[Boatanchors] Collins 516F-2 solid state conversion, use unused 5V windings to buck AC to primary

[Jim Wiley]

Keep in mind that in this case, the 5-volt windings were intended to 
power the 5U4 / 5R4 heaters, which when operating are at several hundred 
volts DC.  In fact, the DC voltage at this point is much higher than the 
120-volt primary winding.  using the 5-volt rectifier heater windings as 
a "buck" connection should be perfectly safe.

That may not be the case with other windings, such as the 6.3 / 12.6 
volt windings intended to power tube heaters.

Look at the specs for a transformer that was intended to power something 
like a vacuum-tube TV receiver or a vacuum-tube audio amplifier.  Those 
transformers had 1500-volt class insulation for the 5-volt windings.

- Jim, KL7CC


On 1/7/2015 7:04 PM, Rick Poole WA1RKT wrote:
> Several times I have seen a modification to the Collins 516F-2 power supply whereby the 5R4/5U4 are replaced with solid state rectifiers, then the unused rectifier filament windings are wired back to the primary out of phase to reduce the secondary voltages.
>
> I came across this article, http://sound.westhost.com/articles/buck-xfmr.htm where the author, Rod Elliott, describes a few different bucking transformer techniques in great detail.  At the end of the article he states:
>
> "Note that great care must be taken with construction and mounting of any transformer used as buck, boost or autotransformer. All parts of all windings are effectively at the full mains voltage, and insulation must be adequate to ensure that the end result is safe under all likely conditions (including faults). If the transformer has additional secondary windings, do not be tempted to use them for anything! The secondary is at mains voltage and the insulation between secondary windings is rarely (if ever) designed to withstand mains voltage, so any remaining secondaries are potentially lethal. Therefore, don't even think about using another secondary to power other equipment (for example). Your bucking (or boost) transformer must be dedicated to one purpose only!".
>
> By definition, a 516F-2 power transformer with 5-volt windings wired to buck the input has "(other secondaries used) to power other equipment", or more specifically, other parts of this equipment.  What are your thoughts as to whether this creates a hazard as Elliott describes?
>
> Rick WA1RKT
> Londonderry, NH
> Catch me on 14.300 MHz Intercon / MMSN most days
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