I disassembled an old radio that I found in my attic and found some electronic components that I can't recognize.

I think that the first component is an inductor. I'm pretty sure about that... But I can't understand why it's made the way it is. There is more than one coil on the same core, so the coils should be coupled... But for what purpose?

As for the second and third component I really have no clue about what they are. However, I tested the third component with my tester and I found out that the resistance between the majority of its pins is really low and many of them are short-circuited on the PCB.

Finally, there's a photo (below) of a speaker. I'd like to know why the connection to the speaker is made with a "coil". There are a lot of connections made like that on this circuit.


  • 1
    \$\begingroup\$ the first is a coil, it has two loosely coupled windings, one of which is used to provide a positive feedback (a reaction) in order to boost the signal coming from the antenna. The second component is a variable capacitor, and the third is a transformer (the screw on top can be used to fine-tune it by inserting a variable metallic core - something done by the manufacturer). \$\endgroup\$ Commented Mar 25, 2018 at 16:11
  • 2
    \$\begingroup\$ The first is a ferrite rod antenna \$\endgroup\$
    – peterG
    Commented Mar 25, 2018 at 16:15
  • \$\begingroup\$ @Elia have searched for an AM block diagram to understand? \$\endgroup\$ Commented Mar 25, 2018 at 21:03
  • \$\begingroup\$ In some of these ancient radios they used an electromagnet for the speaker. The winding's also acted as a ripple filter for the noisy power supply. \$\endgroup\$
    – user105652
    Commented Mar 25, 2018 at 22:51

3 Answers 3


The first is a ferrite rod antenna for MW/LW frequencies.

The second seems to be a variable capacitor: as you turn the knob, the plates will overlap more or less, which changes capacitance. Between the plates is a dielectric, which can be air or plastic sheets.

The third is either a variable inductor or a transformer with variable coupling (variable coupled inductors): turning the screw in the center moves a ferrite core inside a coil. More ferrite inside the coil increases the inductance/coupling. These were most likely adjusted at the factory, so if you want the radio to still work it is best not to adjust them...

I'd like to know why the connection to the speaker is made with a "coil"

It's wire wrapping. Quite fast and convenient to apply during manufacturing, and very reliable. Here's a wire wrap pistol. These days it seems to have been displaced by pre-assembled wiring harnesses with pre-crimped connectors (video because robots), but it was used pretty much everywhere a few decades ago (and is still used in some applications).

  • \$\begingroup\$ Thanks for the answer and the links! I've a doubt about wire wrapping: the coil around the pin is an inductance, isn't it? Doesn't it deteriorate the performance then? I mean, it's like adding a parasitic inductance to each connection! \$\endgroup\$
    – Tripola
    Commented Mar 25, 2018 at 20:00
  • 2
    \$\begingroup\$ no, it's not an inductance, it's just the wire being wrapped around the lug (is it a lug? or a spade? my english is failing). The way wire wrapping works is that the male contact "lug" has square corners (this one looks like sheet metal), so it bites into the wire wrapped around it. This makes a very reliable contact. \$\endgroup\$
    – bobflux
    Commented Mar 25, 2018 at 20:26
  1. The first component is a Ferrite Antenna, the windings service the various bands on the receiver.

  2. The Second is a tuning capacitor and connects to the front panel tuning knob. It has multiple sections one for each of the bands to provide tuning with the Antenna above.
    You find two main type: air cored such as this: enter image description here

And film cored such as this or the one you have:

enter image description here

  1. The third is a coupling transformer in the frequency conversion (Intermediate Frequencies) stages of a super heterodyne receiver (typically 455kHz).
    Typically they have Ferrite cores in them that you tune to the intermediate frequencies.

The third component is a tuned IFT probably at 455KHz .It is complemented by the orange ceramic resonator .This scheme provides selectivity on the AM section of this multiband receiver. Other tuned transformers and sometimes ceramic resonators tuned usually at 10.7 MHz provide FM selectivity,


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