[TenTec] Troubleshooting Argonaut 505 with low output

Neil Martinsen-Burrell nmb at wartburg.edu
Wed Feb 5 09:25:19 EST 2014

I went to try my first CW contact last night on the Argonaut 505 that I
bought recently and when I measured the power output, I was surprised at
just how low it was.  I measured 2.5 V across the diode detector built into
my dummy load for a power output of between 60 and 130 mW depending on what
I use for the forward voltage drop of the diode.  This seems even lower
than it ought to be on this radio and I'd like to troubleshoot and
hopefully repair it.

I have the manual and have been getting familiar with the signal flow and
the voltage measurements in there, but I haven't made any measurements
inside the radio yet.  I did make sure that the output was peaked on the
"resonate" control and that the CW drive was adjusted for full output.  I
did find that if I turned the CW drive down below a certain level then I
didn't get any voltage reading on the diode detector.  The radio receives
well, so I believe that the VFO is working.

I am looking for advice about the general approach to this sort of
troubleshooting.  I have an oscilloscope and a DVM and I imagine that I
will be following the signal flow trying to find the block that is not
functioning.  Should I start at the beginning of the chain with the
oscillator or at the end with the RF power amplifier board (90195)?  Are
there particular components or assemblies that I should be suspicious of on
this radio?

Thanks in advance for any help you can offer.

-Neil KD0UKC

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