Many of the modifications to be described require delicate soldering to the main PCB, which involves dismantling of the machine. It must be stressed that such work might affect the manufacturers guarantee and it is therefore recommended that you should consult your local Acorn dealer before carrying out any of the modifications described.

Before carrying out any modifications it is also advised that the following chapters be studied and fully understood:

  1. Chapter 2 - Basic skills and Techniques Required.
  2. Chapter 4 - Link survey.
  3. Chapter 5 - upgrades.

6.1 The Cassette Interface

Early versions of the BBC Microcomputer had circuit board errors associated with the cassette interface. Current production boards have, however, been suitably modified in order to make the interface more reliable. Details of these errors and modifications are given below.

  1. Due to manufacturing tolerances, there can be an undesirable DC offset voltage on the operational amplifiers inside IC35 (LM324). To check this, measure the voltage with respect to earth at pin 1 of this IC (The case of the UM1233 UHF modulator provides a suitable earthing point). If this voltage is greater than 100 millivolts then the DC offset is excessive, possibly causing the cassette interface to malfunction. Rather than change the IC, which is normally soldered directly to the circuit board, a cure can be effected by connecting a capacitor of value 100 nF in series with resistor R87. The capacitor to be added should be a non-polarised type of small physical size (e.g. RS part number 125-806).

    If a particular machine will not load or save data onto a cassette tape which is known to be OK, then it is most likely that the cassette interface lead is faulty. This can be checked by visual inspection or by testing for continuity between the pins of the 7-pin DIN plug at the microcomputer end and the relevant pins of the connector at the cassette recorder end, using a multimeter set to the 'OHMS" range.

    If the cassette lead is checked and found to be OK, it is possible that the LM324 (IC35) is faulty. This is usually caused by a cassette recorder that has either an abnormally high output level or high voltage transients superimposed on the output signal.

    In most machines, IC35 is not mounted in an IC socket and hence removal. of a faulty device requires very careful desoldering This is best left to a dealer if you do not feel suitably competent. It is advisable, when replacing IC35, to mount a 14-pin IC socket (e.g. RS part number 402-305) in this IC position and fit the new device into it, thus making further replacement much simpler.

  2. The run-in tone detection circuit is incorrect on circuit boards up to and including some issue 4 boards. This can be rectified by changing the value of the resistor (P75 which, along with capacitor C28, forms a CR filter network for the run-in tone detector. R75 may either be a 100 kilohm resistor (early issue PCB) or an 82 kilohm resistor (later issue). Ideally, the value of R75 should be chosen on test as the value that gives the largest pulse on an oscilloscope at pin 15 of the serial ULA (IC7). The value of this resistor should be around 50 kilohms.

  3. Capacitor C32 forms part of the low pass filter of the cassette interface. On some issues of the main circuit board this component was left unconnected at one end, in which case the unconnected end should be connected to the junction of resistors R87 and R88 (see figure 6.1 for details).

  4. When using the microphone input of a cassette recorder, the output (pins 4 and 5 of the 7-pin DIN connector SK5) of the BBC Microcomputer will have to be attenuated. This can be accomplished by connecting a resistor, typically 100 kilohms, in series with the input to the recorder. A convenient point to mount this resistor is inside the 7-pin DIN plug or jack plug of the cassette lead.

  5. The record/playback head of the cassette recorder must be aligned correctly. An audio dealer should have an azimuth alignment cassette to set this up. Alternatively, on cassette recorders which have a hole through which the head alignment can be adjusted, a rather cruder but nevertheless effective way to do this is described below:

    Obtain a good quality, commercially produced, music cassette tape. Whilst playing back this tape through the recorder whic is to be aligned, adjust the set screw with a fine jeweller type screwdriver until the best high frequency response is obtained. The record/playback head should now be in reasonable alignment for use with the computer and should allow you to load commercially produced software reliably.

  6. Only use cassette tapes of the highest quality for saving computer data. Use of cheap, poor quality, tape is false economy in the long run since it will inevitably break or cause data corruption. Cheap tapes will also shed excessive amount; of their oxide coating onto the heads and rollers of your cassette recorder, requiring you to clean them more regularly.

    Oxide coating deposited onto the components of the cassette recorder, particularly the heads, will load to poor playback and recording performance. It is a good idea to remove periodically, any oxide deposits -from your machine. This can be done using a proprietary head cleaning kit or, alternatively, using cotton buds moistened with isopropyl alcohol. It is important that the cleaning solvent is only used sparingly and is removed Using a clean, dry cotton bud immediately after application.

  7. The internal loudspeaker of the recorder can usually be disconnected by plugging a suitable jack plug into the external loudspeaker or earphone socket. with some types of recorder a dummy plug is provided in the accessories pack supplied with the machine for this purpose.


6.2 Sound Output From the BBC Microcomputer

  1. An external loudspeaker of no less than 8 ohms impedance can be substituted for the internal loudspeaker, by connecting it to plug PL15 on the main circuit board of the machine. This requires removal of the keyboard unit (see Chapter 4) * By wiring the internal loudspeaker and external loudspeaker through a switching jack socket (see diagram below), the internal loudspeaker can be muted when a jack ping connecting the external loudspeaker is inserted into this socket. It is best to use a 3.5 mm. jack socket for this purpose. This can either be mounted in a hole drilled carefully in the case of the computer or better still, it can be mounted, with a little ingenuity, in the hole marked "reset" if a reset button is not to be fitted (see section on reset button later in this chapter) . In the latter case be careful that the metal parts of the socket are well clear of the components and tracks on the printed circuit board. A piece of PVC insulating tape can be stuck to the PCB, directly below this socket in order to prevent the connections from shorting to the tracks of the main

  2. A high quality, low level audio output from the sound generating circuits can be taken from PL16. PL16 takes the form of two solder pads situated on the WEST side of the main printed circuit board, near to the -5V power-supply lead spade connector (mauve or brown wire). See link shadow diagram (figure 4.1) for more details.

    The output from PL16 can be fed directly into the auxiliary" input of a "Hi-Fi' power amplifier.
    Connections to PL16 must be made using screened cable and of course this modification requires soldering to the circuit board. The centre (signal) core of the cable should be connected to the SOUTH (bottom) pad of PL16 and the outer screen) wire to the NORTH (top) pad. Again, it is possible to utilise the hole intended for the "reset" switch (if it is not already being used for the external speaker socket or a "reset" button) for mounting a suitable socket for this modification, thus avoiding the need to drill the case of the computer.


  3. The variable resistor VR1, which is mounted on the WEST side of the main circuit board, under the keyboard, adjusts the sound output level (see photograph). Clockwise rotation of this control increases the output level. Note that in some machines this control is omitted and replaced by a wire link. In such cases a suitable 10-kilohm pre-set type potentiometer may be fitted to the PCB in order to provide a volume control, after removal of the wire link. A suitable control is PS part number 186-621.

  4. A buzzing noise may be heard from the loudspeaker of early issues of the BBC Microcomputer. This is due to the audio input (pin 16) on the 1 MHz bus connector picking up noise From the computer busses. This can be cured by connecting a 1/4 watt, 10 kilohm resistor between the audio input and earth. There are three ways of doing this:

    a) Solder the resistor between pin 7 of IC20 (74LS139) and the through-hole plate solder pad immediately below this pin. The connection to IC20 is merely a convenient earthing point.

    b) Solder the resistor between pins 1 and 16 of the 1 MHz bus connector solder pads. Make sure that the leads of the resistor are well sleeved so that they do not touch any other connections on the circuit board. This method does not require such (delicate soldering work as the method described above but does necessitate removal of the printed circuit board.

    c) obtain a 34 way JDC header socket C e.g. RS part number 467-3e2) and connect the resistor between pins 1 and 16. Tte header socket will then silence the audio buzz when it is inserted into the 1MHz bus connector. Make sure when inserting this socket that it is inserted the correct way round, that is, with the "bump" locating into the cut-out in the top of the socket. This method has the advantage that it does not require any soldering to the main printed circuit board. If the 1 MHz bus connector is required for peripheral devices, then the resistor can be connected to the relevant connections inside the device.

6.3 Video Modifications and Hints

  1. Make sure that any monitor used with the BBC Microcomputer provides mains isolation. This is most important and is best checked by consulting the manufacturers of the monitor. Domestic televisions have a special type of isolator incorporated in the lead to the UHF socket. Do not tamper with this or indeed any other part of a television set which you are using as a monitor.

    Specialist firms can convert some domestic television sets so that the RGB output of the BBC Microcomputer can be fed into the television. This can give a very good quality display at moderate cost and has the added advantage that the TV set can still be used to receive normal TV broadcasts. Only have such work carried out by a reputable dealer, preferably one which has the approval of the TV manufacturer.

  2. A reasonable quality display can be obtained by connecting the video output socket of the BBC Microcomputer to the video input of a video recorder. This requires a 75 ohm BNC to BNC connecting lead which is available from video accessories stockists and RS Components (part number 488-539), The UHF output from the video recorder should produce a higher quality display than when using the UHF output of the computer. The reason for this is that the UHF modulator inside the video recorder will, in general, have a better overall specification than the UM1233 modulator in the computer. The audio output from the computer can be connected to the audio input of the video recorder, such that the sound is modulated as well as the video signal. This results in the sound being reproduced by the loudspeaker of the television connected to the video recorder. In this case, the sound level can be controlled using the volume control of the TV set.

    The above connections also allow a high quality video recording of the sound and vision signals from the computer to be made, This is a useful hint when playing computer games, since top scores and so forth can be recorded without the need for a printer. This tip is also useful for indexing video tapes.

  3. If a colour video signal is required from the BNC video connector (SK2) then it will be necessary to connect a 470 pF ceramic capacitor between the emitter of transistor Q9 and the base of transistor Q7. On issue 4 and above versions of the main printed circuit board this modification is made by simply connecting link S39 (see link survey section for more details)

  4. Operation of the BBC Microcomputer in close proximity to a video recorder may cause patterning on the screen of the TV to which the recorder is connected, because the UHF modulators in both the video recorder and the computer are set to almost the same frequency, causing a beat frequency to be produced. To remedy this it will be necessary to set one of the modulators to a different frequency. This is easily accomplished on some video recorders by a screw adjustment (see video recorder operating manual or your local dealer) . The output frequency of the UM1233 modulator inside the BBC microcomputer may be adjusted by carefully rotating the core situated at the bottom right hand corner of the modulator (see photograph)

  5. If possible use the channel marked "VTR" or "AV" of a television set for the computer. This channel is designed to be more tolerant to the synchronisation (sync) signal produced by video modulators.

  6. Some very early machines were fitted with sub-standard video ULA chips. An approved BBC Microcomputer dealer will check this for you, should you be in doubt. A faulty video ULA (IC6) will normally cause random flashes on the monitor screen in the highest resolution modes. Be careful not to confuse this symptom with RF interference, which can give a similar effect. It should also be checked that the metal heatsink, which prevents overheating of the video ULA, is properly positioned.

  7. The UHF cable provided as standard with the BBC Microcomputer is not of the highest quality. This can be replaced with a lead made up with the required length of low loss UHF TV aerial cable. The connectors from the standard cable supplied may be reused on the new lead. It may be necessary to enlarge the hole in the plastic cover of the "phono" plug to allow the thicker cable to pass through.

  8. Inverse video, black text on a light background, can be obtained by moving link S26 from the normal WEST position to tile EAST position. This link is directly below the video ULA (IC6) . For further details refer to the link survey section.

  9. Do not stand a television receiver or video monitor on the case of the BBC Microcomputer. There are several types of stand available which fit over the computer, thus allowing a monitor or TV to be placed directly above !t~ An excellent example of this is the Broady "Owl-Perch" (supplier code 'B" at the front of this book) , which is cast in aluminium and painted the same colour as the BBC Microcomputer.

  10. If instability of the TV screen is experienced when the internal loudspeaker of the microcomputer is in operation, then this is likely to be because the UM1233 UHF modulator is "microphonic". The cure for this is to remove the cover from the modulator and spray the inside with aerosol lacquer (RS anti-corona aerosol, part number 555-617, is ideal for this). Take care that the lacquer does not spray onto the main PCB, and do not forget to replace the cover to the modulator.

6.4 Speech synthesiser Tips and Modifications

  1. Modifications must be made to early (issues 1,2 and 3) versions of the printed circuit board in order to permit the speech interface to function correctly. The modifications required are:

    a) Cut the PCB track leading to pins 13 and 14 of IC98 ci the printed circuit board (underside). Connect pin 13 of this IC to pin B of the cartridge ROM connector PL14 (third pin from the WEST on PL14). This connects pin is of IC98 to 0V instead of +5V.

    b) Cut the tracks, on the component side of the main PCB leading to pins 16 and 17 of the system VIA (IC3). Scrape away the lacquer from the tracks and solder a piece of fin insulated wire to each of the two halves which are still connected to pins 16 and 17 of IC3. The other end of the wire connected to pin 16 at IC3 should now be connected to the cut track which was originally connected to pin 17. Similarly solder the other end of the wire which is connected to pin 1 to the track which was connected to pin 16. Thus the track which were originally connected to pins 16 and 17 of IC3 are transposed.

  2. A standard Texas Instruments VM61002 speech memory ROM may be substituted for the BBC phrase ROM (PHROM) in I position 98. This will produce speech with a Transatlantic accent,

    The use of the VM61(3e2 ROM necessitates that the following syntax be used

    SOUND -16, (any number between 1 and 206),0,0 or
    SOUND -16,GET,0,0

    There are five Texas phrase ROMS available all of which ca replace the BBC PHROM they are:-
    VM61002 Industrial vocabulary 206 words
    VM61003 Weather Vocabulary 150 words
    VM61004 Military vocabulary 150 words
    VM61005 Avionic Vocabulary 148 words
    VM71003 Female clock 35 words

    These all have different SOUND command calls. Thus varying SOUND 1 to -16 will enable the various speech ROMs.

  3. On issues 4 and above of the main printed circuit board, there is a control (VR2) which can be used to alter the pitch of the speech synthesiser. This potentiometer is connected in parallel with R32.

    On early issue boards R32 (which is located between ICs 98 and 99) can be desoldered and replaced with a 220 kilohm pre-set potentiometer. Using this control the pitch of the voice may be adjusted.

6.5 Analogue Port Modifications

  1. The analogue-to-digital converter used in the BBC Microcomputer uses a reference voltage generator consisting of three silicon diodes connected in series. The total voltage drop across these diodes, approximately 1.8 volts, provides the voltage reference source. This does not provide a temperature stable reference voltage but is adequate if the analogue port is used only for games paddles or joysticks. For scientific experiments a band gap reference should be used instead of the three diodes to provide this reference voltage. A suitable band gap reference voltage source is the Ferranti ZN423 (RS 283-223). This device will greatly increase the stability and repeatability of the analogue port. The ZN423 provides a reference voltage of 1.26 Volts, which is somewhat lower than that produced by the diodes but nevertheless provides increased performance over the latter. Consequently, the full-scale input voltage of the analogue input port is now 1.26 volts.

    To carry out this modification, desolder the three diodes (D6, D7 and D8). Connect the positive output of the ZN423 to the pad where the anode of diode DS was originally connected and the negative output to the pad where the cathode of diode D6 was connected. Further information on the ZN423 device is given in figure 6.3.

    When using a ZN423, the value of resistor R.71, which is adjacent to pin-S of IC73, must be changed to a value of 1 kilohm (1/4 watt metal film type). This limits the current through the ZN427 to approximately 4 mA.

  2. The 25-way D-type analogue port socket (SK6) carries the light pen strobe connections as well as the analogue-to-digital converter connections. If simultaneous use of both a light pen and a peripheral which uses the analogue-to-digital converter is required, it is a good idea to connect the light pen connections to a separate socket. To do this, mount a suitable 3-pin socket to the case of the computer and connect this to the relevant connections of the analogue port socket (SK6) using short flying leads. The connections are as follows:

    Pin 9 - light pen strobe input. (LPSE).
    Pin 1 - +5 volts.
    Pin 2 - ground.

    The light pen should be wired to the mating 3-pin plug of the above-mentioned socket.


  3. The inputs at the UPD7002 analogue-to-digital converter (IC73) can be protected from overvoltage by connecting a light-emitting diode (LED) between each of the input terminals and ground, as shown in the diagram below. The LEDs act as voltage "crowbars", thus protecting the sensitive inputs to the UPD7002 from damage due to overvoltage.


  4. It is important that any input leads to the analogue-to-digital converter are adequately screened. With careful screening, up to 10-bit accuracy should be achievable using the ZN423 modification described above.

The manufacturer (NEC) of the UPD7002 analogue-to-digital converter has recently revised the specification of this device to 10-bit accuracy instead of the original 12-bit.

6.6 Printer Modifications and Tips

  1. On some early issue main circuit boards there is an unterminated track on the underside of the PCB, near to the Centronics parallel printer port connector (PL9). This track is from the CAl connection (pin 40) of the user versatile interface adapter (IC69) and should connect to the Acknowledge (ACK) line on the printer port plug (PL9 pin 19). Connection of this pin to the unterminated track, on the underside of the PCB, can be made using fine insulated wire. In order to solder to this track it is necessary to scrape away some of the PCB lacquer coating with a knife-blade, taking extreme care not to cause any damage to the circuit board.

  2. Some printers, for example, the Seikosha GP, require pin 26 to be cut out of the printer port connector. This should be done with extreme care. Pin 26 is normally earthed and since the PCB track connecting it also connects adjacent pins to earth, it is not easy to modify the track.

6.7 Keyboard Hints

  1. Although the keyboard is very robust, refrain from hitting the keys unnecessarily hard. This advice is given since it is possible that some keys, which were not pushed fully home into the metal mounting plate during assembly, might transfer force through to the copper tracks on the keyboard PCB if hit too hard, thus resulting in a broken connection.

  2. On early machines the keyboard connecting cable was soldered directly to the keyboard PCB. Frequent removal of the keyboard to change sideways ROM S or make modifications can result in the wire cores of the ribbon cable fracturing and causing intermittent or non operation of certain keys. A cure for this is to replace the ribbon cable with one that is terminated at both ends with 17-way 0.1 inch pitch "Molex" type connectors. This also requires a 17-pin "Molex" KK connector type 430 to be soldered into the keyboard PCB. These can be obtained from several component manufacturers, for example The Radio Resistor Company (supplier code RR).

    The above modification is present on issue 3 machines onwards.

    The Acorn part number for the ribbon cable assembly complete is 7720-17-2-4 and the part number for the 17-way plug is 800059.

  3. The break key may be disabled by removing the wire link which is directly in line (running NORTH/SOUTH) with pin 2 (WEST end) of the connector on the keyboard end of the ribbon cable. This tip is useful in schools and the like, where accidental operation of the break key is to be avoided. In order to reset ("BREAK") the computer, a normally open push button can be wired into the vacant position on the circuit board, between the "Econet" socket and analogue port socket at the rear of the circuit board. This is marked RST on the circuit board and "reset" on the rear panel.

6.8 Sideways ROM Socket

  1. When replacing a Series 1 operating system contained on two 2764 EPROM devices mounted on a carrier board" with a single ROM version, the ROM may be a loose fit in the IC socket into which the carrier board was originally fitted. This is because the large diameter pins of the carrier board distort the socket contacts. One solution is to desolder and replace the damaged socket. This is, however, fraught with danger since the solder pads on the PCB can lift off, necessitating a replacement PCB. A quick and very effective alternative, requiring no soldering, is to fit a 28-way turned pin IC socket e.g. RS 402-333), which has similar diameter pins to those on the carrier board, into the damaged socket. The ROM can then be fitted into this turned pin socket.

  2. The carrier board mentioned above should be retained as it can be used to hold two 2764 EPPOMS and used as a substitute for a 16 kilobyte 27128 device.

6.9 Disc Interface Modifications

Due to faults on issues 1, 2 and 3 of the main PCB, the following modifications are required to permit the disc interface to function correctly. These modifications are:

  1. Remove the wire link from S9, before the P8271 Floppy Disc Controller (FDC) IC78 is fitted. This is most important, since if left in position this link will short the interrupt (INT) output of the P8271 to 0V, which could destroy the device.

  2. Cut the leg on pin 9 of IC27 close to the circuit board, using a pair of oblique cutters, taking great care not to damage the PCB in the process.

  3. Solder a 2" length of insulated wire to the cut leg (pin 9) of IC27 and connect the other end of the wire to the EAST pad of S9.

  4. Make sure that link 82 is closed, unless an "Econet" interface is fitted.

  5. Make sure that link S8 is fitted, as this links the head-load signal from the disk interface circuitry to the disc interface plug (PL8).

6.10 RS423 Hint

When scrolling through program listings, the CTL-N and shift key facility can be faster than is desired. Using the RS423 port, the screen can be made to scroll through listings at a speed determined by the baud rate of the RS423 port.

To do this, a normally open contact push button switch is wired across the clear-to-send (CTS) and request-to-send (RTS) pins of the R8423 connector (SK4). The commands listed below are typed into the keyboard and when the switch is closed, the screen will scroll at a rate determined by the RS423 baud rate selected. The commands to be typed are:

*FX7,X	<return>	(sets receive baud rate)
*FX8.X	<return>	(sets transmit baud rate) -
*FX5,2	<return>	(sets RS423 serial output).

The value of x in the above changes the baud rate of the RS423 port and should be chosen to give the required scrolling speed.

The command CTRL-C will return the scrolling to the normal rate.