BBC Model B Computer Faults/Cures
Submitted to 8-Bit Software by Paul Martin-Matthews K3B
8-Bit Software will not accept responsibility for any damage you may do to
yourself or your computer after reading this article. If you are not sure,
don't do it! Bear in mind that 8BS now has a repair man. Please examine the
Many long term problems with the BBC are caused by the IC socket connections
breaking down. This is particularly so with the early models. It is very hard
to track down these faults, but with a little effort and care a lot of them can
be dealt with. Make sure that all the power leads from the PSU are a tight fit
on to the spade connectors and that the spades are clean as well. The wires
into these connectors were pre-soldered and this led to problems with
high-resistance joints which caused the supply voltage to drop from the normal
5 volts down to as low as 4.7 volts over a period of time. It is not a bad
thing to pull back the connector's insulation boot and solder the wire to the
spade connector. Repairing the PSU is not a task to be undertaken lightly and
few spare parts are available. Have a look for dry solder joints around the
PCB, particularly by the mains input filter T1. The bridge rectifier can also
be suspect, as can the resistor R12 which is a 0.6w 4.7ohm metal-film resistor.
This resistor provides a feed to the collector of Q2, which is a TO3 device
which is unmarked but understood to be a Hitachi 2SC1942 (a Telefunken BU255
seems to be a suitable replacement although TVT list the BU500 as an
These faults concern issue 2 to 7 boards.
1. Won't start up. The read/write pin 34 of the 6502a chip IC1 shorted to earth
beacuse of a fault in IC33 (74LS04).
2. Won't start up, reset line (RST) incorrect. IC33 (74LS04) faulty.
3. Apparently dead. Port B (in/out) of IC3 (6522) faulty.
4. Large white blocks on the screen. Caused by dry joints on the PSU leads.
Solder the leads to the connectors.
5. Program crashes. A faulty PSU caused Hash on the supply lines.
6. No data loading on tape. IC35 (LM324) chip faulty.
7. No data loading on tape. IC7, the Serial interface chip was faulty.
8. Computer was Apparently dead, but a messed up BBC logo sometimes appeared.
The appearance of random MODE 7 colours was the clue here. A check on the 16MHz
clock waveform showed it to be very distorted, although the frequency was about
right. IC43 (74S04) was the culprit, and it must be a 74S04 as a 74LS04 won't
9. Continuous beep with no keyboard input. IC3 (6522) faulty.
10. No RGB monitor sync. IC48 (74LS86) faulty.
11. Keyboard inputing wrong characters e.g. T instead of 9. Keyboard chip
12. Corrupt data in MODE 7 after warm-up, e.g. characters wobble. A finger on
the 16MHz crystal X1 made things worse. IC37 (74LS04) was defective. It proved
to be heat sensitive.
13. Monitor displays multiple characters. This one was down to IC3 (6522).
14. Intermittent loss of reset. The 55 chip IC16's socket had become faulty:
Direct pcb mounting cured the problem.
15. Intermittent loss of reset. Another cause is a dry joint on R21 (1Mohm),
which is connected to pin 6 of the 55 chip IC16, due to poor flow soldering.
16. Poor Quality IC sockets with thin pins were used on Issue 4 boards. The
pins snap off inside the socket housing - thus when resoldered the broken pins
drop out of the PCB.
17. DFS faulty - computer comes up with the logo and a continuous tone. Duff
8271 (Expensive!!) is generating a continuous IRQ. The fault can be found by
taking out the DFS rom. DFS 0.9 searches for an 8271 FDC and if it does not
find one then it starts to loop and then generates a continuous tone. This can
be used for fault finding. Later issues of the DFS rom don't do this.
18. Won't start up, continuous tone present. No 8MHz output at pin 7 of IC6,
the video processor chip. Replace the chip.
19. No loading from the cassette port. Capacitors C31 & C35 open circuit.
20. Incorrect writing from the keyboard. Keyboard chip 74LS251 faulty.
21. Loss of display when the computer has warmed up. A duff 6845 CRT chip.
22. Shift or Control keys not working. Key f4 permanently on. Replace
23. IRQ line incorrect after a few hours. The fault was pinned down to the IC1
(6502A) becoming heat sensitive.
24. Noisy display with UHF output. Using an ocilloscope at the emitter leg on
Q8 showed that there was little luminance present. Q8 (BC309) had gone faulty.
The emitter is normally at 1 volt and the base at 0.2 volt.
25. Computer slowed down to approximately 700kHz instead of the stated 2MHz. A
duff 6502A (IC1) was bringing the clock pulses down at pin 37. The fault
disappeared when IC1 was replaced.
26. Intermittant lock-up from the keyboard. The socket for the OS Rom, IC51 was
27. Various wierd RAM faults - strange displays, garbage on screen etc. This
may be caused by IC14 (74LS245). This chip is socketed on later issue boards.
28. Mode 7 o.k., all other modes faulty. Input from keyboard prints twice,
with two cursors. IC39 (74LS283)) faulty - pin 4 low. See also (31).
29. Faulty display with higher definition modes. IC39 (74LS283) faulty - pin 1
30. Flickering display in Mode 0. Fault in the video processor chip's palette
(IC6). Chip is earlier ULA type and is overheating. Replace with MkIII version.
31. Mode 7 o.k., all other modes faulty. The disenable input at the video
processor chip IC6 was incorrect. The cause of the fault was found to be an
open-circuit track between pin 3 of IC 38 (74LS86) and pin 9 of IC41 (74LS02).
See also (28).
32. Printer won't work. Pin 40 of IC69's socket was open-circuit. Replace
33. Printer won't work. The computer was a Model A with an issue two board
that had been upgraded to Model B specification. Pin 26 of the IDC connector on
the old Issue two board is shorted to earth - the printer won't work in this
condition. Cut the pin.
34. Modes 0, 1 & 2 faulty - top half of the display flashing. R10 (3.3kohm)
open-circuit. This resistor is connected to pin 11 of IC27.
35. Incorrect data being sent and recieved by the RS423. A comparison with a
fully functioning computer showed that the mark/space ratio of the waveform at
the Baud Rate generator was wrong. IC42 (74LS163) was defective.
36. Interrupt request (IRQ) line down. The meter reading at pin 20 (CS) of the
BASIC Rom IC52 was very low. The 74LS139 Rom select chip was faulty.
37. Will not upgrade to DFS. Pin 12 in IC78's socket open circuit. Replace the
38. Printer won't work. Pin 6 of IC27 (7438) very low. Replace IC27. See also
(32) & (33).
39. DFS faulty: computer loads corrupted data from disc. IC87 (74LS123)
40. DFS faulty: data is corrupted when saved onto disc. IC80 (7438) defective.
41. DFS faulty: index header only is displayed, with corrupted figures. IC27
(7438) defective. See also (49).
42. DFS faulty: read/write incorrect. IC77 (74LS00) defective.
43. 80 Column display at switch on. There are a number of causes for this
fault. Usually the IRQ line goes low because of the 8271 FDC being faulty.
Other possible causes are IC3 (6522) port A (keyboard) and the 6845 CRTC
(c.r.t. controller) chip IC2.
44. The computer ran but gave error messages when programs were loaded into
memory, e.g. "error at line 567.987468". Also the ram test gave a 'no room'
message and the disassembler, when loaded, gave access to only one address at
a time. This fault was tracked down to a faulty IC89 (68B54) Econet chip.
45. RAM fault. CAS 0 disappeared and the computer became a Model A. The fault
was IC45 (74S139).
46. Poor field sync on monitor. This was caused by IC41 (74LS02) which
provides the composite sync from the CRTC chip's seperate field and line sync
47. Loading problems with cassettes on Issue 3 & 4 boards. The resistor R75
needs to be changed from 82kohm to 47kohm.
48. Caps lock won't work, i.e. LED didn't light. IC32 (74LS259) proved to be
49. DFS fault on Issue 4 boards onwards: reads index header only, with
corrupted titles. Link S9 not cut with factory fitted DFS.
50. RS423 fault: the port refused to work properly when a simple acoustic
coupler type modem was used. This modem only supplied the data in/out signals.
The BBC B also needs A CTS signal. The user had used the COMUNITEL system and
used the ?89 procedure to override this need. The B needs a real signal to
operate with a 'dumb' modem. So link the RTS & CTS pins on the DIN connector.
51. No writing from the keyboard. IC4 (74LS30) faulty.
52. Modes 3 and 6 faulty. The effect on the screen was ghost printing under the
required text etc. IC36 (74LS10) faulty, with waveform at pin 8 when it
shouldn't be present.
53. Continuous multi-tone from the speaker. IC18 (76489) was going unstable.
This is a four-channel tone generator.
54. Hangs up for no apparent reason. This may be a fault during production,
the clock pulse being too narrow. Can be cured by adding a 100pf Capacitor from
pin 37 of the 6502A microprocessor chip IC1 to the chassis. With issue 3 & 4
boards there are, near this chip, two plated holes that can be used.
55. Hangs up with a continuous tone. IRQ line and clockpulse at pin 37 of IC1
incorrect. Pin 15 of IC24 (74LS138) was short-circuited to the chassis.
56. Intermittant crashing. replace IC51's socket.
57. Cursor only displayed, flashing at top left of the screen. The IRQ line
was low with incorrect waveforms around IC32 (74LS259) which was defective.
57. Screen filled with half-inch(ish) thick horizontal red bars interspersed
with yellow blobs. Poor chip select signal from IC26 to the video processor
chip, IC6. IC26 (74LS139) was defective.
59. Loud buzz on sound with the screen flashing (no data displayed). IRQ line
to the 6502A chip incorrect. Pin 20 of the Basic Rom chip IC52 read 50ohms to
the chassis. The rom decoder chip IC20 (74LS139) was faulty with a leak
between pins 8 (chassis) and 12.