SX-28A Audio Output transformer needed

[Rick Robinson]

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

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