11/73 into 11/03 chassis?

Robert Jarratt robert.jarratt at ntlworld.com
Fri Dec 11 00:47:24 CST 2015

> Rob,
>     Yeah, I'd actually really like to save this Micro 11's power supply but I'm so
> bad at electronics that I feel hopeless trying. 

I was the same (and still am), but I have slowly learned more and more, and acquired a few more pieces of test equipment.

> It'd be really good, though,
> because I could then just neatly slide the micro into the big cab when I feel
> like having it connected to all the other stuff, or keep it in its little deskside
> cabinet for typical day-to-day stuff.  It has to stay very neat and presentable
> because these machines are actually part of the decor of our den (and with
> my better half's enthusiastic approval, which I realise is quite amazing of itself
> and don't want to blow)!
>     Did you keep notes on how you ascertained what to replace?  Or can you
> summarize in a step by step way that an amateur would be able to follow?
> The whole of my electronics tooling consists of some soldering stuff and a
> multimeter.

This one is very fresh in my mind, so I have no problem describing the failure I had, but of course that does not mean you had the same failure.

First question is, what were the symptoms of the failure? Was there a smoke and a bad smell and the machine actually continued to run, or did it just stop working, or something else?

If you got the smoke and the smell, then it could just be the filter capacitors. You will need to open up the PSU in any case. Have a look near the point where the power cord is attached, there are some relatively large yellow-ish capacitors, if they are all cracked then it is the filter capacitors and you can just replace them. 

If it isn't that then you should inspect the components for physical damage, looking for leaking or bulging electrolytic capacitors, but just replacing any damaged ones may not fix it, as there could still be other damage, depending on what failed.

The failure I had was a loud pop, the machine stopped, and there was no visible physical damage. This was a failure on the half with all the transformers and the actual output connectors. It was the large switching transistor near to the transformer marked High Voltage, located on the part of the board furthest from the output connectors. This transistor had failed short (easy test with a multimeter), and it has also damaged a resistor (it measured open circuit), located between the two large capacitors, and with a black covering.

NB: I am not an electronics engineer, so treat all the above with caution. Remember, there can be lethal voltages even after the PSU has been disconnected. Check the voltage across the two large capacitors on the half of the PSU that has the mains connector before doing anything else.



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