Young electronics hackers today are very fortunate to grow up in an era with both a plethora of capable devices to stimulate their imagination, and cheap and ready access to them. Less than the price of a hamburger meal can secure you a Linux computing platform such as the Raspberry Pi Zero, and a huge choice of sensors and peripherals are only an overnight postage envelope away.
Casing back a few decades to the 1980s, things were a little different for electronically inclined youth. We had the first generation of 8-bit microcomputers but they were expensive, and unless you had well-heeled parents prepared to buy you a top-end model they could be challenging to interface to. Other electronic parts were far more expensive, and mail order could take weeks to deliver the goods.
For some of us, this was not a problem. We simply cast around for other sources of parts, and one of the most convenient was the scrap CRT TV you’d find in nearly every dumpster in those days before electronic recycling. If you could make it from 1970s-era consumer-grade discrete components, we probably did so having carefully pored over a heap of large PCBs to seek out the right component values. Good training, you certainly end up knowing resistor colour codes by sight that way.
In the Age of Analog Broadcasts
A personal fascination of mine as a young radio amateur was with the analog tuner you’d find in these sets. This was the RF front end that converted the signal from the antenna into a 36MHz intermediate frequency for the set to demodulate and display. These tuners were simple enough devices usually having only a couple of transistors, an RF amplifier and either an oscillator and diode mixer or a combined mixer/oscillator.
Where I grew up in the UK they were all for the UHF TV bands (about 470 to 860 MHz), and they were either mechanically tuned with variable capacitors and cavity resonators or electronically tuned with varicap diodes and stripline resonators on a PCB. They were discrete modules inside the set, usually in a screening can about the size of a pack of playing cards. Their stand-alone nature meant that once removed from the set they offered considerable potential for modification and repurposing to other UHF uses, and an accessible way to learn the ropes of UHF construction.
So this article is in a way a condensed version of something I might have written as a series had Hackaday existed in 1987, or even as a slim paperback volume had I been able to find a publisher gullible enough to accept the experimental electronic musings of an unproven teen. Think of it as an homage to a technology now past that doesn’t quite fit our retrotechtacular series, and if your interests lie in that direction and you have a few old TVs come your way maybe you can draw some inspiration from it.
The front-end of most analog radio receivers has three parts whatever the frequency of their operation. The RF you want to receive comes in from the antenna to an RF amplifier which filters and amplifies it. It is then fed to a mixer where it is combined with a signal from a local oscillator to produce both the sum of and the difference between the two frequencies. The output of the mixer has a filter to select the difference signal, which is fed to the rest of the receiver circuitry as the intermediate frequency. In the case of an analog UHF TV tuner, if your TV station is on 600MHz the local oscillator would be tuned to 564MHz and the difference between the two, 36MHz, would be fed to the TV set for demodulation.
TV tuner front ends followed this model closely whatever decade of the analog era they were made in. Examples from the 1950s and early 1960s would have featured a couple of triode tubes, by the 1970s you would have found bipolar transistors and more recently they would have had MOSFETs or even GaAsFETs. Most recently the local oscillator would have been a frequency synthesiser but in the era we are discussing it would have been a free-running oscillator kept in check by nothing more than a simple automatic frequency control circuit.
Digging around the half-forgotten electronic junk at the fringes of my workshop I turned up a generic late 1970s 12″ portable with a typical Japanese UHF tuner of the period. A bit of screwdriver work had the cover off, and you can see the internals to the left.
The tuner takes the form of a series of coupled cavity resonators with a set of linked variable capacitors at the top. On the left at the bottom is the antenna input, and on the right are the gears of the tuning mechanism. The RF amplifier transistor lies on the boundary of the two rightmost cavities while the germanium diode used as a mixer is covered in green lacquer and joins the two cavities on the left. The IF comes out at the bottom of the middle cavity and the leftmost cavity holds the oscillator. Immediately you can see that there is plenty of space in which to modify the circuit, turning it into a UHF hacker’s playground.
The schematics of this type of tuner didn’t vary too much. We’ve found an example with varicap tuning from US Patent 3643168 which uses a diode mixer like the one above, though you may also see tuners that use the oscillator transistor as a combined mixer.
The most striking thing about this circuit and others like it is their simplicity. The performance of these tuners comes not from clever circuitry but from the highly selective design of their cavity filters. This one differs slightly from the picture above in that it has an input cavity and the mixer shares a cavity with the amplifier output, but other than that and the use of varicap diodes rather than air-spaced capacitors they are extremely similar.
Plenty of RF amplifiers and transmitters
If you were an ’80s teenager with a constant supply of these things there was plenty of scope for experimentation. The simplest hack was to turn one into a super-selective antenna amplifier by removing the mixer diode, applying power only to the RF amplifier, and soldering a UHF-sized coupling loop to the IF connector to turn it into a UHF RF output. Put that in the antenna lead, tune it carefully, and suddenly you’re watching somewhere else’s local news without snow on the picture. You had to take your excitement where you could get it, back in the ’80s.
Of more interest to a young radio amateur was the oscillator cavity. A carefully installed coupling loop could draw a few miliwatts of RF, enough to make a small transmitter. Inject a video signal through a buffer to the oscillator transistor and it made a reasonable video sender covering the whole UHF band, definitely illegal but fortunately not powerful enough to bring the authorities down on a young experimenter.
With some careful soldering it was possible to replace the striplines in each cavity with longer ones folded to fit the space, and to drag all cavities down to operate in the 430MHz (70cm) amateur band. Then it was possible to make either a 70cm ATV receiver or a much more useful transmitter for 70cm ATV by replacing the RF amplifier with a tuned and modulated PA in the same cavities. I hasten to add though, none of my transmitters made this way ever achieved a real-world contact outside the confines of a workshop or radio club meeting.
The only kid in Britain with a spectrum analyser
The piece de resistance of my TV tuner hacking came courtesy of a set that had a completely modular small-signal section. Two screened cans, one of which was a varicap TV tuner and the other of which at about twice the size was a complete IF strip and demodulator. With only a small amount of wiring it could become a complete TV receiver with video and sound output, though that was not the use to which I put it. The tuning voltage of a varicap tuner is 0 to 33 volts, so given a 0 to 33V sawtooth generator and an oscilloscope I had a simple but functioning spectrum analyser.
Sadly the spectrum analyser, built on a piece of tinplate salvaged from a coffee tin, has not survived the decades. I remember it generated its 33V from a 12V supply via a two-transistor multivibrator feeding a diode multiplier, and the sawtooth generator was a particularly nasty thyristor relaxation oscillator. With a bit of tweaking of the IF strip to narrow the bandwidth though it gave a good picture of the UHF TV band on the ‘scope, through which you could see all four of the local TV channels and smaller spectra of those from further afield. With the honesty of hindsight, other than allowing me to say I had a spectrum analyser among my test kit it was pretty useless, but hey! I was the only kid in Britain who could say they’d built a spectrum analyser.
It was quite a job to find a tuner for the photograph above, they have so disappeared from the background noise of electronic junk these days. The portable TV I found had my modification from back in the day as a video monitor, probably the reason why I hung onto it. They may be a technology long past, however I still retain a soft spot for these tuners because it was through them I learned my way around the intricacies of UHF construction. I often hear it said that RF design is some kind of black art, to which I would reply that if that is the case then it is one you can only pick up by experience. Perhaps I was lucky to be given the means to gain that experience in every dumpster.