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How to repair the 3 most common power supply faultsThe three most common faults both occur at power on and have tell tale signs which can easily be determined with the aid of a DC multimeter.
Warning! The voltages inside the BBC micro power supply when running are potentially lethal. Do not attempt a repair unless you are competant to do so and have disconnected the power supply from the mains. Remember that the large capacitors in the supply also store charge for many minutes unless discharged safely.
How to read the filenames from a directoryThe OSGBPB (get byte/put byte) filing system call can be made to return the names of the files in the current directory, either one at a time or in chunks depending on how big a buffer you can afford. The call is made with A=8 (on the BBC micro) and will return with C=1 if not supported by the current filing system.The tape filing system and ROM filing system in pre-Master machines are examples of filing systems which do not offer the service. There is an
example to study, which gets the names 1 at a time and prints them
out. How to make use of the temporary filing system support on a MasterIn contrast to the remainder of the documentation about writing filing system ROMs and making use of operating system routines for the BBC micro series of computer the novel concept of 'temporary filing systems' introduced in the Master series is not well described in either of the two Master Reference Manuals.It quickly glosses over the subject on page G.1-4 and F.7-6. The following document covers the three cases when your filing system must respond in order for it to behave correctly, along with some rules about what to do and not do.
Download the three special cases to code for How to connect a standard PC VGA monitor to a BBC microThe following text, which is written to encompass the entire BBC microcomputer family may help the more experienced reader to appreciate some of the challenges to overcome:
The connector pinout is [1] R [2] G [3] B [4] Sync [5] ground [6] 5 volts. Not all monitors are able to deal with the composite sync: it is possible to use an LM1881 sync separator to recover the horizontal and vertical syncs, but infact the separate syncs are present inside the machine from pins 39 & 40 of the 6845 CRTC. They will require buffering suitably before connecting directly to a monitor. As the micro only ever outputs binary levels on the RGB port the loss required to get from 5V peak to peak to nominally 1V peak to peak can be achieved with some small signal diodes, eg.IN4148. With the Microvitec Cub the input stage is a 150R series resistor driving into a logic gate. You might consider a scan rate converter (basically it resamples the incoming video picture then outputs it again at the framerate and levels that the monitor expects) or if appropriate simply fit a TV card to your PC and use the composite video connector instead. Driving a SCART TV is a little simpler, the easy approach being to use the composite video output directly, but there is a notable picture improvement by using the component video. Again, the TV will expect 1V peak to peak when driving into a 75R load, should accept the composite video output as sync information (though a simple R-C low pass filter would be better, or derive a sync from an attenuated digital sync), and already supports the correct field rate and resolution. A charge pump from the 5 volt supply on the RGB connector could also be used to drive the 12V SCART switching line to make the set automatically switch channels when the micro is powered up. How to make a Master battery backup packAfter many years of use the battery backup required to store the configuration values and maintain the real time clock fails, revealing itself by the machine repeatedly forgetting the time or not powering up properly.The very early Master 128 computers were fitted with Lithium cells mounted in a recess next to the cartridge slots, however due to reverse current charging the cells in these machines were prone to bursting into flames so the design was modified to use a shrink wrapped pack of 3 primary cells. Replacing the pack requires two passive components, plus it is advised that a
3AA battery box is used to hold the AA cells to make future replacements
simpler: Note that the pinout of PL8 on the main PCB is reversible, so the 3 pin female plug need not be polarised. Use of leak-proof cells is recommended to avoid damage to the PCB. How to change the composite video output to colourBy default machines output only black and white video on the composite video connector, this is because the digital RGB output was the preferred display method and outputting colour composite video degrades the RGB slightly. Converting the composite video output merely requires mixing some of the colour burst sent to the UHF socket with the other video signals:
How to upgrade a BBC B to use a CMOS processor and BASIC IVThe following text is copyright Geoff Cox.The standard BBC microcomputer has a 6502A at its heart. This has been replaced by the 65C02 in second processors. The 65C02 may also be fitted to the beeb, where it will give 27 new 'legal' instructions, and will also reduce power consumption by around 80mA, a boon for sideways RAM/ROM board users! Fitting the new chip to the beeb is simply a question of switching off, removing the old 6502A and replacing it with a 2MHz 65C02, taking care not to bend any of the pins in the process! In most cases all will be well, but a few beebs will then start throwing out spurious characters in MODE 0. If this happens to yours replace IC14 with a 74ALS245, normally this is just a plug in replacement. All should now be well, but a very few machines may still exhibit problems. If yours is one of these a little surgery is needed in an attempt to correct a circuit fault.
Details of the new instructions are available in the Master 128 Reference Manual, part 2, under the header 'The BASIC assembler'. Alternatively a Rockwell data sheet for part 65C02 will contain opcode details. ©2008 SPROW [Updated 08-Aug-2008] | home |