[Hallicrafters] FPM-300 MK III HELP WANTED

[W4AWM at aol.com]

Hi Hallicrafters folks,

Here is a stumper for you.  I have an FPM 300 MK III that has no RF output
on
any band. The driver and final chacks out as described below. I installed a
new belt and both switches are in the correct position. The receiver works,
except on 40 meters and the 80 meter band is hot as a firecracker.  The
other
bands seem a bit weak, except, as I said for 40, which is kaput.  I would
like to
get the transmit function working properly first, Then I will worry about
the
receiver, which is probably just an alignment problem.

Working with a very technically oriented friend, here is what he has done.
References to "I" relate to my friend, not me. My shop is totally buried and
I
don't have a scope, but that is next on my wish list.

I checked all the voltages to the final tube. Everything is nominal, bias,
screen (it goes down in the tune position to reduce plate current and
prevent
damage during tune up.) on PPT it is nominal.

The drive is at the grid of the final via scope observation, yet no plate
current rise when drive is applied. resting current (when there is no
drive ) is
nominal which indicates that the tube is conducting (not dead). It goes way
up when I reduce the bias.

I even checked all the resistors in the screen, bias and filament circuits
and they are all nominal. There's a 10 ohm to ground in the filament circuit
which is the return path for the final and that resistor is fine (8.9 ohms).

I did some checking of the coupling cap and the antenna relay but it's all
ok. If I hadn't checked the tube I might suspect it but it does conduct
(resting current with no drive). Beats me???

Any help is greatly appreciated.

73,

John,  W4AWM





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