Albert,Albert H wrote:Back when I was experimenting with crude VHF transmitters, I came across the Dutch "Stentor" board. This actually made some efforts to work properly, and had some neat ideas - voltage regulation for the oscillator, using the grounded heatsink of the final transistor to screen the output end from the oscillator, and it even had a (basic) output lowpass filter. At 15 Volts, it ran really hot (which led to drift) but it would deliver 5 Watts or so if all the trimmers were carefully peaked!
The transistor line-up was a BF245 FET for the oscillator, with a 78L09 as its supply regulation, a 2N2219 and an MRF237 (or SD1127) for the final. Just three transistors, and all that power!
I remember putting one on a spectrum analyser, expecting the worst. However, if everything was peaked for maximum power on the frequency you'd chosen, it was remarkably clean! Adding an extra stage of filter to the end made it pretty acceptable! If you kept the supply down to about 11 Volts, it gave about 3 Watts and didn't drift so badly.
Everyone knew that they drifted, so I built one in an aluminium box, with a FLL circuit concealed underneath the board. As you turned the tuning pot (which normally had a span of about 4MHz), it would go up the band in 200kHz jumps. When you stopped turning the pot, it stayed exactly where you'd left it! You could run it for hours without it drifting. I told people that I'd spent hours selecting just the right capacitors and so on to stabilise the oscillator against temperature drift..... Nobody could understand how a "Stentor" could be so stable!
I might still have one here somewhere.....
I've just ordered a 5W Stentor Transmitter kit, again for nostalgic purposes - so will post an image here when it arrives later this week or early next week.
Although this old VFO stuff is long in the tooth now, and I never actually connect it to an antenna, I do find myself impulsively buying it when I have spare money.
