Old Anelex power supply: AC on the DC

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This huge, ancient power supply made by Anelex has been serving from
under my shed workshop bench for about 25 years:
https://www.dropbox.com/s/g11s67bb5jk7bc8/HD-Power-Supply.jpg?dl=0

It supplies a few projects around the house and garden and I generally
just ignore it. But a couple of days ago I found that there is 115V AC
between the case (connected to the 230V AC mains input) and all the DC
outputs. Could have more than a bit to do with the shock I described in
my earlier post 'DC cable puzzle'!

What are the more obvious causes to pursue please?

Terry, East Grinstead, UK

Re: Old Anelex power supply: AC on the DC
On Sat, 25 Aug 2018 08:21:12 +0100, Terry Pinnell

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Water ingress, or corrosion in the power transformer... The
transformer is the only place for that to happen, BUT completely
disconnect the output from the load to make sure it is the supply and
not the load picking up stray leakage from somewhere else.

The primary is probably arranged so you can feed from 120/60 or
240/50.  So a wiring error like grounding the two primary windings at
the midpoint would give you exactly 1/2 your mains supply V and those
symptoms you mention.  If a load resistor between the 115 you are
seeing and case drops the voltage to  zero that may indicate a water
or corrosion malfunction (suspect those same terminals) - but start
with a light bulb not a tiny 1/4 watt resistor (lest if vaporize
between your fingers.  

A hard (wired) connection will not drop the voltage much and should
cause a 240 V incandescent bulb to glow dimly.

Stay safe.  Observe good safety practices, this nuisance has the
potential to turn lethal.  BTW that supply looks like something we
used in the 60's in the Navy, and there's no reason it shouldn't last
longer than your shed if it's in a dry protected location.

Re: Old Anelex power supply: AC on the DC

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Thanks, very helpful. Continuing testing today.

Terry, East Grinstead, UK

Re: Old Anelex power supply: AC on the DC
On Sun, 26 Aug 2018 08:33:46 +0100, Terry Pinnell wrote:

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I had the same thing happen with an old Advance RF sig gen I owned about  
40 years ago. It was fine for years then suddenly started outputting 115V  
on the sig-out socket that was supposed to be 1mV-1V! Blew a lot of  
transistors before I discovered the fault, 'cos I have very high skin  
resistance and couldn't feel it. One particularly dull day I noticed a  
tiny spark jumping from the probe tip to the DUT and on further  
investigation discovered the high V output. The Earth connection to the  
metal case should have prevented this, but the previous owner (a ham  
radio operator) had for some bizarre reason inserted an insulating bush  
between the Earth tag and the chassis!
This may or may not help you but it just reminded me how stupid some  
(supposedly intelligent) people can be.




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Re: Old Anelex power supply: AC on the DC
On Sun, 26 Aug 2018 08:52:36 -0000 (UTC), Cursitor Doom

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There are valid reasons for "floating" the sig gen from ground from a
troubleshooting point of view (like ground loops) but safety and
trouble shooting expediency can be at odds.

One of those 3 wire into 2 adaptors makes more sense so you can float
it when necessary without wiring changes.  Or isolation
transformers...

Re: Old Anelex power supply: AC on the DC
On Sun, 26 Aug 2018 05:28:21 -0400, default wrote:

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Ah, well maybe that's what he did, then. Just forgot to remove the spacer  
after he'd finished doing whatever he was doing with its internals. Easy  
mistake to make.





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Re: Old Anelex power supply: AC on the DC
On Sun, 26 Aug 2018 12:36:08 -0000 (UTC), Cursitor Doom

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In a chemistry lab I worked at they had these tablet dissolution or
disintegration machines.  Anyhow, lots of pumped electrolyte
(hydrochloric acid to simulate conditions in the stomach.  Big metal
cased machine with the works electronics as well as plumbing in one
chassis on a lab bench (with sinks and other grounds nearby).

The old geezer that'd been running that test most of his career swore
he got a shock from a machine... sure enough that whole lab bench was
wired with the hot and neutral switched around, and the case of the
machine was connected to neutral not ground - so I dig out the
schematics and damn if it wasn't showing that on the schematic:
neutral going to chassis and ground connected to neutral.  But like
most accidents, several things have to go wrong before someone is
hurt, it wasn't until the benches were rewired that we discovered the
instrument ground problem as well as the bench wiring problem.

Another time I watched a woman lab rat (affectionate name for the
chemists that worked there) get shocked transferring water from a
plastic spigot to a plastic jug.  I didn't believe it, but of course
checked it out.  Turns out the DI water was so pure it was a perfect
insulator (yeah water can be an insulator, just not tap water) and we
had inadvertently created what was Millikan's water drop experiment...

The friction of the water in the plastic piping was imparting a static
charge and the 3 gallon plastic vessel she was filling was picking up
and storing a hefty static charge.  I put a brass pipe fitting on the
spigot and grounded the fitting and that fixed it.

Re: Old Anelex power supply: AC on the DC
On 26/08/18 15:17, default wrote:
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I once touched the metal front lid of one of our instruments
in a rack, and got a minor shock. On checking, the instrument's
metal case was obviously and measurably connected to the mains
protective earth.

The person who had built it said the shock was impossible,
but still refused to touch it (coward!) :)

Eventually we noticed the front lid's hinges were plastic,
so it was actually floating with a minor induced charge.
Quickly cured by a wire bypassing the plastic hinge.


Re: Old Anelex power supply: AC on the DC
On Sat, 25 Aug 2018 08:21:12 +0100, Terry Pinnell wrote:

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Strange design having all that heat sinking inside the case and so close  
to those caps. :-/




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Re: Old Anelex power supply: AC on the DC
On Sun, 26 Aug 2018 08:56:16 -0000 (UTC), Cursitor Doom

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It looks like a mil-spec supply so may be over-kill on the heatsinks,
and it may also be old enough to have germanium transistors which
don't suffer heat as well as silicon.

Note the fine tuning on the voltage outputs?  That suggests it was
made before they began incorporating remote sensing of voltage to
eliminate voltage drop between supply and load.  Or 1960-70 or so. The
pass transistors may be PNP germanium devices.

Re: Old Anelex power supply: AC on the DC
On Sat, 25 Aug 2018 08:21:12 +0100, Terry Pinnell

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Might be just transformer capacitance, in which case it can't supply
much current.

Load that 115V with a couple uF of film capacitor and see if it
changes.


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John Larkin         Highland Technology, Inc

lunatic fringe electronics  


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