If you’ve been tearing electronic devices apart for long enough, you’ll know that the old gear had just as many mysteries within as the newer stuff. The parts back then were bigger, of course, but often just as inscrutable as the SMD parts that populate boards today. And the one part that always baffled us back in the days of transistor radios and personal cassette players was those little silver boxes with a hole in the top and the colorful plug with an inviting screwdriver slot.
We’re talking about subminiature intermediate-frequency transformers, of course, and while we knew their purpose in general terms back then and never to fiddle with them, we never really bothered to look inside one. This teardown of various IF transformers by [Unrelated Activities] makes up somewhat for that shameful lack of curiosity. The video lacks narration, relying on captions to get the point across that these once-ubiquitous components were a pretty diverse lot despite their outward similarities. Most had a metal shell protecting a form around which one or more coils of fine magnet wire were wrapped. Some had tiny capacitors wired in parallel with one of the coils, too.
Perhaps the most obvious feature of these IF transformers was their tunability, thanks to a ferrite cup or slug around the central core and coils. The threaded slug allowed the inductance of the system to be changed with the turn of a screwdriver, preferably a plastic one. [Unrelated] demonstrates this with a NanoVNA using a nominal 10.7-MHz IFT, probably from an FM receiver. The transformer was tunable over a 4-MHz range.
Sure, IFTs like these are still made, and they’re not that hard to find if you know where to look. But they are certainly less common than they used to be, and seeing what’s under the hood scratches an itch we didn’t even realize we had.
IF transformers are still sold, but finding the correct one for a project isn’t that easy, especially when they’re not the “standard” values like 455KHz or 10.7MHz and bandwidth type. I would encourage people to experiment with alternatives, namely homemade transformers using easily available and cheap toroidal cores and trimmer capacitors. Also, some IF transformers are just used as inductors when one of the windings is left unconnected, and this makes even more easy to replace them with off the shelf inductors and trimmer capacitors after calculating them for the desired frequency (plenty of calculators online will help). Once you become familiar with this practice, it paves the way to things like making a radio with unusual IF, or a converter with a 50 MHz IF that can be fed into a SDR for multiple modes detection, etc.
I would be very interested in modifying these components to work as magnetic amplifiers for my secret project!
A while back I bought an Albrecht AE4200 radio in non-functional condition. As it turned out, one such IFT had developed an internal short (T3 in the schematic below, iirc). I then bought a bag of 10 unknown transformers and built a small circuit to characterize each of them to choose a proper replacement.
To this day I wonder how it could have failed this way, since there aren’t really any high voltages present that could have caused an insulation breakdown.
AE4200 schematic: https://elektrotanya.com/albrecht_ae4200.pdf/download.html
Oh well, elektrotanya….a very familar name for me :)
Thanks!
Elektrotanya is now bookmarked!
I cant believe there is a multi band Ham HF Radio transceiver kit on eBay that supplies all the cans and spools and you are expected to wind every transformer yourself. I have built hundreds of complicated kits over the last 50 years but none required me to literally build the IFTs.
My electronics teacher in high school in the late 60’s ranted on about those single slug IF transformers. Only double tuned transformers were used in a good radio. For both AM and FM some bandwidth is needed for best response. One simple bell curve response may be good enough for AM talk or police FM, but a square curve is needed in a good radio. Tune every single one dead on and suffer. Stagger tuning of these one sluggers was used as a compromise for poorer quality. He called it a diddle-stick, the wooden or plastic blade or hex shaped tool used here.
Then came crystal filters first big and expensive then a little transistor sized 3 terminal device available in different bandwidths. Put two in series (crammed in) and make a highly selective improvement to your radio. These filters have gotten even better lately.
Yeah, my High School Electronics teacher (may he rest in peace) also taught us about staggering the IF peaks.
Some older and larger IF transformers had cores with a hexagonal hole and were tuned with a plastic hex key, available at Radio Shack and other places. The hexagonal portion of the key was only about 3/8 inch long on a thinner shaft. The thin shaft allowed the hex key to be pushed entirely through the core, so the key could be turned without moving the core. Why?
Because some transformers had 2 cores and 2 separated windings that could be tuned independently.
Back in the days of tube shortwave receivers, the stability of parts was not good. If you wanted top performance from your receiver, every few months you’d open it up and retune the IF transformers, and the RF transformer if there was one.
I remember one component that had me scratching my head as a child, those piezoelectric quartz delay lines that held a scan line or so in televisions and video recorders. They still feel quite futuristic even now.
best sounding cd player I encountered was an early Yamaha that had IF transformers in a LC low pass filter after the DAC… I imagine there was a lot of hand tuning of them on the production line. But the audio, in the upper range, was magical