Quote:
Originally Posted by Robert Grant
I saw many of these at secondhand stores and hamfests in the early 1980's, and used one regularly back in those days.
They actually found new popularity after the all-channel act was passed, as new UHF stations came on the air and people were stuck with VHF-only sets.
Most UHF converters had a very simple circuit. A power supply powered a 6AF4 oscillator (in later models, a transistor or tunnel diode) which, 76 or 82 MHz below the frequency of the desired station, mixed, in a germanium diode with the station's signal, converting the UHF station to channel 5 or 6.
As one might imagine, this essentially passive conversion took a LOT of UHF signal to produce a usable picture.
One good question: Is it legal to possess a UHF converter today, post-ECPA? (since it is a device designed to receive frequencies that have since been reallocated to Part 22).
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If you study the schematics of any television's built in UHF tuner you'll find they work in much the same way. IIRC the succeeding "IF" stage is the VHF mixer stage, with the VHF oscillator disabled and that stage tuned to the same frequency as the remaining pixIF chain.
With an outboard converter you add the benefit of the RF stage of the VHF tuner and the VHF mixer and oscillator are working as they normally do, making for an additional stage over the built in configuration, and "dual conversion" to boot.
I could be wrong about all this, as it has been a long time since I've delved into this stuff, but that's how I remember it.
The outboard converters seemed to work very well for me on pre '64 sets I've had. I suppose that there are some that are better than others. The only gripe I'd had was that there were often spurious image signals. But as long as they didn't share the same frequency with a desired station one was OK.