Trouble Shooting

By John Brown

Originally published in EUG #02

Trouble Shooting

Having almost completed rebuilding an Electron, these notes are given in the hope that they may be of some use to others. I will start with the power transformer.

Power Transformer

This is internally protected with a cut-out, so any serious fault will blow it. The output of the transformer is between 16 and 18 volts. This is fed to the socket on the right hand side of the Electron.

Inside the computer are two 'boards', the one on the right is the power supply system board. The incoming Alternating Current (AC) is rectified and modified to give 5 volts Direct Current (DC) positive and 5 volts negative DC to supply the needs of the computer. The power supply could quite easily be modified to run from a 12 volt DC input (like a car battery).

These voltages are connected to the main board by a three-wire connector. One wire is Positive, one is Negative and the last is 'Common' (this is best explained as being Positive to the Negative line and Negative to the Positive). Coming to the main board, you will see a small metal box on top; this is the modulator which converts the incoming signals to video composite at Radio Frequency (RF). This will be acceptable to any carefully tuned television set.

This composite signal is comprised of two digital signals called the "Video" and the "Sync", the latter being required to keep the video in step with the TV. The sync itself needs two types of signal: "Frame", which produces the vertical pulses, and "Line", which produces the horizontal pulses. You need not concern yourself with these as they are produced internally by the computer's electronics.

Some Snags Encountered

  1. Screen lit with 'noise' (white dots, etc) = Modulator fault (No data from the computer)
  2. Screen lit with a square raster but no data = ULA - which stands for Uncommitted Logic Array - (large IC marked "ICL") could be faulty or, in my case, just dirty contacts.

    Release the spring clip that keeps the metal square in place.

    This must be done carefully with preferably no leverage. Remove the metal plate and the chip can be carefully unplugged. Important - Be sure to note the exact position of the ICL before removing as there are 68 connections!

    Holding lightly, clean the connections with a pencil eraser (use a new, clean one - they're cheap!). After cleaning, replace the chip in the correct position and re-fit the metal plate. Having switched on, you should see the "Acorn Electron" logo. If not, you may have a faulty chip. These are now obsolete and unobtainable as far as I know, but if a stock does exist somewhere you may be asked to pay the proverbial arm and leg for it.
  3. No colour from the RF socket = In my case, IC11 was faulty; this supplies the pulses to drive the colour burst. IC12 can produce a similar result.

Please remember, the above is only for the electronically minded and should not be attempted by the faint-hearted!

Happy Computing...

John Brown, EUG #2