BBC Master Compact Troubleshooting
Fault-finding for the BBC Master Compact, following section 6 of Acorn's Compact service manual. The Compact's custom chips and fault-finding steps resemble the Master 128's, but every IC number is different, and most ICs are soldered to the board.[1][2]
Test points
[edit | edit source]Acorn put six test points on the board.[3]
| Point | Signal | Expect |
|---|---|---|
| TP1 | MAX232 negative output (serial upgrade fitted) | −5 to −10 V, typically −9 V |
| TP2 | MAX232 positive output (serial upgrade fitted) | +5 to +10 V, typically +9 V |
| TP3 | CPU NMI | Generally 5 V while running; low only during disc and Econet use |
| TP4 | CPU IRQ | Not stuck high or low when free running |
| TP5 | CPU SYNC | Changing; stuck high or low means the processor has stalled |
| TP6 | CPU read/write | Changing frequently between 0 and 5 V |
TP1 and TP2 are near IC5 at the north of the board; TP3 to TP6 are south of the CPU IC28. For an oscilloscope ground use the north end of C1 in the north-west corner. The large copper area in the south-west of the board is +5 V, not ground.[3]
Switch-on
[edit | edit source]Connect an RGB monitor and switch on.[1]
| Symptom | Probable cause | Action |
|---|---|---|
| Noise on the monitor, no beep, keyboard power LED off (6.1.1) | No 5 V | Check that the supply gives 5 V, centre pin positive; if the supply has failed, see the disc unit below |
| Power LED lit, nothing else (6.1.2) | Board fault | Core checks below |
| Rolling lines or diagonal stripes (6.1.3) | CRTC or sync | MA lines (IC34 pins 4–17) and RA lines (pins 35–38) to the CRTC multiplexer IC46; VSYNC (IC34 pin 40) every 20 ms; HSYNC (pin 39) every 64 µs; CSYNC at IC10 pin 10 and pin 4 of the RGB socket; CSYNC polarity link PL10 |
| Cursor stuck at top left (6.1.4) | Keyboard | Keyboard connections and the connector; the TTL ICs IC43, IC44, IC32, IC33 and IC45, which Acorn calls reliable |
| "Acorn MOS" and "Acorn ADFS" only (6.1.5) | ADFS waiting for a disc, or bad configuration | CTRL F BREAK, then a formatted disc or *CONFIGURE NODIR; otherwise switch on holding R, then CTRL F BREAK
|
| "BASIC" but no > prompt (6.1.6) | IRQ stuck | IRQ at IC28 pin 4; address and chip select lines of the system VIA IC25 |
| > prompt, keyboard dead (6.1.7) | System VIA, keyboard or IRQ | IC25; the keyboard connections; another keyboard; IRQ (IC28 pin 4), which the memory controller IC48, the ACIA (if fitted), the user VIA IC7, IC25 and the expansion connector PL13 all drive. The system VIA interrupts every 1/100 second |
| Some keys dead (6.1.8) | Keyboard matrix tracks | Several keys: the connection or a break mid-row or mid-column. One key: a break at the end of its row or column. Individual keys cannot be replaced on this membrane keyboard; if no track fault is found, replace the keyboard |
| Random dots or characters (6.1.9) | Memory system | CPU address lines (IC28 pins 9–20 and 22–25) to the multiplexers IC37, IC39 and IC40 at CMOS levels; address stable at least 15 ns after RAS; noise on lows under 500 mV; data bus to the DRAMs |
| Two or more copies of what you type (6.1.10) | CRTC addressing | IC34 pins 4–13 and 35–38; MA13 (IC34 pin 17) static in a given mode; IC34 to IC46; C0 and C1 from the 74LS259 IC31 (pins 4 and 5) to IC46 (pins 38 and 39) |
Core checks: dead machine
[edit | edit source]Acorn's sequence for a board that powers up but does nothing (6.1.2):[1]
- Check +5 V and 0 V at every IC and make good any dry joints.
- Feel for very hot ICs; the 6845, the VIDPROC and the DRAMs always run quite warm. Look for shorted tracks, and replace any IC that got abnormally hot.
- Check the 16 MHz oscillator at IC10 pin 2 and that it reaches the VIDPROC at IC16 pin 8: about 4 V peak-to-peak. The oscillator is half of IC10 with crystal XL1.
- Check 1, 2, 4 and 8 MHz from the VIDPROC at IC16 pins 4, 5, 6 and 7; replace IC16 if any is missing or not clean.
- Check 2 MHz at the CPU, IC28 pin 37.
- Check that the system ROM is in its socket.
- Check the buses for shorts to each other, to +5 V and to ground.
- Check RAS (pin 5) and CAS (pin 16) at the DRAMs IC35 and IC41: 4 MHz square waves. RAS comes from the D-type IC8 pin 9; CAS from 2 MHz through IC10, a NAND gate in IC18 and another in IC27.
- Check the DRAM address lines (pins 6–8 and 10–14 of IC35 and IC41) switch after RAS with about 20 ns hold time.
Warning: if the 16 MHz clock fails, RAS and CAS can be held low and the DRAMs overheat and fail. Do not leave the machine on for more than 30 seconds at a time while the clock is dead.[1]
Continuous tone and no picture
[edit | edit source]This is the common Compact fault reported on stardot, and on several machines it has been the DRAMs:
- One owner who found the VIDPROC clocks and the RAS and CAS signals present, and the 6522, 6845 and WD1772 not responsible, cured it by replacing the RAM.[4]
- Another needed two DRAMs, a new MOS socket (some pins were not making contact) and a new configuration EEPROM; his remaining instability was the external 5 V supply he was testing with.[5]
- Socket the replacement DRAMs; they are 4464 64K × 4 parts, 120 ns.[6][5]
A Compact that worked on the bench but failed once screwed into its case was traced to a case stand-off pressing on two pins of IC22, the I/O controller.[7]
Configuration memory
[edit | edit source]The 128-byte EEPROM IC24 replaces the battery-backed CMOS RAM of the Master 128. Acorn's checks (6.11):[8]
- Try a replacement PCD8572 in IC24 (it is socketed).
- Check 5 V at IC24 pin 8 and 0 V at pins 1, 2, 3 and 4.
- Check that IC24 pin 6 connects to the system VIA IC25 pin 15, and pin 5 to IC25 pin 14.
- Check the system VIA IC25 and change it if necessary.
- Finally check the RC timing circuit R1 and C2.
The EEPROM has a limited number of write cycles, which is why Acorn socketed it.[8] Because there is no clock chip, the date is always 31 December 1999 unless the machine gets the time over Econet; that is not a fault.[9] If the machine starts at a * prompt with "This is not a language", type *CONFIGURE LANG 14 and *INSERT 14.[9]
Video
[edit | edit source]Connect both an RGB and a composite monitor.[1]
| Symptom | Check |
|---|---|
| Picture rolls vertically | VSYNC at IC34 pin 40 through the exclusive-OR gates IC20; on one output only, PL10, or R51 to R54 for composite |
| Picture broken into diagonal lines | HSYNC at IC34 pin 39 through IC20 |
| No colour on RGB | RGB from the VIDPROC IC16 pins 14, 12, 10; in modes 7 and 135 only, from the SAA5050 IC15 pins 24, 23, 22 to IC16 pins 13, 11, 9 |
| Only modes 7 and 135 fail | 6 MHz into IC15 pin 19 from IC20 and IC21 (no part narrower than about 40 ns); pull-ups R29 to R31; data bus to the PBC IC26 pins 21–28; VSYNC for half-height text |
Interfaces
[edit | edit source]| Interface | Checks |
|---|---|
| Joystick port | Continuity from PL8 to the user VIA IC7; 5 V at PL8 pin 7 and 0 V at pin 8; software that sets port B of IC7 to outputs (writes to &FE62); another joystick; IC7 last |
| Serial port | The basic machine has no serial interface: check that the upgrade was fitted correctly; TP1 and TP2 voltages, then the supply to the MAX232 |
| Disc | Signal and power connectors home; another drive; select inputs of the 1772 (IC6 pin 1) and 74LS174 (IC1 pin 1), driven by IC22 pins 1 and 4; Read Data at IC6 pin 19; pull-ups R9 to R12; 8 MHz at IC6 pin 18 from IC19 pin 8 and IC16 pin 7; DRQ (IC6 pin 27) pulsing every 32 µs and inverted at the CPU NMI (IC28 pin 6); INTRQ (IC6 pin 28). Corrupt catalogue entries: C10 (220 pF) from IC6 pin 27 to 0 V, and only one drive with termination resistors. Wrong data: memory controller IC48 address inputs (pins 21–28). Unreliable writing: WD and WG at IC6 pins 22 and 21. If *CAT only clicks and the drive does not spin, Sprow's fix is *CONFIGURE FILE 13 and *INSERT 13
|
| Printer | Strobe pulse at the user VIA IC7 pin 39 for every character; inverted at pin 11 and true at pin 8 of the 7438 IC3 |
| Expansion connector | Does the machine work without the peripheral? Check external hardware and software carefully; no shorts on the edge connector; the plug the right way round; worn gold plating |
| Sound | Speaker connected to PL1 and not blown; more than four envelopes use serial buffer space; strange sounds are usually software writing the sound buffer directly |
| Econet | Both module connectors fitted (the longer one has two spare pins to its left); NMI at IC28 pin 6 not held low |
Sources: Acorn's service manual and Sprow.[1][9]
Disc unit and power
[edit | edit source]Acorn's monitor-stand guide limits dealers to first-line work: check the plug fuse (5 A) and the mains lead, the mains switch, the mains fuse (anti-surge 3.15 A HBC 250 V, type T), the 5 V lead (under 100 mΩ per wire at full load) and the disc data cable, and replace the supply board or the drive as a unit.[10] Repeated disc errors are a reason to try another data cable.[10]
| Symptom | Probable cause | Action |
|---|---|---|
| Supply takes a minute or more to come up | The supply board's start-up capacitor C10 (220 µF; not the main board's C10, a 220 pF ceramic) lost value, or its charging resistors drifted high | Replace C10; measure the resistors of 100 kΩ or more that charge it[11] |
| Fast clicking from the supply and the power LED flashing in step | Short or overload on an output, a failed start-up capacitor, or a shorted output rectifier | Disconnect the floppy drive and test again; replace the start-up capacitor; measure the outputs for a short with the power off; check the 115/230 V wire link[12] |
| Smoke and a smell from the disc unit | Failed RIFA X2 capacitor | Replace it; see BBC Master Compact Capacitor Guide[13] |
To test the keyboard unit away from the disc unit, owners use any regulated 5 V supply of 2 A or more with the centre pin positive.[14][15]
Related pages
[edit | edit source]- BBC Master Compact
- BBC Master Compact Maintenance
- BBC Master Compact Capacitor Guide
- BBC Master 128 Troubleshooting Guide
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Acorn Computers, British Broadcasting Corporation Master Series Compact Microcomputer Service Manual, part no. 0458,002, issue 1, October 1986, hosted as File:Acorn BBC Master Compact Service Manual 0458,002.pdf, 6 "Fault finding", printed pp. 26–38: switch-on symptoms 6.1.1 to 6.1.10, sideways RAM, video, joystick port, serial port, disc interface, printer port, expansion connector, sound, Econet and non-volatile memory.
- ↑ stardot.org.uk, "Master Compact Repair Help/Continuous Tone of Death" (January 2014, retrieved 2026-10-02). AndyGarton: most ICs soldered; continuous tone and no display, then display and prompt but dead keyboard apart from BREAK; the keyboard worked in another computer and the NVRAM chip had been swapped. 1024MAK: BREAK is only a reset; how to test /IRQ with a scope, logic probe or meter. hoglet: the system VIA IC25 is not socketed. Secondary source.
- ↑ 3.0 3.1 Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, 4 "Selection links and test points", printed pp. 9–10 (PL7, PL9 to PL12, VR1, FS1, L1 and L2; TP1 to TP6), and 7 "Hints and Tips", p. 39.
- ↑ stardot.org.uk, "Acorn Master Compact - blank screen and beep" (February–March 2019, retrieved 2026-10-02). nekroskop: no display, endless beep, power LED lit, Caps Lock flashing; VIDPROC clocks present on pins 4 to 7 (1, 2, 4, 8 MHz), RAS and CAS present on IC35 and IC41; removing the 6522 and WD1772 changed nothing; fixed by replacing the RAM. Secondary source.
- ↑ 5.0 5.1 stardot.org.uk, "Master Compact - Faulty - FIXED" (December 2023 – February 2024, retrieved 2026-10-02). DaveLecky: two DRAMs, a MOS socket with dead contacts and the configuration EEPROM replaced; the last fault was his bench supply, cured by using another 5 V supply. maniacminer: a dead Compact whose RAM had failed, probably after RAS and CAS were held low. tom_seddon: the Compact MOS appears to boot with default settings if the EEPROM does not acknowledge. Secondary source.
- ↑ Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, parts list, printed pp. 43–47: 4464 DRAM 120 ns IC35, IC36, IC41, IC47; PCD8572 IC24 (socketed); system ROM IC49 (socketed).
- ↑ stardot.org.uk, "BBC Master Compact - faulty when in case [SOLVED"] (December 2024, retrieved 2026-10-02). dsdf323: the board worked out of the case but failed consistently once seated in the bottom case; a stand-off was crushing two pins of IC22. Secondary source.
- ↑ 8.0 8.1 Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, "Non volatile memory", printed pp. 19–20, and 6.11, p. 38.
- ↑ 9.0 9.1 9.2 Sprow, "The most common beeb power on problems", Sprow's webpages, updated 9 November 2010 (retrieved 2026-10-02). Community source: the Compact's fixed time of 31 December 1999; *CONFIGURE LANG 14 for BASIC; *CONFIGURE FILE 13 and *INSERT 13 for DFS on the Compact; the ADFS mount hang.
- ↑ 10.0 10.1 Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, section 2, monitor stand service guide, PDF pp. 50–58: first-line servicing; mains lead, switch, auxiliary output and input; mains fuse; changing the supply; the +5 V lead and DC cable assembly; the disc data cable (try a known working cable if disc errors keep occurring); changing the drive (Sony F6 3W set as drive 0).
- ↑ stardot.org.uk, "BBC Master Compact PSU very slow to start" (January 2022, retrieved 2026-10-02). C10 measured 65 µF and caused a one- to two-minute start; a new 220 µF part cured it. 1024MAK: also check the high-value resistors that charge it. Secondary source.
- ↑ stardot.org.uk, "Master compact disk unit power supply" (August 2025, retrieved 2026-10-02). turboman: clicking about ten times a second, LED flashing in time, unchanged after a recap, computer fine on another 5 V supply. 1024MAK: disconnect the floppy drive; the primary start-up capacitor is a common failure. JonnoW and 1980s_john: usually a short on the output; one had a shorted output rectifier diode; check the 115 V/230 V jumper wire. Secondary source.
- ↑ stardot.org.uk, "Refurb process for Master Compact Base Unit PSU" (February–March 2025, retrieved 2026-10-02). 1024MAK's supply after "the white smelly smoke"; C1 0.1 µF 275 V AC X2. Secondary source.
- ↑ stardot.org.uk, "BBC Master Compact Power Supply?" (February–March 2023, retrieved 2026-10-02). 1024MAK: 2 A at 5 V regulated is enough; egel: the original is printed 5V 2A; jgharston: a 5 V 2.5 A adaptor, tip +5 V. Secondary source.
- ↑ Acorn Computers, Master Series Compact Microcomputer Service Manual, 0458,002, connector pinouts, printed p. 41: PL2 power input, outer 0 V, inner +5 V.