Printed circuit boards are a very cheap, convenient and compact way to assemble a variety of electronic components so as to form a working electronic unit. The components are mounted on the board by hand or by robot depending on quantity and are then soldered by lowering the relevant side of the board on to molten solder thus making all the soldered joints at once. The board is then connected to custom-made test equipment to check that it works OK.
The problems start when the PC board fails in service. Finding where the problem lies can be difficult and if a complete working replacement can be obtained at reasonable cost this is the best solution. There are several good sources in the Sales Page of this website. The following text cannot be more than an introduction to faultfinding and repairing PC boards; there have been plenty of books written on the subject. I can only write about my own experiences when I have had a few successes and a lot of failures. The old adage "if it ain't broke don't mend it" is true but if a PC board is already "broke" then there's not a lot to lose, particularly if you think you know what needs doing. But repairing PC boards usually involves some pretty delicate soldering so if your previous experience and soldering iron has been mostly concerned with plumbing you will need some practice and a new soldering iron before even thinking about it.
Handling PC boards
PC boards are not usually fragile but neither can they
be handled carelessly. Bending a board even slightly can result in fracture
of the tracks which can be hard to detect subsequently. When unplugging connectors
or chips from a PC board make sure that the board is well supported and if much
force is needed prise the connections apart instead of pulling them. Where PC
boards are held in place by screws or clips, ensure that the board can be removed
easily; it's all too easy to overlook a fixing and strain the board.
When possible work with the board lying on a flat surface, preferably resting
on a soft cloth. This is particularly important when plugging ICs into sockets.
If this isn't possible then ensure that the board is supported behind the socket
with the fingertips.
Damage caused through static electricity
We have all experienced shocks from static electricity
caused, for instance, by walking over nylon carpet, particularly in warm and
dry conditions. You go to open a door and there is a discharge from your fingers
to the metal door handle. These static charges can measure 1000s of volts and
therefore many people go through a solemn ritual of earthing themselves before
even taking the back off their machine.
I have handled plenty of PC boards without taking any such precaution and consider
that PC boards are immune to such disasters. However, when handling active components
such as transistors or chips I avoid touching the pins as much as possible and
keep myself earthed with a wrist strap or by touching a metal radiator occasionally.
I'm not convinced that it's necessary but it's easy enough to do. Most repair
shops take special earthing precautions and so who am I to argue they're wrong?
It possibly comes down to the make-up of the individual; for example some people
have damp hands others very dry. It must make a difference to the conductivity
through the skin. Once components are fitted into the PC board they can be handled
with impunity. I don't know why new hard drives and other units are often supplied
in special "anti-static" bags. Could it be that it looks impressive?
Finding the fault
If possible get a circuit diagram of the PC board and this,
together with your knowledge of the symptoms, is a start. Don't overlook the
obvious. Power failure may be caused by a blown fuse!.
The professional has an array of test equipment but the home hobbyist will probably
have to get by with a multimeter and maybe a logic probe. The latter is quite
inexpensive, around £15 from Maplins for example, and very useful. Many components
on Amstrad PC boards are TTL and won't tolerate more that 5 volts, so be careful
of this. However, I've used many types of multimeter and have never had problems
even though the battery is often more than 5 volts.
Part of the faultfinding will have to be done with power on which can be dangerous
if you're checking the power board so be sure to take all precautions. Don't
try to check the voltage on the UHT lead of the flyback transformer with your
multimeter; you'll probably fry it. I use insulated tools and thick rubber gloves
when fooling with mains voltage. Laughable, I know, but I'm still alive (I think).
Other boards such as the CPU board and the boards in the keyboard and printer
are less fraught, being of lower voltages, but I still use screwdrivers insulated
to the tip to help prevent short-circuiting components. These boards are best
powered from the machine when testing live. Don't underestimate the power; even
12 volts can give you a nasty burn if you're unlucky. It's the current that
burns, not the voltage.
Fault-finding, particularly on a board for which you have no circuit diagram
is difficult. If you're lucky you may find a pretty obvious fault, but moving
through the board from component to component can give false results. For example,
if you think that a resistor, diode or capacitor has gone down the only way
to check for sure is to unsolder one pin from the board for a true test. If
the flyback transformer is not delivering UHT, the fault may lie further back
with the supply to the transformer. In some cases PC boards have test points
where voltages can be checked and this can be helpful but not conclusive.
Sometimes the fault is obvious; maybe a wire has broken away from the PC board.
It is not always possible to resolder the wire back in the same position, but
by using the meter a common point can be found where it is easier to make the
connection. Similarly, when tracks are fractured, it is not always possible
to solder connecting wires at the point of fracture but by using the meter suitable
points can be identified where wires may be more easily soldered. Fine insulated
wire is not always easy to find but one way is to strip down a piece of IDC
cable. Steps in faultfinding are as follows.
1) Inspection; look for broken wire links, fractured tracks,
signs of overheating and dry joints i.e soldered joints which have failed. Use
a magnifying glass.
2) With power on check out known voltage outputs. e.g on the output plug of
a power supply board from a PCW8256/8512 you should get +5, +12 and +24 volts.
If one is missing check the 78XX chips,described later
3) If the board uses plugged-in chips try prising them out a little then pressing
them back in. Occasionally a pin or two becomes very slightly corroded, spoiling
contact. Incidentally note that all chips on a board are plugged in the same
way round as shown by a depression at the pin 1 end. Useful to know when changing
a chip!
4) If you're knowledgeable a lot can be found out using the logic probe; that's
a bit beyond me but I like looking at the pretty lights.
5) Pray
Soldering
This is aimed at the person who doesn't have experience
of fine soldering but would like a starting point. In the end there is no perfect
way; everyone finds their own way of doing things. Your first consideration
must be safety; a hot soldering iron can inflict a painful burn. The second
is to keep your workplace organised, clean and tidy so that you know where to
find things in a hurry. The two things are really the same; in particular keep
your iron on a stand or hanging in a safe place when not in use.
The professional repair shop has all manner of soldering and desoldering aids;
the following is what you can do as a hobbyist with low cash limits. For working
on PC boards a 15 or 18 watt soldering iron with replaceable bits and a selection
of bits is desirable; get a well-known make with a standard bit then get extra
bits once you feel the need. A larger iron of 25 watts is useful for larger
components. Various pieces of kit to be used as heat shunts will be needed;
a common trick is to use bulldog clips, although purpose-built kit is available
and is cheap. The shunt is clipped to the component to protect the component
from excess heat whilst soldering or unsoldering. Cored solder in various gauges
will be required; get the finest gauge you can; hopefully 22 gauge plus a more
robust 18 gauge. You'll only need the smallest packs - solder goes a long way.
You'll also need some sort of flux; I don't know what's available now, I'm still
using the remnants of a tin of Fluxite from many years back. Other requirements
are a desoldering pump for removing excess solder and desolder braid for mopping
up small amounts of surplus solder.
PC boards are usually coated with varnish so before attempting to solder, this
must be scraped off locally to get down to the copper. The copper tracks are
fragile and can separate from the board under excess heat so always endeavour
to use the minimum amount of heat. Solder melts at a specific temperature and
the sooner this temperature is achieved the less the amount of heat generated.
So if you have to melt a large blob of solder the 25 watt iron is the one to
use; the 15 watt iron with a 1mm tip is useful for very precise work and so
on. Don't apply the iron for longer than is needed. As soon as the solder is
seen to be molten do your job of soldering or unsoldering and leave it.
Before starting a specific job, perhaps soldering a wire to the board, do a
dry rehearsal to ensure that everything is in the optimum place for speedy work
and that the board is easy to work on.
Removing soldered components from a board can be difficult. Components such
as resistors and diodes which have only two connections are quite easy if you
heat the solder around one leg whilst levering the component gently from the
board. Then grasp the component between your finger and thumb and pull it as
you heat the other leg. You'll soon know if you're using too much heat! Two-pin
components, such as electrolytic capacitors which are usually mounted close
to the board or components with four legs will have to be rocked whilst heating
each solder joint in turn until the component can be released completely. Components
with 3 pins in line are particularly difficult to remove and you may have to
cut one pin close to the board with side cutters before unsoldering the other
two. Components with multiple legs, 6 or more such as chips really have to be
destroyed in order to avoid damaging the board (unless you have access to specialised
(expensive) equipment. Cut off each leg with side cutters then remove each leg
individually. If you want to save the chip you'll need to destroy the board.
When a component has been removed solder usually remains to block the hole and
this can be cleared by reheating and clearing the hole with a sewing needle
(to which solder will not stick) or I prefer to use a matchstick on which the
end has been whittled to a point which will go through the hole. The desolder
pump and braid can also be of service here. To remount a chip it is better to
solder in a low-profile socket and plug the chip in, assuming you have the headroom
when the PC board is replaced.
Components
Many components on Amstrad boards were special to Amstrad
and are not, as far as I know, available now, but others are standard components.
Among these are the 78XX range of voltage controllers. Those I've seen used
are the 5volt(7805) the 12 volt(7812) and the 24 volt(7824) Make sure you get
the 1 amp version or better. They may be found bolted to a heat sink with their
three legs soldered into the PC board, although they are often used without
a heatsink. Other ICs can sometimes be replaced by alternatives if you know
what they do in the circuit, but in this case you should ensure, even if the
circuit appears to be working OK, that excessive heat is not being generated
before you put the cover back on.
Resistors, capacitors and diodes are all standard items but do your best to
get the same spec. for safety reasons.
Summing up
For the average hobbyist faultfinding and soldered repairs
to PC boards are confined to the more obvious faults. It is better to obtain
a complete working board to fit, or you could spend hours with no benefit at
the end.
Faultfinding a board with an obscure fault needs serious equipment such as an
oscilloscope, the knowledge to use it, a circuit diagram and pinouts of the
transistors and chips on the board. However if you're that way inclined, as
I am, the exercise can be pleasurable and if success crowns your efforts then
it's time to celebrate.
I hope that some of the above waffle may be of help if you intend to make small
projects on Veroboard such as the 12 to 5 volt power conversion board described
in the 3.5" drives section.
Ron King
Andover, England Email ron@king27.freeserve.co.uk