This chapter is intended to provide the less experienced reader with some of the information which he or she may require in order to carry out some of the more advanced upgrades and modifications described within this book. Those readers who already have some practical experience with a soldering iron may find some of what follows a little tedious, although they should still read the sections on anti-static precautions and perhaps de-fluxing.
Before going on to describe the precautions that should be taken when handling certain integrated circuits it is prudent to spend a little time considering the environment in which any work on the BBC Microcomputer should be carried out.
If you do intend to carry out any work on the circuitry within the microcomputer, whether to simply plug in additional chips or to perform soldering operations, you must first of all give some thought to the manner and place in which the work should be carried out.
You must have a solid work surface which should ideally have a free area of no less than one metre square. It should be set at a comfortable height such that you can sit down whilst working. It is advisable to use a conductive cover placed over the surface of the work bench, the reasons for this are detailed in the section below on anti-static precautions.
There should be plenty of light, a desk top lamp fitted with the highest wattage bulb that it can safely hold is an ideal light source since it can be adjusted into position above the work. For some of the more delicate work a magnifying glass or eye glass may be required, these can be of great help when working on small or delicate parts. The best place to obtain a suitable magnifying glass would probably be your local philatelists or hobby shop.
Before commencing work on the machine be sure that you fully understand exactly what it is you are doing. This is especially true if you have not tackled this type of work before. You should be totally familiar with the particular section in the book that deals with the work you are doing, if you are less experienced this will involve reading the section a number of times. All of the tools, integrated circuits, sockets and connectors etc, which you require for the work should be on hand ready. It is advisable to check that you do in fact have the correct devices before fitting them, this is especially true if the parts were purchased by mail order since more mistakes seem to occur when parts are bought in this way.
It is important to allow yourself plenty of time in which to carry out the work, do not be tempted to rush because this usually results in poor quality workmanship. Also if you rush work of this type it is very likely that mistakes will be made and these could turn out to be costly.
A systematic and orderly approach should be adopted since this will minimise the likelihood of doing something incorrectly or omitting something. In practise this will mean planning ahead by mentally going through the procedure before you commence work. Try to keep the workbench tidy and do not clutter the space up with unnecessary objects, leave yourself plenty of room in which to work.
It is also important to seat yourself in a comfortable position at the workbench. If you are not comfortable whilst you are carrying out the work you will be tempted to rush the job and hence run a higher risk of making mistakes. If you do rush the work the end result will probably not be of a good standard.
Make sure that you have plenty of light on the work area, a strong light will make most of the work which you will be carrying out on the machine far easier.
Once you have completed the work on the printed circuit boards it is essential that you conduct a detailed visual inspection of both the work which you have carried out and the remaining areas of the PCB. It is quite likely that small pieces of wire etc., will have fallen onto the PCB whilst you were working on it. Under no circumstances should you reassemble and switch on the machine without having carried out this inspection, in fact it is advisable to inspect the board at least twice using a strong light and an eyeglass. As mentioned in chapter 5, be sure to check that the heat sink on IC6 (Video Uncommitted Logic Array) is firmly seated and that non of the resistors and capacitors have become bent over and shorted. The importance of this inspection cannot be over emphasised.
Many of the integrated circuits used within the BBC Microcomputer are either Negative Metal Oxide Semiconductor (NMOS) or Complementary Metal Oxide Semiconductor (CMOS) types. These types of IC's are susceptible to damage by static discharge and special precautions are, therefore, necessary when handling these devices.
Opinions on exactly what precautions should be observed when handling the devices seem to differ but the procedures outlined below, if observed, will ensure that the risk of damage to the IC's is very minimal. Although many people believe that with present day MOS devices the possibly of damage due to static discharge is hardly worth considering recent research has proved otherwise. Recent investigations have revealed that damage caused to MOS and CMOS integrated circuits may not be immediately apparent, that is to say that the device may function perfectly satisfactorily but will fail after only a relatively short time in service.
The number of MOS devices which you will be handling whilst upgrading or modifying your machine does not warrant the expense involved in purchasing any sophisticated anti-static handling equipment but a few simple precautions should be observed;
In order to carry out many of the modifications described in subsequent chapters, certain basic tools are required. If possible, obtain good quality tools, as these should last a lifetime if treated with care.
The following list of tools will allow the reader to carry out all of the hardware upgrades and modifications which are described in this book:
No.2 cross-point screwdriver.
No.3 cross-point screwdriver.
Flat bladed terminal screwdriver.
Pair of thin nosed pliers.
Pair of oblique cutters.
Soldering iron, preferably temperature controlled (see section 2.4).
Desolder pump (see section 2.5).
Solder-22 S.W.G. rosin cored, 60/40 tin/lead alloy.
Bench lamp (e.g. "Anglepoise" lamp) .
Many newcomers to electronics and computing are under the misconception that the soldering of components to delicate printed circuit boards is a very difficult technique to master. Although good quality soldering does require practice and patience, it is a task which should be mastered fairly easily by the majority of readers. This is especially the case if the general guidelines which follow are carefully observed.
A good quality soldering iron is, of course, a necessary requirement. Poor quality soldering irons can easily cause damage to the main PCB of the BBC Microcomputer, since the board has fine copper tracks on both sides as well as through-hole plating. Ideally, a temperature controlled iron which is fitted with a fine pencil" tip, should be chosen. This type of iron may have a heating element power of up to 50 watts, the higher wattage types having a much faster temperature recovery than the lower wattage types. Temperature controlled soldering irons are, however, relatively expensive and if the price is considered to be excessive, then a good quality "thermally balanced" type is an inexpensive be alternative. This type of iron is designed such that the be heating element and tip assembly combination is only capable of attaining a certain maximum tip temperature, in the steady be state condition. For work on the BBC Microcomputer, a thermally balanced soldering iron of power between 15 and 25 watts should be chosen.
Before the soldering iron is used to make a joint, the heated tip should be thoroughly cleaned on a damp sponge and then "tinned" using rosin cored solder. For soldering delicate PCB assemblies, such as the main PCB of the BBC Microcomputer, a good quality, rosin flux cored, 60% tin, 40% lead solder of 22 SWG. should be used for both tinning the bit and soldering the components. To solder a joint, place the fully heated tip of the soldering iron into position and allow the area to heat up for approximately two seconds, before "feeding" in the cored solder. When the solder has flowed uniformly around the joint, stop feeding in the solder and then remove the soldering iron. After the soldered joint has cooled, inspect it to see that the solder has formed a good quality joint, which has a bright, shiny appearance. Dry-joints, which are usually caused by insufficient heating of the joint or contaminants on the component leads, can quite often be identified by their dull, matt appearance.
When soldering multi-leaded components such as integrated circuit sockets and some types of connector, it is recommended that soldering is alternated between diagonally opposite pins, so that localised overheating of the component and PCB is avoided.
Occasionally, it will be necessary to desolder components or clear PCB holes which are blocked with solder. This task can be successfully performed using a desolder pump, which should be available from most electronic component stockists. Most desolder pumps consist of a spring loaded plunger inside a cylindrical barrel. The spring is held compressed by a catch, which when released causes the plunger to travel rapidly along the barrel, thus sucking the molten solder through the fine, heat resistant plastic nozzle.
To remove excess solder using a desolder pump, firstly "arm" the pump by depressing and locking the spring loaded plunger. Position the nozzle of the pump close to the solder to be removed and whilst keeping your thumb positioned over the trigger button, melt the solder using a soldering iron. When the solder is fully molten, depress the trigger button such that solder is sucked up into the barrel of the desolder pump. The molten solder forms a pellet inside the barrel which should be discarded by re-arming the pump. This operation should not be carried out whilst the desolder pump is positioned over the PCB, since the solder pellet or any solder flakes which have been formed can easily fall onto the board, thus bridging tracks.
After soldering, flux deposits will be left on the printed circuit board, which look unsightly as well as possibly causing high impedance tracks between soldered joints. These deposits can be easily removed, using a proprietary defluxing solvent (e.g. RS stock number 555-134) , which should be applied using a stiff brush. It is recommended that the manufacturers instructions supplied with the defluxing fluid are carefully read and understood, since the solvents used can be harmful, are particularly if defluxing is not carried out in a well ventilated area. It is also important to ensure that the point. solvent does not come into contact with the bodies of the components on the PCB, since the solvent may dissolve certain types of component marking and some thermoplastic.