On Tue, 2007-07-31 at 15:33 +0100, Bob Towers wrote:
> Dr. Gerald N. Johnson wrote:
> > On Mon, 2007-07-30 at 16:26 +0100, Bob Towers wrote:
> >> Some time ago the power output on my Argonaut V dropped to nearly zero
> >> on all bands.
> >>
> >> TenTec say the most likely cause is
> >>
> >> a) one or more of the switching diodes in the LPF circuits
> >> or
> >> b) blown finals
> >>
> > Open switching diodes in the LPF circuits could lead to blown finals by
> > letting the transistors develop excess voltage when not loaded. An open
> > makes a high SWR for the transistors.
> >
> >>
> > 73, Jerry, K0CQ
> >
> Thanks, Jerry, for your comments.
>
> I'm beginning to wonder whether the finals might be blown rather than
> the diodes because the low power is on all bands, not just some of them.
> Presumably it's rare for all the switching diodes to be blown.
>
> 73
>
> Bob
> MM0RKT
Depends on the switching circuit and how the diodes failed. I usually
find failed semiconductors have failed shorted, not open and that could
have two filters in parallel on all bands with some odd network results.
If there are both series and shunt diodes, a shorted shunt diode could
kill the RF output depending on where it is in the circuit.
Its sure the PA transistors are worked the hardest, are affected by a
high load SWR, and so are most vulnerable to abuse. But, in a linear
radio like a SSB multiband radio which describes the Argonaut, every
stage from the first audio to the PA affects the output. Low gain in any
stage or any interstage network or low local oscillator drive to any
mixer or low supply voltage results in low power to the antenna. To
isolate the failed stage, its necessary to check the signal levels
throughout the transmitter, its not necessarily the PA or the low pass
filter. I doubt there is any indicator available from the front panel
meter about PA drive power or any RF or audio voltages so you will have
to open the radio up and trace with a suitable RF voltmeter or scope
with adequate bandwidth.
But then Tentec has probably worked on more than a few of these radios
and has that direct experience that I don't have and either LPF
switching diodes or PA transistors fixed most of them.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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