On Tuners and UHF Mods/Adapters

[VintagePC]

Perhaps this is a fuitless pursuit, but I'm curious to know whether there is any surviving information on UHF tuner add-ons. By this I don't mean standalone UHF converter boxes, but rather the type that would have been sold as an optional "feature" of one's television set.

There's some interesting information in http://www.antiqueradio.org/art/RCAC...nerArticle.pdf which describes a combination VHF/UHF tuner and how they are two separate parts combined.

The service data for my set even makes reference to an attachment point for a UHF signal in the tuner (http://vintagepc.site50.net/site/fle...ice/TV_004.png).

Presumably if you went for the option when purchasing the set, your dealer could then install the additional tuning mechanism (for sets with separate knobs for each) and special UHF mixer strip in place of one of the VHF channels, and it would feed the output of the UHF tuner to your IF strip.

My curiosity in how this works has me wondering if one could cobble together a functional UHF adapter in this same fashion with the correct reference literature. A schematic of an external converter could be of significant value since it would essentially be doing the same thing.

Was the crude principle behind it (excluding tuning mechanics and such) simply to frequency-shift the UHF station down to VHF frequencies with a simple superheterodyne receiver, or is there more to it than that?

Looking on a certain auction site the standalone ones seem deceptively simple with only one or two tubes. If shipping from the US wasn't often 2-3x the price of these I'd be tempted to pick one up just to see what the insides are like.

Edit: I see a few BTC-99s floating around for a reasonable price. Are those any good? Is there service literature available?