The types of capacitor that age the worst (and therefore degrade over time) are the Electrolytic
capacitors. These are the large cylindrical ones. These are the ones that should be replaced to extend the life of a power supply unit (PSU).
Any replacement capacitor MUST
have a voltage rating the same as, or greater than the component that it will replace. The amount of electrical charge that a capacitor can hold is measured in Farads. Nearly all capacitors (except for memory back-up) are smaller than 1 Farad, so a smaller unit is used. For normal power supply electrolytic capacitors, these come in ratings measured in µF (micro Farads). A replacement capacitor should have the same value. If you are unable to get the exact value, it normally does not cause problems if you fit a slightly larger value (if it will fit on the circuit board).
The types of power supply unit used in modern computers (including Atari machines) are the switch mode type. These operate by rapidly switching the power so that the average voltage produced at the output terminals is the correct value. In order for the power supply to work correctly the electrolytic capacitors have to act as temporary stores for the electricity. This "smooths" the voltage. As the switching action is fast (value depends on the type of PSU, but can be from 20kHz to 200kHz) the capacitors have to be able to cope. Because the electric current flows in high frequency bursts, to be fully effective only use the "low impedance
" types in switch mode power supply units.
Power supply units normally get warm during operation. Heat is produced from the other components around the capacitors and from within the capacitors. Electrolytic capacitors age and degrade quicker as they get hotter. I recommend that you use the 105°C
To sum up, in the Atari ST power supplies use 105°C Low impedance electrolytic capacitors
capacitors cause problems. In some power supply units I was repairing (these were not Atari units) one Tantalum had failed short circuit in each PSU resulting in the PSU shutting down as soon as it started up!
These look like this
Do note that normal electrolytic capacitors and all tantalum capacitors MUST
be fitted the correct way round. If you fit one with the wrong polarity, when you switch on there may well be a BIG BANG
as your nice new capacitor explodes.
Most of the other smaller capacitors do not cause too many problems, so do not need changing. However, if you find any that run warm, or live next to a component that runs warm, it would be a good idea to renew it.
Any good electronic supplier should be able to supply these components. Some of the big companies that supply the UK market are
Rapid Electronics Limited, Severalls Lane, Colchester, Essex C04 5JS United Kingdom http://www.rapidonline.com/
Premier Farnell UK Limited http://uk.farnell.com/
RS Components Ltd. Birchington Road, Corby, Northants, NN17 9RS, UK http://uk.rs-online.com/web/
I have used all three of the above companies. You do not have to be a business, they will sell to individuals (they take credit card orders over the phone).
There are also some smaller firms, but they have a more limited range. Some of them can however help with sourcing components.
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