Authenticity opinions

Chris Elmquist chrise at
Sun Jun 6 17:05:22 CDT 2010

On Sunday (06/06/2010 at 06:54PM +0100), Tony Duell wrote:
> > That sounds very low indeed.  I wouldn't expect a transformer secondary 
> > to drop below rated voltage with only half load on it!  Are you sure you 
> > haven't got a shorted turn somewhere?
> My experience is that a single shorted turn in a mains transformer will 
> cause enough primary-side current to be drawn to blow the fuse. Of course 
> that's assuming there is a primary-side fuse -- but if there isn't there 
> darn well should be :-)

Yes... this is a fuse but it has not blown...  ever.  The machine got
pretty extensive use from early 1976 until 1984 or so and even with the
low 8V rail, it did heroic efforts for me.

> Still, it can't hurt to measure the primary current with no load on the 
> secondaaries. If you have a wattmeter, masure the power consumed by the 
> trransformer with no secodnary load (note that this is _not_ the product 
> of the primary corrent and the mains voltage, they will not be in phase). 
> If there is significant power drawn on no-load, then suyspect a shorted turn.

Yup.  Have a wattmeter and can make this measurement...   however only
after desoldering all the connections from the transformer to the
chassis mounted bridge and the bridge mounted on a PCB since none of
this is connectorized.

> > I agree with Tony that you should do some more tests to see if there's 
> > something wrong with the transformer or rectifier.  For a start, look at 
> > the secondary voltages and current drawn from the supply with all the 
> > secondaries open circuit.  Excessive no-load current is a reasonable 
> > indicator of a shorted turn.
> I should have read on before writing the above :-)

I completely agree... although, I am now prepared to replace the transformer
so once I have desoldered the original I suspect I'll be putting back the
new ones.

> > If you eventually do need to upgrade the power supply, could you fit 
> > just one switcher in the cabinet with that transformer?  Use it for the 
> > 8V line, and keep the old PSU for the rest?
> My personal view (if I had such a machine) is that as the originial PSU 
> wasn't a switcher, you shouldn't use a switcher to do any form of upgrade 
> or repair. But it's really up to you.

I agree with this too absolutely.

I have another homebrew S100 machine from the same era and it too has
a marginal power supply...  but I won't think twice about rebuilding
that one with a set of switchers.  It has no pedigree or authentity to
be preserved... other than it is built in a wooden box with a bright
orange Formica front panel...  a piece that was left over from my mom's
new kitchen in 1979 ;-)


Chris Elmquist

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