Use a variac, an AC ammeter along with an inline fuse . You know what the primary voltage is and the corresponding secondary voltage. Just scale down from that once the turns ratio is know and "tickle" the primary with some fused low voltage and measure the corresponding secondary voltage. If the voltage ratio is correct based on the turns ratio the transformer is probably good. The only other issue is a breakdown within the transformer that does not show up with the low voltage applied. Make sure if the transformer has been stored in less than a great place that you bake the transformer first. An oven works good as long as your wife is not using it!! If you have a good sunny day placing it on a driveway will bake it nicely. Hy Pot testers are great buy many do not have them. An old oil burner ignition transformer current limited under variac control works well.
Joe, W3GMS
I like Joe’s suggestion – particularly using the lower voltage and calculating the output winding based on turns ratio. As stated by a previous post, a resistance check would not help with shorted turns or a insulation breakdown.
A quick check I used for my failed AL-82 power transformer was to use a 60 watt bulb in series in conjunction with a Variac. No breakdown = dim light bulb. A breakdown = sporadic flashing of light bulb as the breakdown occurs and transformer recovers from the reduced voltage applied to primary due to light bulb. You might even hear the arc if you have a good set of ears – I did.
Al
