Mark,
You may have already seen this link that discusses replacement of the Alpha
86's PIN T/R circuit with a pair of vacuum relays:
http://volim63gtenet.mydomain.com/webservice3/old-mike/Mike/n3eaq_files/Alpha86/Alpha86.htm
The Alpha 86 went through three evolutionary production changes and to the best
of my knowledge, Alpha didn’t publish these changes in later owner's manuals.
Ultimately, the fourth 86 iteration became the Alpha 89.
The primary change relates to the addition of high-voltage bias on the receive
PIN diodes. Late 86 models have extra PC boards. This change, together with
the pre-existing high-speed SWR protection circuit helped but did not eliminate
all high SWR PIN failures.
In my opinion, Alpha didn't have a complete grasp on the protection problem
until the late '90s when the last PIN circuit updates went into 87A and 89
production. Post Y2K 87A models seem to have very stable T/R circuits.
Paul, W9AC
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