1970s Ajax Commander S-74 Transistor Radio Repair

A few months ago bought my first ever vintage radio, a 1970s Ajax Commander S-74 “solid-state” AM/FM set, which is among the oldest items in my collection. I’ve been wanting to get into vintage radio restoration for some time now (after watching YouTube channels such as the excellent Mr Carlson’s Lab), as I find the electronics fascinating.

I bought it in unknown condition from a YMCA charity shop in Middlesbrough for £25.00, with its original box, user manual, warranty card, and headphone.

The S-74 is a reasonably simple six-transistor superheterodyne radio receiver, powered by four 1.5V AA batteries, with a channel switch and an internal antenna.

If you’re interested in how “superhet” radios work, “Transistor Superhet Receivers” by none-other than Sir Clive Sinclair himself is an excellent book.

On inspection, the radio had suffered battery leakage in the past, which had left remnants in the case interior and had severely corroded the battery holder, to the point where one of the terminal cables had been eaten away completely.

The original battery holder was pretty much irreparable, so I cut it out and threw it away. It seemed to be a somewhat standard design, holding six four 1.5V AA alkali batteries in series, providing 6Vdc to the radio via a power switch.

I actually managed to source a replacement battery holder with roughly the same dimensions as the original which seems to fit well into the same space, and features a standard PP3 battery connector (which is typically used on 9V cells), making it removable.

I simply replaced the original fixed battery leads with a PP3 snap-in battery clip lead, by desoldering the original cables from the circuit board, and soldering in the new ones (taking care to ensure that they were installed in the correct location).

The battery holder then simply clips into place, and fits into the case.

I also took this opportunity to clean all of the controls and switches with contact cleaner. There is only one electrolytic capacitor in this set (100uF 12V, the round metal can next to the speaker) and it tested OK in-circuit, so I wasn’t going to replace it just yet.

With a set of batteries installed, I started up the radio and it seemed to work great, as you can see in this video that I took. A nice easy repair, fantastic!

Published by themightymadman

My name is Adam Wilson - I'm an electronics engineer based in the North East of England, UK, and I like tinkering with old junk. In my spare time, I collect, repair, refurbish, and (sometimes) sell vintage computer systems and peripherals, typically from the 1980s (the likes of Commodore, Sinclair, Acorn, Apple, Amstrad, and Atari).

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