uhf converters

[Adam]

My current daily watcher is a vhf only set, and I thought rather than using a vcr to get uhf, why not try a uhf converter. I have a B-T 99, that was new in the box, that I replaced any possible bad caps in, and it doesn't work very good on anything but the strongest signal. Most of the problem is due to all the FM radio interfearence comming through because this thing works on channel 6. Is there any particular model uhf converter to look for that can pull in a weak signal, or at least one that doesn't operate on channel 6?

[Carmine]

I had one that worked on ch. 5.

[gadget73]

I had one, don't recall what channel it worked on, but I remember a similar thing: ie poor sensitivity. I found mine at a flea market for $2 and had it hooked to my 20" Motorola VHF only set. I also won't claim to even remember what make it was, or the model of the Motorola, its been a long time.

[jonman]

If I remember correctly, we had a UHF converter in the early sixties that worked on channel 4/ The model was an Areostar Satelite I think?

[Dave S]

Originally Posted by Adam

My current daily watcher is a vhf only set, and I thought rather than using a vcr to get uhf, why not try a uhf converter. I have a B-T 99, that was new in the box, that I replaced any possible bad caps in, and it doesn't work very good on anything but the strongest signal. Most of the problem is due to all the FM radio interfearence comming through because this thing works on channel 6. Is there any particular model uhf converter to look for that can pull in a weak signal, or at least one that doesn't operate on channel 6?

I will probably see Ben Tongue next week and I will ask him about this.

--Dave

[Phil Nelson]

I have a Radio Receptor UHF converter that I purchased NIB. You can use either channel 5 or 6.

http://antiqueradio.org/RadioReceptor.htm

I haven't used it regularly, since it has all the original caps and the UHF stations in this neighborhood are uniformly awful. However, I have tried it out with my Predicta and my RCA 630TS and it seems to work just fine.

You could try sending an email to Mark Nelson. His website shows his collection of something like 100 converters and boosters.

http://www.geocities.com/k8zhd/TVBox.html

According to Mark, the RR converter was sold under several different brand names, including Silvertone, DeWald, and Truetone, so it might be fairly easy to find.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Steve McVoy]

I think all of the old UHF converters would work on either channel 5 or 6 output, depending on which channel was vacant in your area.

They had no RF amplifier, and had horrible noise figures, only good for very high input levels. Also, they had almost no bandpass filtering on the input (not much on the output, either). With today's UHF and FM spectrum so full, these converters will rarely work well.

[Phil Nelson]

Maybe I just have low standards :-)

For kicks, I hooked up the RR converter to a 1970s solid state TV to see what would happen. Picture & sound were not significantly different from what I got when using the TV's built-in UHF tuner. Tuning pushbuttons made it easy to flip back and forth between the two for comparison.

I imagine your mileage may vary depending on your local UHF stations, type/orientation of antenna, etc. I used an old amplified antenna from Radio Shack, which works well but is twitchy. Walking around the room and waving your arms will affect reception, for instance. That was even more the case when I used this setup with my 630TS, which lacks automatic gain control.

Anyhow, these old converters typically sell for next to nothing at swap meets, so if you want to pick one up for $5 and play with it, you're only risking the price of a boutique coffee.

Phil "looks OK to me" Nelson

[Adam]

Some of these uhf converters seem to have an amplifier tube and some don't. This one I have doesn't. I'll have to see if I can pick up that does, I'd think that would make quite a difference.
Here's a pic of what I'm getting now on ch 56. If I just tap the dial on the uhf converter slightly (I'm talking about just touching it, not turning it) I get FM radio. I also tried stacking 2 (even 4) of those "bowtie" antennas, and it didn't help.

[Phil Nelson]

Do you have a local (or nearby) station that broadcasts on channel 6?

What sort of UHF reception do you get on channel 56 using a modern TV with a UHF antenna, not cable?

Does your old TV need alignment? Before it was aligned, my 630TS got wonderful FM radio reception on channel 6, but no TV picture or sound.

Phil

[Adam]

There's nothing around here on channel 6. 56 isn't super local, some sets will get a clear picture on that channel, some won't, without an antenna I can't get it at all, on some sets I need to stack 2 of those antennas to get clear sound. That tv doesn't need allignment, and I get FM radio through the uhf converter no matter which set I hook it to. Actually that set in the picture there is the one that works best with the converter. The converter says it also works on channel 5 and 6, but there being a local ch 5, all I get on ch 5 is ch5, I don't see the output of the uhf converter at all. And that interference on the screen in the picture doesn't show up too well, it's not just a snowy picture there's a definite pattern to it, it's strange looking, which makes me think it has to do with the FM radio signals interfearing with the output of the converter more than how well the converter picks up ch 56 in the first place.

[N9ZQA]

Adam -

I have the same converter and in addition to replacing the electrolytic capacitor in the power supply, I also found it necessary to clean the tube pins/socket as well as the brass fingers in the tuning section - contact cleaner and qtips work well.

After these repairs I was able to get several local stations using a loop antenna; I'm located about 25-30 miles from the station(s). Maybe these repairs will help.

Jim

[Dave S]

Originally Posted by Steve McVoy

They had no RF amplifier, and had horrible noise figures, only good for very high input levels. Also, they had almost no bandpass filtering on the input (not much on the output, either). With today's UHF and FM spectrum so full, these converters will rarely work well.

I asked Ben Tongue about this last night. It's hard to keep up with him sometimes, but if I understood him correctly, he designed converters to work as well as possible in the prevailing conditions of the day at realistic price points. The UHF tuners built into sets after that feature was mandated were usually more selective and/or sensitive than the converters. However, the FCC-mandated spacing of signals today should allow a properly functioning vintage converter to work as well as it did when new, which should actually be reasonably well. I suspect that converters were used mostly in higher signal strength areas.

He warned that a functioning but weak tube would definitely cause problems and to make sure the tube was strong.

He also cautioned, of course, that other brands might not perform as well as the B-T converters

I'm pretty sure some did have an RF amplifier stage.

Ben indicated that he might join us here. If he chimes in, I'm sure we'll receive a much more knowledgable answer than my layman's interpretation of his comments.

--Dave

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/

[Adam]

Welcome to Audiokarma, and thank you very much for the information on the BT uhf converters. That was very interesting, although I'm going to have to go over it some more before I can understand all of it. I'll look around for the BTU-2, and see if it works any better.
I did pick up a philco uhf converter with an IF amplifier, it currently has some problems with the power supply (bad selenium rectifier, filter capacitors, etc.), but when I get it going I'll post up some pictures and see if a converter with an amplifier makes a difference.