On Tuners and UHF Mods/Adapters

[VintagePC]

Sorry for the double post... anyone have a schematic for said BTC-99? Web tells me it is Sams 259-3.

Thanks in advance. Should be simple enough to do without but I like having reference material on hand, and so I can pre-pick my parts drawer for the correct caps it needs replaced.

[wa2ise]

Just to see if it would work at all, I connected a tube based UHF to VHF converter to an antenna, and the output to a HDTV converter box tuned to channel 6. The tuning was rather fussy, I think because the converter box doesn't expect the digital carrier to be that much off in frequency. Also the UHF converter may add too much noise. With the UHF converter, the resulting digital carrier on channel 6 could be anywhere. A vintage TV set with fine tuning could cope easily.

[dieseljeep]

Originally Posted by Dave A

The venerable CT-100 had spaces for UHF strips usually installed with blanks unless you had a UHF station in your city. The channel dial will confirn that. And I have about a dozen NIB of the pre-tuned strips somewhere. Channels 33 and 51 I think. I think the paperwork shows how to re-tune. Not that we could see the results anyway unless you have the ultimate stars-are-in-alignment setup of a working CT-100 and a BT modulator with UHF. I will try to find one and post a photo.

Slightly before the CT100 came out, the RCA all channel sets were equipped with a tuner similar to it. It either was factory, or field converted. They were equipped 19, 25 and 31 strips. Back then, channel 31 was only proposed and never materialized.
The tuner had, 2 6BQ7's, 1 6AF4 and a 6S4, voltage regulator. The 6AF4 was used as an oscillator for both, VHF and UHF and lasted a lot longer, than the ones in the UHF only tuners.
I like the tuner a lot, truly overkill at it's finest.

[jr_tech]

Originally Posted by wa2ise

Just to see if it would work at all, I connected a tube based UHF to VHF converter to an antenna, and the output to a HDTV converter box tuned to channel 6. The tuning was rather fussy, I think because the converter box doesn't expect the digital carrier to be that much off in frequency. Also the UHF converter may add too much noise. With the UHF converter, the resulting digital carrier on channel 6 could be anywhere.

Were you able to get any sort of stable reception of the DTV channels? I tried the same thing with no success.
It would be *really fun* to be able to turn the UHF knob or Converter tuning on an old tv to tune the digital channels.
jr

[VintagePC]

Originally Posted by jr_tech

Were you able to get any sort of stable reception of the DTV channels? I tried the same thing with no success.
It would be *really fun* to be able to turn the UHF knob or Converter tuning on an old tv to tune the digital channels.
jr

Don't most converter boxes act just like a cable box in that you tell it what channel and it tunes to that and outputs on CH3? From what it sounds like you are trying to do... you'd need a box that "de-digitizes" the DTV streams on the fly and outputs them to their appropriate frequencies...

[wa2ise]

Originally Posted by jr_tech

Were you able to get any sort of stable reception of the DTV channels?

Think I should have said "extremely fussy". Think I only got the strongest signals about 5% of the time. Not at all a practical thing to do. Otherwise I would have though of using an old TV tuner inside a vintage TV set, then feed its IF to the ATSC converter (upconverted to a channel it could tune, or maybe find the IF strip inside the box and feed it directly), and take the resulting demodulated video and audio and feed it to the audio amp and video amp of the TV set (bypassing the TV set's IF amps).

[jr_tech]

Originally Posted by VintagePC

Don't most converter boxes act just like a cable box in that you tell it what channel and it tunes to that and outputs on CH3? From what it sounds like you are trying to do... you'd need a box that "de-digitizes" the DTV streams on the fly and outputs them to their appropriate frequencies...

And indeed, many set up a number of ATSC converters feeding B-T modulators to provide an in-house source of converted-to-analog ASTC channels for vintage TVs to "tune" on the "correct" channels with their own tuners.

What I was considering was "bass ackward" from the normal configuration, something like this:
1. Antenna connected to vintage UHF converter.
2. Converter down converts the digital signal (most are UHF) to channel 5 or 6 (common output channel).
3. Output from UHF tuner is connected to ATSC converter box, fixed tuned to ch 5 or 6.
4. ATSC box then converts the ch 5 or 6 digital signal to audio & video, which feeds the TV set.

The novelty here would be that the ATSC converter could be mounted out of sight (and out of mind) inside the TV, while the digital channels would be tuned with the UHF tuner. TV would "appear" to function as designed, without resorting to a rack full of ATSC converters and B-T modulators.
jr

[David Roper]

Originally Posted by dieseljeep

I heard that Zenith made UHF strips for their early sets, such as the porthole models. There was always an empty slot in the turret, next to the channel two slot. I've never ran across one.

Yet another reason I could kick myself for letting this set slip through my fingers: I accidentally discovered a Zenith 19" porthole I once had would receive UHF channel 35 when tuned to channel 2 (or was it the empty slot? Can't verify--set's gone ).

[VintagePC]

Originally Posted by jr_tech

And indeed, many set up a number of ATSC converters feeding B-T modulators to provide an in-house source of converted-to-analog ASTC channels for vintage TVs to "tune" on the "correct" channels with their own tuners.

What I was considering was "bass ackward" from the normal configuration, something like this:
1. Antenna connected to vintage UHF converter.
2. Converter down converts the digital signal (most are UHF) to channel 5 or 6 (common output channel).
3. Output from UHF tuner is connected to ATSC converter box, fixed tuned to ch 5 or 6.
4. ATSC box then converts the ch 5 or 6 digital signal to audio & video, which feeds the TV set.

The novelty here would be that the ATSC converter could be mounted out of sight (and out of mind) inside the TV, while the digital channels would be tuned with the UHF tuner. TV would "appear" to function as designed, without resorting to a rack full of ATSC converters and B-T modulators.
jr

That's an interesting idea. In theory there's no reason it shouldn't work... in practicality, however, I suspect that the analog first stage just isn't clean enough for what the ATSC box is expecting.

Re channel strips: It's odd you don't see more info on them floating around, but then again they're also not something that went bad easily so it's not like repair shops had jars of them on hand to service customers. OTOH... you'd think they would pop up occasionally since dealers and service folk did replace or customize them for the local stations and would therefore amass a pile of spares.

[wa2ise]

Originally Posted by jr_tech

What I was considering was "bass ackward" from the normal configuration, something like this:
1. Antenna connected to vintage UHF converter.
2. Converter down converts the digital signal (most are UHF) to channel 5 or 6 (common output channel).
3. Output from UHF tuner is connected to ATSC converter box, fixed tuned to ch 5 or 6.
4. ATSC box then converts the ch 5 or 6 digital signal to audio & video, which feeds the TV set.

That's essentially what I did in post 32, but I found the tuning very fussy to get the ATSC box to do anything. Seems the DTV signal needs to be nearly dead on frequency for an ATSC box to lock onto it and decode it properly. I don't think any DTV set comes with an analog tuning front end.

[ChrisW6ATV]

Originally Posted by jr_tech

The novelty here would be that the ATSC converter could be mounted out of sight (and out of mind) inside the TV, while the digital channels would be tuned with the UHF tuner.

This is definitely a fun concept, but it would not work as described even if/when the UHF converter was correctly tuned to an ATSC channel, unfortunately. The catch is that ATSC stations have digital codes as part of the signals, to identify the main channel as well as any additional channels in the signal (the 4-1, 4-2 and so on you may have seen on a digital TV or converter). Normally, this information is stored for each channel during the "channel scan" typically done with a new TV set. Even if some digital tuners can re-check for these codes each time a channel is selected or re-selected, almost certainly none can do a search "on the fly" when a signal is lost (such as when turning the knob on the UHF converter in the proposed setup). It would be cool if they could, though.

[old_tv_nut]

Originally Posted by wa2ise

Think I should have said "extremely fussy". Think I only got the strongest signals about 5% of the time. Not at all a practical thing to do. Otherwise I would have though of using an old TV tuner inside a vintage TV set, then feed its IF to the ATSC converter (upconverted to a channel it could tune, or maybe find the IF strip inside the box and feed it directly), and take the resulting demodulated video and audio and feed it to the audio amp and video amp of the TV set (bypassing the TV set's IF amps).

The digital decoder has enough pull-in range to handle expected offsets:
the frequency steps of a synthesized tuner; transmitter co-channel offset (+/- 10 kHz); crystal tolerances. These offsets add up to considerably less than 1 MHz (probably on the order of +/- 100 kHz max). You could get there reliably with an analog tuner system with AFC (which would have to include the analog IF), or with a synthesized analog tuner, but not with a simple continuous-tuning analog tuner running open loop.

[VintagePC]

I've received my UHF converter box and finished recapping it (also replaced the selenium with a modern diode+100 ohm dropper).

It appears to work, but not particularly well. Incoming signal should be strong (fed via cable). Schematic is available on last page of Sams 259-3, which is on the ETF page here (http://www.earlytelevision.org/tv_sc...s_postwar.html) - Tip of the hat to Tom Albrecht for that!

Symptoms are as follows, all resistors check OK but haven't tested the capacitors. Electrolytics replaced, others are ceramic and can't be measured in-circuit. 6AF4 is also unknown state.

1. The entire tuning range of the converter spans about 8 channels. (62-64, got 70 once).

2. Some regions on the dial pick up FM radio stations. I attribute this to not having a proper tube shield on the 6AF4 since it is much less if I temporarily wrap it in grounded tinfoil, and said regions are likely multiples of the station's frequency. (e.g. 88.9 FM when Ch55 (~716Mhz) is onscreen).

So far there are a few things I suspect it could be, and I was hoping someone here could point me in the correct direction. As I'm interested in learning, even if you don't have an answer but just some basic circuit theory explaining how the setup works, I'm happy to listen. I do get the higher-level principle of superhet down conversion/how an oscillator works but I don't think I grasp all of the circuit blocks quite yet (e.g what the intent of each of the traps are).

I gather from the tuning range that the oscillator isn't spanning to the correct frequencies (based on the channel seems to be stuck in a narrow band of them.

B+ measurements seem reasonable. They do seem to vary a bit depending on the tuner, from about 115V to 135 as called for on the schematic. Voltage drop across the dropper resistor indicates we're drawing about 25mA B+.
The 6AF4 plate voltage (at 80v in the Sams) varies from 70-90 in a similar way as the B+. Grid voltage I have seen go from near zero up to -10 as I tune the dial.

1. Bad 6AF4? I don't have a replacement on hand, but that might be the most logical, but not the cheapest place to start... They're plentiful and can be had for a few $ but since they come from the US I pay upwards of $10 for shipping a single one... which isn't particularly cost-effective.

2. Bad UHF diode? I really don't suspect that's the case given I do get reception, but I'm curious what kind of symptoms a bad crystal mixer (1N82) would cause. I remember reading about it in a thread either here or elsewhere, but can't find it any more.

3. The tuning 'gimmick' (labelled as such below L3 in the schematic). It's comprised of two wires, one insulated, twisted together. Any idea how sensitive that is to misalignment?