+2 crashes upon pressing some keys

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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1024MAK
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Re: +2 crashes upon pressing some keys

Post by 1024MAK » Thu Apr 12, 2018 12:29 am

Some suggestions and hints and tips:

Only the electrolytic capacitors normally need to be changed.

Don't try to desolder the capacitors unless you are an expert and have suitable tools. Instead cut the leads. Then you can heat the solder pad, and use a pair of long noise pliers to gently pull the old wire out. Then clear the hole of solder using a solder sucker / desolder pump or good quality desolder braid. Don't even think about trying to use cheap stuff.

The advice about taking photos of each capacitor before starting work is an excellent tip that I recommend.

Polarity is very important with electrolytic capacitors. The negative lead is next to the side of the capacitor marked with - signs (radial types). But most circuit board markings use + signs to indicate where the positive lead goes.

Silicon heatsink paste is recommended. The adhesive type was not used when they were manufactured.

It's always a good idea to clean the old heatsink paste off and then apply a thin smear over the surface of the chip and the heatsink before reassembling. You only need a thin smear, as the idea is to fill in any unevenness in the surface, NOT to form a sandwich filling!

With the ULA, the metal heatsink parts fit together in such a way, that they are supposed to securely stay in place. You may however have to adjust the parts, or in some cases, bend the clip very slightly.

Have you ran a simple BASIC program (that shows that it is running, say by printing a counter on screen) for one or two hours?

A dry solder joint can also appear to be a heat related fault. Especially if physical disturbance of the PCB effects the operation.

Mark
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Alessandro
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Re: +2 crashes upon pressing some keys

Post by Alessandro » Tue Apr 17, 2018 9:53 am

Thanks everyone for the answers. Now, I'll try to summarize my answer as much as I can :lol:

Actually I have 10 Spectrums. Maybe I unconsciously included the Next in them! :mrgreen:

Further examinations of the ULA showed that the heatsink is firmly kept in place by a clip; I prefer not to take it apart, at least for now because I would not want to break it or damage the circuit board by applying strength to it. I still suspect the cause of the strange behavior could be a cold or dry soldering of the largest keyboard ribbon connector, as Mark suggested in fact. Therefore, no need to apply thermal paste, adhesive or not, at least for the time being.

After some advice from Dad, I purchased a soldering station like this one and a soldering pump. However it will take a very long time before I feel confident enough to even try to substitute the capacitors. I am starting from zero and am afraid I could butcher my Spectrum beyond repair.

Brendan Alford's test can be run continuously, but it won't demonstrate anything by itself, apart from the fact that the ULA and RAM seem to be in good order when the problem arises. The only way to invoke the strange behavior is to press the N, M and Space keys in fact, which which occurs after about 20-25 minutes of continuous operation.

Again, due to my complete lack of skill in soldering, it's going to take a long time before I attempt to check the keyboard connector and maybe resold it. I will make some practice with old cards when I have time to spare, then re-solder the largest ribbon cable connector, and see if the problem has been resolved. Then I plan to take note of all electrolytic capacitors on the +2's board and buy the new ones, then try to substitute them one by one. It will take me ages, but at least I have no hurry. I am doing this for my own personal enjoyment - like my other retrocomputing/retrogaming activities actually. I have no kids and my niece is not interested in present day computers, let alone 30-35 year old ones. This means I'll most probably leave them in my will to a computer museum or some other academic institution :lol:
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