First of all something besides the final was bad which caused the new
final to fail. The resistor is a good suspect. Do you have a scope?
That would be a help in finding out if you need the "fix" for excessive
low frequency gain, ie any off 6m signals.
What was the purspose of the suggested series capacitor on the antenna
connector, (I am thinking you said series). AC (and RF) coupling by the
cap should not hurt. It does put a reactance in series with the
antenna, but maybe they were worried about the final collector shorting
to an external ground if an antenna cable failed?
I would talk to Ten Tec again and determine the "why" of each mod.
Obviously, some of these rigs worked without them. That need for a mod
can come about because the gain of transistors may vary from unit to
unit due to production variations. They may not find that out in
prototype testing, but only after customer experiences are totaled.
If you wanted a "beefier"
transistor than the NTE replacement, you need to find the same
polarity of transistor, NPN for example, equal or greater collector
power rating, about the same Beta and F tau, to insure duplicating the
operation parameters of the original. The voltage ratings need to be
about the same, although higher Vce voltage rating can be useful if it
does not increase the capacitance greatly. Not being able to look at a
schematic right now, these are general comments aimed at any replacement
of an out of production transistor. You could also check with RF Parts
Co. who handle some power transistors that are less common now.
One of the issues with power transistor troubleshooting, is that you
must have sufficient accuracy in measurements to recognize an abnormally
low resistance. A good visual inspection of all components for signs of
heating should be the first thing to do; with high intensity light and a
magnifier, upon starting troubleshooting before even powering on a
circuit. After replacing components, do resistance checks to see it you
have reasonable values, just to catch the rare solder bridge, or bad
joint that can happen in replacement soldering.
Good Luck to you; there should be many hours more fun with the 1260,
once you get the final sorted out.
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