[AMRadio] Modulation transformer ID and repair

[Donald Chester]

If you can ID the primary by the markings P-B-B-P, put 110v into the 
primary.  Measure the voltage that appears across the secondary.  The ratio 
of voltages will give you the turns ratio.  Square the turns ratio to get 
impedance ratio.  Look for any markings that distinguish the secondary B+ 
lead from the class-C plate lead.  Getting those reversed can crap out the 
transformer, since many transformers have less insulation on the B+ side of 
the winding, since it only carries the DC plate voltage, while the class-C 
end carries the DC plate voltage PLUS the AC audio voltage.  A capacitance 
meter should show more capacitance between the core and B+ lead than between 
the core and class-C plate end.  Once the configuration and impedance ratio 
is determined, only trial and error will tell if it is a satisfactory 
transformer, since you don't have data on the nominal impedance values or if 
the secondary is designed to carry DC.  Use a reactor regardless, if one is 
available.

The best way to drive moisture out is to keep the transformer in a warm, dry 
environment for an extended period.  You could try baking it out in an 
electric oven (gas ovens tend to have warm moist air, since water is a 
by-product of the combustion of methane).  After the drying out process, 
hi-potting should be ok.  You want to dry it out first, so that you don't 
risk damage to the insulation with hv from the hi-pot machine.  Depending on 
the hi-pot device used, that may not be a danger, since a good hi-potter 
limits the current flow to a few microamps, not enough to damage damp but 
otherwise undamaged hv insulation.

Don, K4KYV

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