AJAX COMMAND Radio Is An Oldie But Goodie

If you are a certain age, it is hard to wrap your head around the fact that an old radio might have transistors — the old ones all had tubes, right? But a radio from the 1960s or 1970s is reaching the 60+ year mark and people are restoring old transistor sets. [Adam] picked up his first old radio, a 1970s vintage Ajax Command S-74.

He was fortunate. The only repair needed was to replace a corroded battery holder and clean up the mess from the batteries. You can hear the radio doing its thing in the video below.

As repairs go, it wasn’t very difficult, of course. But seeing the inside of this radio si bound to bring back some nostalgia if you are at the right age. The PCB looks like a typical mass-produced board of the era. You’ll quickly recognize the IF transformers and the loopstick antenna.

It was almost a shame that the radio worked so readily. These old sets were actually fun to troubleshoot using a signal injector or signal tracer. We recently covered a modern take on a signal tracer, in fact. Back in the day, it wasn’t uncommon to have one of these or even a premium one from Heathkit.

32 thoughts on “AJAX COMMAND Radio Is An Oldie But Goodie

  1. Radios like this were primary tinker training for scamps with a screwdriver of my age. What can the kids take apart these days that will learn them something?

    1. Taking apart a radio as a kid teaches only why you shouldn’t ruin the end of a screw. You need to be a born wizard to understand anything that is going inside. So no worries if todays kids aren’t disassembling radios, they are on their computers writing games on Unity ..

      1. That’s about as dumb as saying “It’s fine that zero kids are brought up on a farm these days because they know how to work the menu at McDonalds.”

        1. I developed my EE skills writing code for microcontrollers and developing application circuitry to do things like run robots completing task courses in competitions. Go buy yourself some esp32s and start learning, analog radios are boring

          1. One of the nice things about analog radio is learning the physics behind it. On the other hand, programming does teach deeper math (depending on what you are doing), both teach debugging (troubleshooting) skills.

            Software Defined Radio combines the best of both worlds.


      2. There was a time, ca. 1920 to 1999, when radio construction sets were a thing. They could be found in most toy stores. These kits included fine manuals that explained the basic about electricity, electric components and radio waves.:

        Radio types like the crystal radio (detector radio), the regenerative receiver, the superhet etc. After reading those books, reading electronic schematics was no problem anymore.

        Some also were full-fledged electronic construction sets, with a lot of other experiments, besides radio technology.

        Those kits were enjoyed by generations of open-minded, patiently kids capable of at least some minimum cognitive self reflection, mainly young boys age 7+. Not just geniuses or prodigys. Rather the contrary. That audience was rather modest and not rich.

        Those that couldn’t afford the kits, -say a poor family in the 1950s-, were usually building simple things from scratch (crystal radio) on a kitchen table, with the aide of radio hobbyist books and their parents/grandparents. These books were also popular.







        1. I had one of those Electronics Kits toys, with the components connected with wires between springs that you bent over.
          A transistor, some resistors and capacitors, a dial and a coil.
          Then there was a cassette tape with Johnny Ball telling me how to make various AM radios and other things.

    2. >Radios like this were primary tinker training for scamps with a screwdriver of my age
      Agreed – my first memory of working with electronics was taking apart my grandmother’s broken radio (very much like the style of radio shown) and using the volume control to control the speed of my Meccano motor. Scrapping kit for parts was very much the thing to do in the 70s, but as you suggest, there’s not much equivalent nowadays.

  2. In germany, as a kid, you took an old radio like this, opened it up and turned everything that was turnable. Then after 30min of turning you could receive police communications perfectly.

        1. You can tune them to receive Geman police four meter band, but usually only the upper half. That’s 84,015–87,255 MHz. The lower half is too far down (74,215–77,475 MHz.) The upper half ist usually all you’d need, though. The relay stations all transmitted in the upper half – you could hear both sides of a conversation just by listening to what the relay transmitter sent.

          You could also tune an FM broacast receiver to pick up at least the lower portion of the aircraft band (118MHz to 137MHz.) Aircraft communications are AM, but you could twiddle the FM discriminator tuning to make it better for AM – it wasn’t really good, but it did work.


          That kind of thing is, naturally, easier if you have access to a good radio test set. The place I used to work had some Motorola R2600 test sets. Just set one to transmit a modulated signal on the correct frequency, then tune the radio for best reception.

          Doing it blind was kind of difficult – it’s hard to tell if you aren’t receiving anything because you haven’t got the tuning right or because there’s just nobody talking. Easier, of course, near the relay stations for larger towns and cities.

    1. You can twiddle all the knobs you like on this one, you’re not getting police bands… it’s AM medium wave and long wave…. possibly off the top end of the AM you’d get more into the shortwave bands, and off the bottom end of the long wave, it might get the time signals, but that would be really stretching it.

          1. Dang, return hooked the post button before I was done…

            I have had several radios and cheap stereos where any taxi, police or utility trucks on the block would break through on the amplifier.

      1. Agreed.. but the slightly later ones with FM could be used. In college, about 1972 We used a slightly modified home quality Sony Stereo FM receiver with a modified IF strip to pickup the satellites in LEO .. the NOAA series at somewhere like 137.3 MHz, and detect the images from weather cameras .. . Images were made on a textronix scope modulating the Z beam and a slow time base as line scan x.. the frame scan needed an 3 minute integrator . Then the image was photographed off the screen and developed in the Darkroom.

    2. All my relations used to give us their broken tech in the 70s and I’d break it down for components if I couldn’t fix it. German radios were great as they all had a circuit diagram stuck to the inside of the case.

  3. The two transformer circuit is the audio amplifier to drive the speaker. A single transistor gain amplifier stage drives the first transformer that generates two 180° out of phase replicas of the audio signal, that drive two transistors used as power amplifiers, each of which amplifies only half of the wave, one the positive, the other the negative (I think that makes it a push-pull configuration). The two half signals are then recombined by the second transformer.

    I also think it was done like that because you can use two identical transistors (both PNP, or both NPN) for the power amplification. When opposite type transistors, one PNP and one NPN with good enough matched characteristics became available, they ditched the transformers and went to a more classical push-pull configuration.

    I should really document somewhere how to design these circuits, they’re fun and nowadays you can buy hundreds of transistors for peanuts from china, it’s a golden age to experiment with these circuits but it looks like few have noticed

    1. What stops it being a golden age to do it, is that ferrite loopsticks and tuning capacitors can be hard to find. Relative to the transistors… there would be 50x as many places to get the transistors from as those parts. Sure, WE know the usual suspects, but as a bootstrap project for someone, they’re gonna be floundering trying to dig up the parts.

        1. Variable capacitors maybe were common at hobbyist type stores. But their need has declined, varactors taking over for many applications.

          There was a time when r.adio building was sort of mainstream, but that’s in the past. So you’ll haveto look for inductors and variable capacitors. You’re even less likely to find scrap radios to supply such parts.

  4. It sounds like it has a intermittent tuning capacitor, or maybe a couple bad solder joints. I’d probably replace all the electrolytic caps and then touch every solder joint with a soldering iron and a little flux… The cheap pots used for volume controls often went bad and became intermittent or very scratchy. Slide switches also failed intermittent (actually I think they were like that when new- they really were awful).

  5. Dr who the sea devils john partner returned me radio by fiddling with local oscillator to call for help he turned transistor radio into vhf tranceiver how did he do that impossible adca child I knew this was a joke must of been done in April fools day

    1. There was a scene in Gilligan’s Island where the professor states every radio can be a transmitter and then hacks a portable radio. It works but then …this is Gilligan’s Island.

      1. There’s an episode where the Skipper starts having dreams and reliving an incident during the war (WWII) in which he had to convert a receiver into a low power transmitter.

        The others try to watch him as he sleep walks and carries out the modification on an (imaginary) radio and see what has to be done.

        This being Gilligan’s island, of course things go wrong.

  6. When I was at school I used to repair radios like this for other children I saved pocket money to buy tools and components children today can’t do practical hobbies anymore as the police state government prohibits people under 18 from buyingtools and the component shops have closed they just sit in front of screens now how boring

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