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For a few pennies, the whole shebang can be saved from this failure mode. The external cap charged to a higher voltage is an easily imagined scenario.
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Al,
Scroll to page 29, the third to last page of this article:
http://worldphaco.com/uploads/THE_GREBE_MU-1.pdf
You will see the diodes in series with the emitters on the schematic. What happens here if say you have the radio's HT plugged on to the 135V output (giving the filter caps in the radio that potential), and while its running, then say pull out the plug and plug it on to the 90V connection, the energy stored in the radio's capacitors destroys the pass transistor. So you can see the diodes there to save them.
Hugo.