SX-28A Audio Output transformer needed

[Roderick M. Fitz-Randolph]

>The very first thing I do to any old receiver I buy is to change the
>coupling caps going to the grid of the audio output tube or tubes.  It
>doesn't take much dc leakage from the B+ to bias the grids too positive
>causing plate current to rise to unacceptable levels.  A large increase in
>plate current can cause the primary of an audio output transformer to fry.
>I've seen this in more than one BA with open output transformer primaries.
>
>I'm not an engineer, just a technician,  so I don't understand how an
>unloaded transformer secondary can cause spikes in the primary.  Primary
>voltages seem to me a function of the B+ and the drive to the audio output
>grid.  I can understand how a dead short could cause problems in the
>primary but not an open.
>
>Please explain.
>
>73,
>
>Rick kf4ar
___________________________________________________________________________
Rick, an unterminated secondary on an audio output transformer allows it
to act like a flyback transformer, resulting in horrendous voltages, even
if the primary is tied to B+.  It can result in a shorted primary (turn-to-
turn or turn-to-core).  I've seen in happen in the "old days" enough times
to have made such an indelible imprint on my memory that I always assure a
resistance across the secondary.

Rod, N5HV
w5hvv at aeneas.net

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