I have repaired dozens of GE Mastr II 100 Watt UHF PAs, and a few VHF
ones too (the VHF ones have reverse power protection and do not fail as
often). I have opened up the PA transistors (not recomended as you and
inadvertantly inhale Berilium Oxide dust which is toxic) and looked at
them under a magnifying glass. Every RF PA transistor I have looked at
has multiple emitter wires and multiple collector wires connected to the
silicon chip. It appears that they are actually several transistors on
one chip, connected in parallel to the outside container package leads,
or maybe just multiple parallel wires to a single transistor in order to
reduce lead inductance. Often transistors that still work, but not up to
par, have some of the little wires fused. My experience has been that
when one transistor in a PA goes bad (or just partially bad) the other
transisitors get stressed and you usually see toasty resistors in the
Wilkinson power combiner. If you only replace the really bad
transistors, the other ones go bad very soon thereafter. Replacing all
of them at once saves enough labor that the parts cost is justified.
Perhaps this is not the case with HF PAs.
DE N6KB
These transistors have a
multi-emitter arrangement that enhances bandwidth. I
believe that if one or more of these multi-emitter
paths is open the transistor will still work but on
the mid or lower HF bands only. I don't think you can
tell the difference with ohmmeter checks.
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