with build-date string, total free user memory, and the language prompt within 1 s.
3. Drop into the BASIC II prompt (>) on a 32 K shadow-screen MODE 7 display.
4. Respond to keyboard input within 1 character cell of typing.

If any of these does not happen, stop and diagnose at that stage. Don't blindly press Break multiple times — repeated breaks can mask a CPU fault by re-running the boot ROM after a partial failure.

Power-On Symptoms

No power LED, no beep, dead

• PSU fuse blown — the in-line 250 mA T (slow-blow) fuse on the live mains feed has popped. A blown fuse usually means C1 or C2 (RIFA mains-suppression caps) have failed short. Do not just replace the fuse and re-power — investigate the PSU first. Smell-test the unit; a fishy / acrid smell confirms RIFA failure.
• Mains lead fault — uncommon but possible; verify continuity on all three cores.
• Astec PSU module dead — measure +5 V, +12 V, −5 V at the four-pin Molex; if any rail is absent or low, the PSU itself needs service. Recap the secondary side first; replace the 7805 / 7905 regulator only if recapping doesn't restore the rail.

Power LED on, no beep, no display

• CPU clock not running — scope the 2 MHz clock pin (Φ2) at the 6512A; should show a clean 50 % square wave at 2 MHz. If absent, suspect the System ULA (IC15) or the 16 MHz crystal X1.
• MOS ROM dead — pull IC51 (HN613256 MOS) and verify the chip with a programmer. The MOS is on a 32 K EPROM mask in the B+; replacement EPROMs can be programmed from the freely available MOS 2.00 image.
• DRAM fault — a single dead 4164 will prevent MOS boot. Diagnose by pulling DRAMs one at a time and probing for the standard 6502 reset sequence on the address bus.
• Reset line stuck — measure pin 40 of the 6512A; should idle high. A stuck-low reset usually traces to the System ULA or a failed reset capacitor.

Power LED on, beep, no display, no banner

• Video ULA dead — IC6 generates the video bitstream. No output indicates ULA failure; ULAs are unobtainable as new parts and donor B+ boards are scarce.
• CRTC fault — HD6845 (IC2) failure produces no sync signals. Scope HSYNC / VSYNC on the video DIN socket; if both absent, suspect the 6845.
• Composite output dead but UHF works — the composite output buffer transistor (TR1) has failed. Replace with any small-signal NPN equivalent (BC548 family).

Banner displays but garbled

• Shadow RAM fault — the B+'s 20 K shadow screen RAM is separate from main DRAM. A bit fault in shadow RAM produces vertical bars or skewed glyphs in MODE 0–6 but a clean MODE 7 (because MODE 7 uses character cells from teletext ROM, not framebuffer pixels). Reseat the shadow RAM ICs (IC57–IC59); replace if reseating doesn't fix it.
• Main DRAM fault — bit fault in main RAM corrupts both user program and (on B+) MODE 7. Diagnose by running the MOS self-test (Ctrl + Break + R if available) or simply by writing values via *FX and reading them back.
• ROM mis-checksum — *HELP at the prompt should list MOS, BASIC, DFS, and any sideways ROMs. Missing entries indicate a sideways ROM not being recognised — usually a bent socket pin or backwards insertion.

Floppy Disc Diagnosis (WD1770)

The B+ ships with the WD1770 as standard. Common faults:

• No disc activity at all — verify 5 V and 12 V at the floppy edge connector; verify the 34-pin ribbon is the right way round (red stripe to pin 1, which is on the side of the connector marked with a triangle on the B+ PCB).
• Drive spins, no head movement — WD1770 stepping logic failure. Read the WD1770 status register via *FX or a short BASIC program — a stuck busy bit indicates the controller itself has died.
• "Disk fault 18" or similar — DFS read error. Could be the disk itself, head alignment, or a failing 8 MHz oscillator feeding the WD1770. The B+ uses a separate 8 MHz crystal X3 for the floppy controller.
• Reads single-density only — DFS 2.10 ROM corruption or a WD1770 with a degraded density-select line.

Sideways ROM Diagnosis

The B+ has five sideways ROM sockets (vs four on the Model B), relocated to the top-left corner of the board adjacent to the keyboard cable. Numbered 4–8 left-to-right.

• ROM not in *HELP listing — either the ROM is bad, the socket is dirty (clean with deoxidising contact cleaner and reseat), or the ROM is in a socket that requires a specific MOS configuration.
• B+128 won't recognise the additional 64 K sideways RAM — sockets 4 and 6 must be populated with the 32 K static RAM modules (HM6264 family) — confirm both are seated. The B+128 firmware tests both banks at boot; a missing or dead RAM module shows up as a reduced free memory figure in the boot banner.

Keyboard Diagnosis

The B+ keyboard is essentially the same as a late Model B — 73 keys plus the Break-Reset key. Common faults:

• Single key not responding — corroded or worn key switch. Most B+ keyboards use SMK or Futaba switches; both can be cleaned by disassembling the switch on the bench and lightly polishing the contact dimples.
• Whole column dead — broken trace on the keyboard PCB, or a dead column-drive line from the System VIA (IC3). Trace with a logic probe.
• Whole row dead — IC1 (74LS151) row-mux failure on the keyboard PCB.
• Ghost keys — diodes on the keyboard matrix have failed (rare). Replace with 1N4148 equivalents.
• No keys respond, banner present — System VIA dead, ribbon cable broken, or the keyboard ROM (IC2) has died. Swap the keyboard with a known-good Model B keyboard for triage (the cable is identical).

Refer to the keyboard schematic for matrix pin assignments.

Cassette / Tape Diagnosis

• Tape won't load — verify the motor relay clicks when *MOTOR 1 is issued. If not, suspect the motor relay drive transistor.
• Loads at 1200 baud but not 300 — the ACIA (68B54) baud-rate divider may need adjustment; or the saved tape itself may be at 1200 baud.
• Random load errors — failing cassette input opamp on the main board; or a worn tape head on the cassette deck (most B+ machines were used with budget cassette decks where this happens regularly).

Serial / Parallel / 1 MHz Bus

• Centronics not printing — User VIA (IC69) bit failure, or a bad printer cable. The B+ Centronics port is on a 26-way IDC connector (different from the Model B's pin-out at the latched output stage).
• RS-423 not communicating — ACIA or the MAX232-equivalent line driver dead. Verify the RTS/CTS lines at the DIN socket.
• 1 MHz Bus add-on not recognised — buffering chip IC11 (74LS245) has failed; or a stuck address-decode line on the page-select logic.
• Tube not recognised — Tube ULA failure or an absent / bad Tube ROM in one of the sideways sockets.

Common Field Symptoms

• Smell of fish on power-up — RIFA X2 cap (C1 / C2) venting. Power off immediately at the wall.
• Random reboots when warm — PSU electrolytics aged, particularly C9 (primary bulk). Recap the PSU first.
• MOS self-test passes but disc reads fail — WD1770 8 MHz clock fault; check crystal X3 and the surrounding oscillator network.
• Keyboard inputs occasional ghost characters — failing diode in the keyboard matrix, or a flaky ribbon cable.
• Screen flickers in MODE 7 only — SAA5050 teletext IC clock fault; verify the 6 MHz line feeding the chip.
• B+128 boots as B+64 (only 64 K free) — additional sideways RAM banks not detected; reseat the HM6264 modules.
• *HELP shows DFS but no disc activity at all — WD1770 itself dead, or the 8 MHz crystal X3 has fractured.
• Screen rolls vertically — vertical sync issue from the 6845 CRTC; clock fault or a dead 6845.

Diagnostic Workflow Summary

1. Power on; observe LED, beep, banner.
2. If no power: check PSU rails; suspect RIFA caps and C9.
3. If power but no beep: scope CPU clock; suspect ULA / DRAM / MOS.
4. If banner garbled: pull DRAM and shadow RAM; reseat ROMs.
5. If banner OK but no floppy: check WD1770 8 MHz clock; verify floppy cable orientation.
6. If banner OK but no keys: System VIA and keyboard ROM checks; swap keyboard with known-good unit.
7. If banner OK but B+128 reports B+64 memory: re-seat sideways RAM modules in sockets 4 and 6.
8. Always recap PSU before chasing intermittent boot failures on a 40-year-old machine.

⚠️ Power supply — recap before switching on

The BBC Micro's switch-mode PSU is notorious for its RIFA X2 mains-suppression capacitors (a 10 nF/0.01 uF and a 100 nF/0.1 uF across the mains input), which crack and fail with age, sometimes explosively, with plumes of acrid smoke. Standard practice is to replace them before the machine is switched on for the first time in years. A small 220 uF electrolytic in the supply also causes intermittent or failed start-up; recap the whole PSU.

Continuous tone, no display

A BBC that emits a continuous tone with no picture — often first seen after a PSU recap once the supply is healthy again — is commonly a faulty video ULA (the Ferranti video processor). Reseat it first; if that does not help, the ULA itself is suspect (a known failure part, with replacements and modern recreations available).

Core logic

With a healthy PSU, work the BBC's socketed logic: reseat the 6502 CPU, the OS and BASIC ROMs (sideways ROM sockets), the RAM, the 6845 CRTC and the SAA5050 teletext generator (Mode 7). A garbled or wrong-colour display points to the 6845, the SAA5050 or the video ULA; a dead machine with good rails points to the CPU, the ROMs or the system clock. Cross-check the +5 V and other rails at the PSU connector first.

Related Pages

• BBC Micro Model B+
• BBC Micro B+ General Maintenance
• BBC Micro B+ Capacitor Replacement Guide
• BBC Micro Model A/B Troubleshooting Guide