[GreenKeys] Off topic: variable transformer

[Craig Sawyers]

Alternatively you can just cross your fingers and turn it on, and wait for sparks and smoke.

There can be problems that a variac will help diagnose well before any valve rectifier and
regulator.  Such as transformer problems like shorted turns (had that) or open circuit (like on a
Quad II power amp) or perished wiring.  Or if solid state, a bridge rectifier with an open or
shorted diode (had that), or a switched mode supply with a shorted bridge (had that too).

And if a switched mode supply sets fire to itself with the wrong voltage, what happens in a brown
out as the mains fails during a storm, or when there is a power failure and the mains cycles on and
off several times before it comes back up?


> -----Original Message-----
> From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Jeffrey D Angus
> Sent: 28 July 2016 05:29
> To: greenkeys at mailman.qth.net
> Subject: Re: [GreenKeys] Off topic: variable transformer
>
> On 7/27/2016 10:09 PM, Paul Heller wrote:
> > I have some electronic and computing equipment that I want to safely
> > power up. I'd like to use a variable transformer. Can anyone recommend
> > a particular model to buy?
> My recommendation: None.
>
> This a bad idea despite what you've been told.
> With regards to vacuum tube gear, there will NOT be enough filament voltage on the rectifier tubes
to
> conduct until you reach 80-90 VAC on the variable transformer. Then you will "hit" the equipment
with
> 60 to 70% of full B+ voltage. However, anything that requires the correct B+ and any biasing
voltages is
> going to be way off.
>
> If you're doing this to "reform" electrolytics, you're wasting your time.
> They're either good or bad, if the "reform" then they'll probably fail shortly afterwards.
Electrolytic
> capacitors are cheap. The stuff they damage when they fail are not.
>
> One "computer gear" what ever that means.
> If they have old linear power supplies, the voltages are going to be way off until you hit a
certain
> percentage of the line voltage. Having wrong or miss-matched voltages is a bad idea.
> On the other hand, if they are switched mode supplies, they tend to set fire to themselves when
they
> don't have the right voltages on the inputs.
>
>
> --
> Jeff-1.0
> wa6fwi
> http://www.foxsmercantile.com
>
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