There are lots of great looking vintage electronics out there that sadly no longer function. Restoring the original circuitry can be painstaking and still leave you with a device that relies on outdated components. Here we take a great looking AM/FM radio from the 1950s and replace its innards with a modern transistor radio.

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Getting started

We start by choosing a classic AM/FM radio. You can find these at vintage stores, yard sales, or in the family attic. Because we are not concerned with whether the circuitry works, you can choose a radio based on appearance. Make sure that the radio you chose has all of its knobs, that any fabric speaker covers are in good condition, and that the frequency knob moves the radio station (frequency) indicator.

For this tutorial, our vintage radio will be a Zenith model K731 from approximately 1951. We will be using a digital pocket radio from Radio Shack as the new voice of this retro beauty.

We begin by removing the back of the radio, and seeing how the controls operate. You may set aside any tubes and loose components (and you probably won't need them later). Knobs are often attached through the front faceplate, and attached to components with metal bands. You will have to remove these metal bands before detaching the knobs through the front of the radio.

With vintage radios, the enclosure is a truly separate piece from the electronics inside, so you should be able to entirely remove the electronics from the wooden enclosure. Here is a picture with the electronics sitting on top of the radio enclosure. The knobs were taken off and set aside during disassembly. Note the pulley and string system that moves the frequency indicator. Interfacing with this will be a fun challenge.

We now have a better idea of how the radio controls operate. The tuner is a mechanical assembly, as opposed to an electronic knob. AM/FM selection is controlled by a slide switch. Power and volume are controlled by a single knob which clicks on/off, and then rotates further to control the volume. Finally, there is a knob for "tone" which we will not use.

Interfacing with the controls

Now it is time to open up the handheld radio and see how we can interface with the controls. The handheld is opened by removing the visible screws (remember to check uunder the battery door, and/or under stickers.) After the screws are removed, all that holds the two halves of the radio together are plastic hook tabs.

Pro tip: A good trick for getting the halves apart is to use a piece of plastic from the radio's packaging, or any "bubble-pack" type packaging. Cut a piece into a rectangle, wedge it into the seam, and then side it along the seam. This method is superior to using a screwdriver, because it doesn't chip the plastic.

After disassembling the handheld radio, we see that the power and AM/FM functions are controlled by (momentary SPDT) pushbuttons. Tuning is controlled by a rotary switch with three data pins. Volume is controlled by a potentiometer dial with five pins.

Having opened both devices, we can now begin to plan out how we will interface the two radios. After some investigation, using a logic analyzer, it was determined that the handheld radio tuning knob was a pulse encoder. Therefore it could not simply be replaced by a variable resistor (potentiometer). The solution is to de-solder the pulse encoder and physically attach it to the vintage radio's tuning knob. Due to space constraints, we cannot mount the pulse encoder directly on the vintage radio's tuner shaft. Using some posts and gears from Lego Technic, we build a drive system that transmits the turning of the vintage radio's knob to a point in the interior of the radio. It is important to use gears in a 1:1 ratio, otherwise the vintage radio's tuning display will not reflect the station that the handheld radio is tuned to.

The pulse encoder is de-soldered, and extension wires are soldered to the five pins. (Three data pins, plus the two mounting pins, in case they are important as grounds.) The whole thing is covered in hot-glue, so that the soldered wires are not stressed, and a mounting post is added.

After the handheld's tuning pulse encoder is linked to the vintage radio's tuning knob, the extension wires are soldered to the handheld, allowing the pulse encoder to function as it was orginally mounted.

The handheld's power and AM/FM selectors are momentary push buttons, whereas the vintage radio has persistent selectors (power is a rotary on/off knob, AM/FM selection is a slide switch). In order to interface the vintage radio to the momentary contacts, we first solder wires to the push buttons, and then trigger them with a 555 timer chip in "one-shot" mode.

The AM/FM slide switch on the vintage radio triggers the one-shot, which mimics the momentary pulse of the handheld's buttons. The two radios' power buttons are interfaced in the same way.

The volume knob is de-soldered and the vintage radio's potentiometer is connected in its place, via soldered extension wires. (Note, if the two potentiometers have different values, there may be a point beyond which turning the knob will not result in any increase in volume.

Hooking up the speakers

Now that all of the controls are interfaced, all that is left to do is solder the speaker wires from the handheld to the speaker of the vintage radio. You may encounter that the vintage speakers no longer work or offer poor sound quality. You have two options. You can solder a replacement speaker to the handheld, or you can use the headphone-out jack of the handheld and plug in some computer/portable speakers.

Pack everything back up in the stereo enclosure, and enjoy your newly-resuscitated vintage radio!

Original article and photos by Ari Douglas, for Wired.com.

 

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