[Milsurplus] Audio Output Tranny- Mystery Failure Mode

[Bruce Gentry]

I restored an REP from two scrappers.  Like anything I have ever seen 
made by Crosley the transformers in them are about as cheap and 
cut-corner  as they can be.  Even properly restored, the output 
transformer in mine gets rather warm after a few hours of use outside 
the cabinet. I think heat has caused a short in a winding of the output 
transformer in yours, probably the primary because the layers of 
windings are thicker and the heat generated would be trapped. Even a few 
shorted turns can drastically alter the sound, and not cause much of a 
change in a resistance measurement.  The power transformer is even 
worse, it gets too hot to touch and drips pitch all over the cabinet.  
One of the two I used for the restoration had a shorted power 
transformer, the other was covered in pitch that had oozed all over the 
chassis. These receivers were designed for 110/220 volt power, and no 
more. The cheap transformers had minimal everything, and if 120 volts is 
used, they get HOT. If the radio was powered on 50 cycles, especially at 
120/240 volts, it would be barbequed. I mounted a small 120/240 to 12/24 
volt transformer under the chassis, and hooked it up as an 
autotransformer so each primary winding on the power transformer gets 
108 volts.  Reducing the voltage did not affect the performance at all, 
and the reduced voltage and current in the output transformer  could 
only do it good. I tried mounting the power transformer on spacers to 
get airflow over it, and noted the temperature after a few hours of 
operation in the cabinet. I noticed the chassis was rather cool, even 
when operated in the cabinet, and hit on the idea of using heat sink 
compound to mount the transformer to the chassis instead of on the 
spacers. It worked well, the transformer is noticably cooler and the 
chassis near it  warmer.  The receiver sounds good for a table radio, 
but there are only two AM stations that play music here, and I only 
enjoy one of them, playing oldies.  One great thing about morale 
receivers is that most have a phono input, fine for amplifying receivers 
that only have a line or headphone output.

      Bruce Gentry,  KA2IVY


On 8/2/13 7:03 AM, David Stinson wrote:
> Working on the Navy REP Moral(e) receiver.
> It has a single-ended class A 6V6 output.
> Audio level is about half what it should be and distorted.
> Substitution shows the problem to be both the output transformer and 
> the internal speaker.
>
> I pulled the output tranny.  DC resistance checks show what
> one would expect for good windings.  Used a DC supply at
> 90 volts between points and had no detectable leakage between input 
> and output windings or to ground.
>
> Ideas as to what the failure mode in this tranny might be?
>
> 73 DE Dave AB5S
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