Saturday, 19 February 2011

A "Doing" Day (in the shed)

A few days ago a Skype message from one of my small group of "Skypees" alerted me to the fact that an equipment failure had occurred. Jim's 30 amp power supply was no more. I was being asked what I thought about a couple of possible replacements, but as I enjoy the challenge of "fixing things", and also generally far too parsimonious to countenance replacing something old with something new without at least putting up a fight, the Skype conversation quickly got round to the faulty equipment and to what might be wrong with it.

With almost astonishing speed I was appraised of fault symptoms and then a copy of the circuit diagram. It was completely dead. Nothing coming out of it, apparently. Nil. Zip. Nada. Not even "magic smoke".

The circuit diagram was examined and I figured out some likely causes, all which seemed to be fairly straightforward to resolve, and so it was arranged that I would call in to see my friend when I was passing his home a few days later.

In the meantime, my friend proceeded to purchase a replacement unit (a hundred pounds, near as dammit!) and was very pleased with it.

The broken power supply was duly collected yesterday afternoon, and today I decided to take a look at it.

The power supply is called a "Nissei DPS-300GL" and is quite a hefty beast. Opening it up I found it to be quite nicely constructed, albeit rather grubby as the cooling fan had been dragging unfiltered air in and across the internal heatsink for many an hour. However, no matter what the outcome of my efforts was, the fan would drag no more air as it was completely seized up beyond redemption.

Inside the Nissei PSU (quite neat isn't it?)

My earlier deliberations had led me to suspect a problem with the overvoltage circuit, but I adopted a technique I have used on many occasions to prove or otherwise the general health of power supplies, and that was to connect a current limited supply at the correct DC voltage across the main rectifier output. Encouragingly this brought the majority of the power supply to life, at the same time lending weight to my original theory about the overvoltage tripping circuit which was fed from a completely separate supply via its own transformer.

Next I moved the external supply to the rectifier in the overvoltage circuit, and to my slight surprise the "enabling" relay (the overvoltage trip drops out the relay which removes the AC mains supply to the main power supply transformer) was heard to operate. My original theory was that there was a fault with this relay, or its associated driving circuitry.

Fortunately this piece of circuitry is built on its own separate board and was relatively easy to extract and examine.

It didn't take long with an eye-glass and a multimeter to get to the root cause of the problem - see the attached photograph.
Underneath the Overvoltage Board ...

A nasty looking joint at the relay contact which switches the AC mains to the main part of the power supply. Bingo!

The joint was cleaned up and resoldered, the overvoltage circuit returned to its rightful place, the PSU powered up from its own mains supply, and lo and behold the PSU is now working, albeit temporarily without a cooling fan.   I realised at this point that the description of the original fault symptoms must have been slightly wrong in that the trip relay would have been heard to operate upon the application of mains, and I didn't verify this myself. Having said that this might have made me more worried about what the cause of the fault was!

Having spent a few frustrating minutes wrestling with Maplin's web site looking for a replacement,  EBay was consulted, and a 92mm 12V DC fan was quickly found and has been ordered for the princely sum of £4.99 (including postage!).

Soon Jim will have a spare power supply.

Right!  On with the next job ...

Today is the day I finally decided to start building my "X-lock", a VFO stabiliser kit from local firm Cumbria Designs.   I've had this "in stash" for quite a while as other projects have got in the way.  Once built I have then to decide which rig of my various"old ladies" will get the benefit.  At the time of writing I think it will be my FT-107, but that is by no means certain.   I also have it in the back of my mind to build the version described by Eamon Skelton EI9GQ which seems to be a variation upon the same theme.   Still, that will be a way off as I still have to "cut my teeth" properly with PIC programming, another medium-term objective of mine.  It will be interesting comparing the performance of the two units, and the advantage of the EI9GQ version is that the PIC source code is in the public domain, and I would have the opportunity to experiment with it.
X-Lock construction under way

Anyhow construction of the X-lock has now commenced, hopefully it can be finished off tomorrow, with luck.

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