The only one I've tried to build is the telebox derived one.
Of the three I tried (for Brum), one works perfect, one has some spurios AM (crystal fundamental) and the third looks like a blocking oscillator. For ch4 you also need to remove a turn or two from the oscillator coil, which is a teenie weenie little thing and I can hardly see the ferrite !
With the second modulator, I think I ran out of adjustment on the coil (as I adjusted it the spurious AM reduced) and I bet if it had been for channel 1 then it would have been as good as the first one. The problem with the third one may be due to a fault, though I couldn't find it.
I ended up scrounging a built modulator (thanks Malcom!) which works fine and I believe uses the same chip as the David Looser design (and is the same as the one in the telebox).
There is a modulator kit available form I think the States - perhaps someone can chip in a link here - and I know of someone who has modified it to add diode switching between ch1 and ch4.
I've not built Darius's design but I definately intend to when I find the time.
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I wonder if a wide bandwidth PA device on the output would be a reasonable idea, to bring the output up to 500mw - this would allow more than one set to benefit from the signal......
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Not needed ! To run my HMV2805, I feed the set via a fixed 18dB attenuator followed by a variable attenuator set to "lots" so you should be able to knock up a distrbution system from one modulator.
Unless you mean to actually transmit the signal (naughty, naughty
)
TTFN,
Jon