Hi Adam,

Dave S made me aware of your post re UHF converters. I'd like to give you a little background on the approach Blonder-Tongue used for UHF converter design.

The first UHF converter we manufactured for home use was model BTU-2. This unit used a low loss, low cost variable inductance tuner invented and patented by Ike Blonder. It was a three gang unit enabling the design of a UHF converter using a well-matched double-tuned preselector along with an oscillator section, tuning the then 470-890 MHz UHF band and converting it to an IF between 76 and 88 MHz.

I invented and patented a well balanced, low loss UHF balun made of twisted and formed wires to drive the double-tuned preselector from 300 ohm twinlead. The preselector was coupled to a General Electric germanium diode designed especially for UHF mixer use. I think the RMA registration was 1N72. This diode was mounted, one end grounded, in a Tinnerman clip of the type used to mount cardboard forms for permeability tuned coils to a chassis. The clip provided a very low inductance-to-ground coaxial connection for the cold end of the diode. This moved the parallel resonance formed by the to-ground inductance and stray capacity-to-ground from the diode-case to well above the frequency of about 808+890 MHz (LO+RF). This resonance caused a bad on-channel RF impedance mismatch, distorted the RF bandpass and caused some extra insertion power loss (read noise-figure degration). The IF output frequency (76-88 MHz) that covered channels 5 & 6 was coupled from the diode to a 6AB4 (1/2 of a 12AT7) low-noise, grounded cathode, neutralized triode amplifier that fed a double-tuned output transformer.

The original BTU-2 sold very well, but when we became aware of consumer demand for a lower priced unit, we designed the BTC-99 to meet this demand. The single-tuned RF preselector in this product was very broad and was used mainly for assuring a good impedance match from the 300 ohm input to the diode mixer. The IF filter was also very broad, with the main consideration being a good impedance match to the 300 ohm IF output (again, for channels 5 & 6). No IF amplifier was provided. The design was balanced RF to balanced IF, no baluns required. The FM interference you have observed with your BTC-99 converter might be caused by the reception of two strong signals, one an FM station, mixing in the diode mixer. I have never heard of your problem before. We must have gotten the design right, however, because we sold many more '99 than BTU-2 type converters and had very few returns. BTU-2 units turn up every so often on eBay. You might want to try one of them.

BTW, in the persuit of low cost design we were stimulated by a GE Application note to design and manufacture our BTD-44 UHF converter that used one self-oscillating tunnel diode acting as both the mixer and local oscillator, and powered by one D cell battery (good to last for about one year). That model sold very well for us until GE decided to stop subsidizing the manufacture of the diodes and gave up the idea they probably had for high volume consumer product sales for tunnel diodes.

On another subject, if anyone has and wants to use a Blonder-Tongue B-9 Audio-Baton (9 knob inductor-less graphic equalizer), please take note: The seven single-tuned banpass filters are realized using a single triode (half of a 12AT7), two capacitors and two resistances, one of which is the plate resistance of the 1/2 a 12AT7. The circuit is described in my U. S. patent #2,983,876. The resonant frequency of each bandpass filter is equal to the reciprocal of the square root of the product of the values of the 4 components, all divided by 2 and by pi. The two capacitors were a mix of rather high value ceramic discs, conventional paper capacitors or metalized paper capacitors, all of 5% tolerance. These components passed all tests except one, the 'test of time'. The main bad actors are the metalized paper capacitors (the higher valued caps). Some of the aluminum metalization slowly, over the decades, oxidized into an insulator, aluminum oxide. The capacitance therefor dropped, by now to maybe 85 to 65 % of the original value. As a result, the tuning of the filters was ruined. If I were rebuilding a B-9, I would at least replace all the tuning caps (those connected plate-to-cathode and cathode-to-ground) as well as the caps in the low-pass filter labeled 40 cps. A side note: Henry Kloss, the founder of the Advent Corp., marketed an inductorless solid-state stereo eq while we were selling the B-9. Investigation disclosed that they were using, for their bandpass filters, a circuit that infringed on our patent. Fortunately, our Patent attorney had suggested that I design a transistorized version of the filter for inclusion in the patent. I did. Result: Advent paid us royalties on the patent.

If anyone needs a manual for a B-9 or B-9b, or for that matter a BTC-99 or BTU-2, e-mail me and I'll mail you a copy. Over the years our UHF converters were updated to improved versions. If the model number of a converter you have includes a suffix, please state it.

Best regards,

Ben Tongue
http://www.bentongue.com/