[turretslug]

Hi Neil,

Good luck with that re-stringing- I get your drift there, when overhauling my 750, I was struck by just how fine the bronze (?) wire was, they must have had robot spiders spinning it in Alvechurch!

Taking a "sniff" from a classical simple valve HFO like this takes a little care and thought- enquiries on the 'net will throw up innumerable gurus who will glibly say, "just stick a J-FET source-follower buffer on the LO grid". That's not good enough, maybe it would serve on emitter/source/cathode of a Colpitts/Hartley-type VFO but the stability of the type of oscillator here is very, very critical indeed on both stray capacitance and impedance characteristics of anything connected to it. At the top end of the highest HF band, i.e. around 30MHz with sets like this, even moving a finger around the oscillator components can produce a shift of tens of kHz! Even if this is trimmed out when connecting a buffer, the tiny change in characteristics of any connected device with voltage and temperature can ruin the good oscillator stability of a previously dependable radio. I found a thread on just this subject on Antique Radio Forums regarding an SP-600 that was badly compromised by what seemed to be a "light-touch" oscillator sniffer some time back, but I've been unable to re-find it.

As well as minimal deleterious loading, there's the question of reverse-isolating from what is likely to be a noisy digital device. Simple valve oscillators can have a very clean output and it would be a shame to stir wide-band noise into the mixer from an external device. Cathode/emitter/source followers are poor from this point, boot-strapped buffers are pretty much ruled out and anything using feedback may require care. In the end, I decided that a lightly coupled small pentode with a low anode load would be the best initial approach. (Small implying both low power and low circuit capacitance). I used a DF60 pentode- these were available very cheaply and plentifully a few years ago but this may have changed, my heart sank when I once saw a charleton describing them as "substitute for EF86".... It has a 1.25V (not 1.4V) 50mA filament and is about the size and shape of one of those "Cola bottle" jelly sweets that have been rotting kids' teeth for a few generations now, with a screening coat of paint connected to a filament pin. I fitted it over the gang capacitor oscillator section secured by pushing through a grommet in an oval hole in the piece of angle steel I used, the 0.5pF input capacitor going straight down to the oscillator gang tab by the EF91 LO. I was going to add a two-transistor buffer to low-impedance coax powered by the second coax within a 4-pin S-VHS lead going to the frequency counter (lovely VFD module from China Inc.) but the circuit as illustrated worked fine with a short screened lead from the 750, feeding about 60mV RMS of buffered LO to the counter. Switching the DF60 filament on and off resulted in a pitch-change in 10m SSB reception, implying a few tens of Hz shift but I thought, that's pretty good and now subject to the curse of diminishing returns.... I "stole" about 1mA HT supply from the first mixer's screen grid feed of approximately 70V- this is pretty much negligible, if anything pentode action via the added buffer's screen grid will tend to stabilise the mixer screen-grid supply.

Unfortunately, I didn't take any photos as it was a quickie proof-of-concept that I knocked up a few years ago and then dismantled, the 750 having good enough resolution for HF broadcast usage. I was going to make up a DC filament feed from the 750's 3.15-0-3.15VAC heater supply but prototyping used a single NiMH "D" cell that lasted many hours between charges!

Hope this at least provides a pointer,

Colin