The repair centre: Acorn Atom

1. TV and monitor output, cassette input: Electrolytic capacitors drying out

Over time, electrolytic capacitors dry out, and will have a lower capacity than originally. On the Atom, this leads to distorted video output and not being able to read tapes...

1.1 Smeared video output (composite video)

Smeared video output in text mode and a distorted graphics mode screen, like this:

1. Very strong ghosting and distortion in graphics mode (CLEAR 4):

2. A good picture on my TV, but the image is not quite ok an a video monitor (distortion at the top of the graphics screen, dark areas to the left/right of the graphics screen:

These problems were all fixed by replacing capacitor C3. You can find this capacitor on the top right of the Atom's PCB:

So, if you have a similar problem, just replace C3 and your output should have better contrast, no smearing and no graphics mode distortion. Should this not fix the problem, you could try to replace C2 and/or C4 but these seem to be far less critical in my experience (if C4 is bad, the image will not synchronise properly, so if you get no image at all, try this one first). Q1 and Q2 should also be checked if replacing C3 doesn't help, but the capacitors are usually the problem and should always be checked first.

1.2 Bad TV output

The TV output is even more critical w.r.t. the capacitors. If the TV output is not ok, and you want to use this output, it's best to just replace C2, C3 and C4 with new capacitors (22 µF axial, these cost hardly anything and are available from any electronics shop). In the past I replaced capacitors with those from a scrapped Atom. This did give me good composite output, but TV output was rubbish in some cases... I did this to keep the machine all original (newer capacitors look different, colours esp.), so there's a trade off: Keep it all original, or get good TV output. In the end all electrolytic capacitors will need replacing, as they all deteriorate over time...

1.3 Won't load programs from tape

This can be caused by defects in:

  1. capacitor C8
  2. The LM358 IC
  3. The 8255 IO chip

Replacing C8 is the first thing you should try. It's quite useful to have 2 Atoms btw, so you can verify if an IC is bad by plugging it in another Atom. In that case, try switching ICs first.

2. Damaged tracks/IC sockets caused by battery leakage: ROM/RAM switch card

I fixed an Atom with a 19" rack with a power supply, ROM/RAM switch card (a non-commercial product that one had to solder all the sockets etc. on, PCBs were made by the Dutch Atom club), and RTTY decoding/encoding card (the latter is untested, power supply not connected to it, and I don't have the connections to the IO socket; I could probably find out, but, I'm not really interested).

The ROM/RAM card did not work, so I removed it from the rack, and found a hidden battery, that had leaked all over the card!

I had to clean it all up, remove 5 IC sockets by force, cutting up the plastic with sharp nose pliers, then desolder the leftovers of the pins from the component side, as they were so badly corroded that the metal had disintegrated in places. No, I could not easily desolder the sockets otherwise, as the solder itself on the solder side of the PCB was badly corroded by the acid and putting a soldering iron to it would do nothing. From the other side it was not much of a problem and a solder sucking pump would remove most of the junk from the corroded side. More bad news: some ICs were dead... Eventually I got it all working again, but I often thought of just throwing it away as it took so much work...

The Atom itself was in a unusable condition too, as the EPROM installed to modify the OS and the modified FP ROM had a few bad bits... I finally fixed this when I got a HCR EPROM programmer (able to program 2532s).


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Last modified: Tue Mar 17 03:57:22 CET 2009