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LF: Computer hash reduction

To: "LF-Group" <[email protected]>
Subject: LF: Computer hash reduction
From: "Alan Melia" <[email protected]>
Date: Wed, 8 Aug 2001 23:27:27 +0100
Reply-to: [email protected]
Sender: <[email protected]>
Hi all
I have been complaining about increased SMPSU has on 136 recently, relating
it my mind to the beginning of school holidays. I have now realised that
some of it was self inflicted !!  After nearly 3 years on this computer with
little in the way of trouble except a locked 90Hz line from the VDU
occasionally, the PSU fan stopped about 2 weeks ago. The easy solution was
to swap out the whole power supply unit (an old AT format unit) This was
quickly completed and an extra block of memory inserted  (now 64
Mbytes....well I am an old meany) What I did not realise for some time that
the extra drifty lines where the new power supply that now switches on a
frequency that puts a hamonic in the top half of the 136 band. This computer
is in a different room to the radio gear, but does have an audio connection
via an isolating transformer.

It is not an original idea but I decided an 'easy' solution might be to fit
a filtered IEC socket into the power supply box. A quick look at the spec
suggested that a 1 Amp or 3 Amp rated unit would be best as the chokes have
a higher inductance in the lower current rated sockets. I decided that
although a 1 Amp rated socket should be OK for a 250W supply I would play
safe and install a 3Amp unit (I also just conveniently happened to have one
in the junk box!!). It is a fairly tight squeeze. The best way of doing it
is to extend the earth wire from the stud and release the grommet holding
the lead to the power switch, then the soldered connections can be made
outside the power supply box, and the socket slid into place and screwed in.
I suggest the earth tag is orientated at the bottom to keep the live tags
away from the PCB. (I bent the 6mm tag up slightly). When screwed into the
metal housing of the socket is very close to comonents on the PCB. I was
initially intending to wrap the body in tape but the size of the hole in the
box did not allow enought clearance for even a layer of tape. I finally
placed a short length of OverHead Projecter foil (Copier/laser printer type
which will stand a higher temperature, not the hand draw acetate sheets)
between the socket body and the components, trapping the edge in the case
top to hold it in place.  This was done without removing the PSU from the
case, as all the screws holding the lid where accessible.

The results are quite pleasing. The clutch of six or seven 50Hz spaced
lines, with the strongest at S9 have disapeared and I can just see two or
three weak lines (<S1) on the waterfall display when the background is quiet
during the day. I suspect I may be able to get rid even of these by a bit of
careful re-routing of the leads and aerial feeds.

So if you want to run a computer even if its only to check the reflector (or
let the kids play games) whilst you are listening to the band this could be
a useful mod. The problem is that all you with it up-to-date blokes have
probably got ATX style PSUs now and I dont know what the inside of those
look like, but the same solution may well work.

Cheers de Alan G3NYK
[email protected]

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