When I worked for the Post Office Telephones in the 1960s we had
a valve amplifier with a bridge rectifier connected to its input
to detect intermittent faults on our amplifiers. When used with
amplifiers in our 12-108kHz carrier systems, you could hear the
Light Programme on 200kHz if you touched the top-cap
(control-grid) of one of the valves in the amplifier under test.
Regards, Ian Brooks, Verwood, Dorset.
On 18/03/2023 15:43, Alan wrote:
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Dave, the 2SC5551 is a bit faster than 300MHz!? A spectrum
analyser is a very useful tool but not necessary to detect
spurious oscillation. Measure the current consumption on an old
analogue multimeter and run a portable broadcast VHF receiver on
a distant weak signal. Touching different parts of an
oscillating amplifier will generally cause a change in current
draw and an audible difference to the broadcast signal.
Another useful trick is to use a simple diode demodulating
probe and audio amplifier to listen to the amplifier output,
touching an oscillating circuit will? be clearly heard. Making
fault conditions something you can hear is very useful as you
can then devote your eyes to what you are touching.
I would agree with the point about high Ft devices, unless you
have a lot of experience in UHF/Microwave thinking there are
sound reasons in using older slower devices. A 1dB difference is
insignificant to most users. The reality is that most users are
in Urban locations with high noise levels slight differences in
noise performance will not be noticeable.
73, Alan G8LCO
Ian Brooks