BBC MICROCOMPUTER A AND B FAULTS -------------------------------- POWER SUPPLY PROBLEMS --------------------- Fault: Dead Fix: Check fuse in plug and inside PSU. If fuse keeps on blowing then an excessive current draw is taking place. Check current limiting resistor R1 ,1 OHM by mains input. If this resistor is blown then bridge recifier will be blown SHORT. Check power transistor TO3 package. If blown fit a BU105. If none of these fixes, replace capacitor C9 220uF at 16V. Fault: Overvoltage output / PSU constantly tripping out. Fix: If tripping out then either a short circuit exists on the output of the switching transformer (check RECTFIERS and SMOOTHING CAPACITORS) or the PSU is overvoltage. If overvoltage check for poor solder joints on small 6 pin feedback transformer. The poor joints are between the legs and the windings of the transformer. VIDEO PROBLEMS -------------- MODE 7 FAULTS ------------- Fault: Random characters on screen - Ok in other Screen Modes. Fix : IC15 74LS273 DATA BUFFER. IC10,IC11 81LS95 or 74LS465. SAA5050 TELETEXT IC. Upper half of RAM - prove failing ram by removing link S26 altogether as this keeps the machine at 16K but swaps the lower half with the upper half of RAM. Fault: Flickering characters - Ok in other Screen Modes. Fix: IC15 74LS273 DATA BUFFER. 6845 VIDEO IC. C37 (33pF). C41 (220pF) Fault: DATA LINE D6 appears to be stuck when in MODE 7 Fix: REPLACE IC36 74LS10. GRAPHIC MODE FAULTS ------------------- Fault: Screen flicker, odd dots, no display, cursor changing size. Fix: 6845 VIDEO IC. VIDEO ULA. If ok in MODE 7 replace IC8, IC9 81LS95. Fault: Only some (not all) Screen Modes working. Not including MODE 7. Fix: Video ULA ALL SCREEN MODE FAULTS ---------------------- Fault: No colour. Fix: Dry joints near MODULATOR. MODULATOR. XTAL. TRANSISTOR Q10. VC1 adjustment. Fault: Snowy picture or no R/F output. Fix: MODULATOR. Dry joints near MODULATOR. MODULATOR. TRANSISTOR Q8. Fault: Incorrect screen addressing. Fix: IC39 74LS283. IC39 can also cause odd effects in MODE 7. IC39 has been known to cause problems in MODES 0-6 and cause problems to MODE 7 in only the 32K setting. Fault: Machine appears to initialise Ok but SYNC on picture is incorrect. Fix: Not talking to VIDEO ULA. Check VIDPROC signal. Check IC21 74LS00 Pins 4,5,6 for VIDPROC in and out. Check IC41 74LS02 pins 2,3,1. Check Pin 1 for output. Check data lines on VIDEO ULA. Check IC25 74LS20 pins 9,10,12,13 and 8. Fault: Machine powers up in incorrect Screen Mode. Fix: Check links on bottom right hand side of KEYBOARD. Replace KEYBOARD. Replace 6522. KEYBOARD / SOUND FAULTS ----------------------- Fault: One line of keys not working: Fix: Broken track on keyboard. 6522. Replace KEYBOARD. Fault: AUTO REPEAT set to maximum. Fix: Replace 6522. Fault: Machine powers up ok but has either a continuous tone or gurgles and keyboard may not operate properly or sound gurgles in sympathy with keyboard. Fix: IC32 74LS259 or 9334. 6522. Replace KEYBOARD Fault: No Sound output. Fix: Check SPEAKER / SPEAKER leads. Is there output on PL15 ? Check for VSS (-5V) and VCC (+5V) to be present and correct voltage. If no output check IC19 LM386 Pin 5 which is the output. Check Pin 8 IC17 LM324 which feeds IC19. Check Pin 7 IC18 74869 which feeds IC17. Check Pin 14 IC18 74869 for 4MHz Check Pin 6 of IC6 ULA for 4MHz output. Replace IC18 74869. Fault: Noise from speaker. Can make different buzz depending on program. Fix: Old type LINEAR PSU fitted. Poor connection on DC power leads to motherboard. On Issues 1 to 3 motherboard the following modification needs carrying out to remove noise. Fit a 10K RESISTOR between pin 8 of IC20 and the thru plated hole just south of that pin. CASSETTE INTERFACE NOT WORKING ------------------------------ NOTE: Always use known good TAPE DECK, TAPE and CABLE as customers units are guaranteed to have a fault. Fault: Cannot load program from TAPE. Fix: LINKS S28 must be WEST. LINK S29 must be EAST. Replace SERIAL PROCESSOR and 6850. Both these are very reliable. Check for data on Pins 1,8,14 of LM324. Should be 1 VOLT p/p on Pin 1 of LM324. Is data present on Pin 12 of SERIAL PROCESSOR ? - If not replace CAPACITOR C30. Is SERIAL PROCESSOR SELECTED ? Check for SERPROC signal. Is 2MHz clock on pin 10 SERIAL PROCESSOR present ? Is clock on Pin 25 of SERIAL PROCESSOR present ? Is it a good square wave ? Replace CAPACITOR C28 for precaution. Replace 6502 MICROPROCESSOR. Is 6850 chip selected ? Check ACIA on pin 9 of 6850. RXD and DCD should both be running when loading a TAPE. Is 1MHz on Pin 14 of 6850 ? Check R/W on Pin 13 of 6850. Replace IC26 74LS139 for precaution. Replace IC24 74LS138. Check DATA / ADDRESS lines on 6850, SERIAL PROCESSOR, IC22, IC24, IC26. ANALOGUE TO DIGITAL FAILURE --------------------------- Fault: Not working on channel A Fix: Fit IC77. Replace IC77 NOT COMPLETING RESET CYCLE (Continuous Tone on Power Up) -------------------------------------------------------- Remove all of the following devices (if socketed) KEYBOARD, Both 6522's, SERIAL PROCESSOR IC7, 6850 IC4, UPD7002 IC73, ALL ECONET CHIPS, ALL DISC CONTROLLER CHIPS, Make Link S9, All ROMS except IC51, IC71, IC72, IC98, IC99. The machine will still power up with a Banner with all these devices removed. -------- Check RESET pulse on pin 40 of 6502. When pressing BREAK it should go low and then stay high. If incorrect then replace C6, C8, C10 and C24. If still faulty replace 555 chip (IC16). Check Chip Select on O.S. ROM IC51. Fit LINK S26 to SOUTH to set to 16K. Remove LINK S26 to select 16K (this swaps the top and bottom RAM banks to allow fault finding to determine which bank is faulty). Check for 1MHz at Pin 4 of VIDEO ULA, Pin 9 of IC34 and Pin 8 of IC34. Check for 2MHz at Pin 5 of VIDEO ULA, Pin 37 of 6502, Pin 10 of IC37. Check for 4MHz at Pin 6 of VIDEO ULA. Check for 8MHz at Pin 7 of VIDEO ULA. Check for 16MHz at Pin 2 of IC42. Check / replace IC14 (74LS245) Check / replace IC12 , IC13. Check CAS 0 Pin 7 of IC45 and CAS 1 Pin 5 of IC45 74S139. Check Pins 9, 10, 11, 12 of IC45 74S139. Check R/W on Pin 34 of 6502. Fault: Machine only works in 16K Mode. Ok with Link S25 in or out. Fix: IC39 74LS283 failing can cause this effect. NOT PRINTING ------------ Enter the following program..... 10 ?&FE61=RND 20 GOTO 10 This will randomly toggle all 8 Data Bits on printer port. It will also allow the checking of CS on Pin 23 of IC69 6522. Check Data Lines and Address Lines on IC69 6522. Check 2MHz on Pin 24 of IC69. Check 1MHz on Pin 25 of IC69 Check RESET on Pin 34 of IC69. Check R/W on Pin 22 of IC69. Check STROBE on Pin 39 of IC69 and on Printer connector. Check IC70 74LS244 Data Buffer. Check ACK is reaching Pin 40 of IC69.