While I was in the process of installing a new bandswitch
in our club's Alpha 78, I noticed that the plate overcurrent
shunt resistor R111 was cracked into two pieces, and
that the G-10 PWB underneath this resistor was burnt
pretty badly. It looks as if the resistor was running too
hot which in turned slowly cooked all the epoxy resin in
the G-10 laminate. When I scrapped away the soot, all
that remained was the criss-cross pattern of the fiberglass
layup weave.
In any case, I don't think this was a "big-bang" kind of
failure. The resistor was not blackened at all. Instead
the resistor's ceramic form material is "crazed"
indicating that was overheated. Presumeably the hot
resistor proximity baked the resin in the adjacent laminate
material over time. The resistor is a 25 ohm x 25 watt
unit, which in my mind is too small. This resistor carries
the full supply current of the amplifier which is probably
close to 1.4 amps when running full bore!!
Anyone run into this problem before with Alpha 78?
By the way, this might be related to the problem that
Jorge, EA2LU is having with his Alpha 76.
73 de Mike, W4EF
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