The Armstrong 424 FM Tuner

The above picture shows the 424 tuner. This was VHF(FM) only. The 423 Tuner looked almost identical but also included MW/LW AM bands. The tuning dial arrangement was a rotary dial with a central control which turned the dial via a step-down reduction drive. This meant that as the user rotated the knob at the center of the dial, the scale would rotate more slowly than the knob, making fine tuning easier. The system employed was to rotate the dial so that all the displayed values went around. The dial scale was translucent purple with a vertical cursor and was lit from behind. The reduction ratio was approximately 4.25:1, so you had to make around four complete turns of the knob to tune from one end of the scale to the other. This arrangement was (then!) a modern restyling of a form of rotary tuning indication that was quite common with valve radios in the 1950's and earlier.

The image above indicates the layout of the controls, etc, on the 424. The tuning arrangement has already been described. However the unit also had a mono/stereo switch which could be used to defeat (bypass) the optional stereo decoder to obtain improved signal to noise ratio from weak stations. In addition, near the bottom-right of the facia is another control which can easily be mistaken for a button or indicator light. This is actually a variable ‘squelch’ control. The legend below this look similar to that often used for volume, so may be mistaken for a volume control. The 423 had extra buttons beside the mono/stereo and on/off to select between VHF/MW/LW bands.

Only one tuning meter was used on the FM tuners/receivers and the AM/FM tuners/receivers. When tuning on the VHF/FM band, the tuning meter operated in a center-zero mode to help the user tune on-station with precision. The tuner did not have AFC so relied upon its inherent stability to ensure that the tuning did not drift once a station was selected. Although no signal strength meter was fitted for FM reception, the amount of swing off-center as a station was off-tuned gave an indication of the FM signal level. When tuning to MW/LW stations, the same meter acted to show signal strength, with the meter indication rising vertically with increasing input signal level.

400 Range tuner specifications

FM/VHF Sensitivity	Mono 1.5 microV, Stereo 5 microV (30dB SNR 75kHz dev.)
FM Frequency Response	30Hz - 15kHz +/-1dB	Coverage	86 - 109 MHz
IF bandwidth	220 kHz (-6dB)	centered at	10.7 MHz
AM coverage	Medium Wave 510 - 1625 kHz, Long Wave 150 - 300 kHz
AM Sensitivity	5 microV for 20dB SNR	IF Rej	80dB
IF bandwidth	+/- 4kHz (-6dB)	centered at	430 kHz
Audio Out	Variable 0 - 1 V, source impedance < 1kOhm

The squelch circuit suppresses low-level signals and prevents them producing any audible output. The control allows the user to choose the input signal level below which the squelch operates. This serves two purposes. The main purpose is to ‘mute’ any inter-station noise and prevent a loud hiss being heard as the tuner is tuned from one station to another. The secondary purpose is to enable the user to suppress any very weak stations from producing noisy, distorted output. The control can be adjusted down to zero to prevent any squelch operation if the user does not mind inter-station noise and wants to be able to detect and hear even the weakest stations.

The tuner sections of the 425 and 426 receivers (tuner amplifiers) were exactly the same as used in the 423 and 424 tuners. Electrically, the 400 range and 500 range tuners were also essentially identical.

The M4 Stereo Decoder could be fitted to the tuners or receivers to enable them to receive VHF Stereo broadcasts. This board could be included when the unit was purchased or added at a later date by the owner as it was a simple ‘plug in’ board. The M4 was essentially the same as the M8 decoder that was sold for fitting to the 500 range.

In the mid to late 1960's stereo radio broadcasts were much rarer in the UK than now. For some years the only stereo broadcasts in the UK were on Radio 3 (Third programme), were only for a few hours per week, and only available from a few of the main transmitters. In those days it was quite an event when the light on the tuner lit up to tell you that you were hearing stereo radio. Hence at first many owners did not bother to buy and fit a decoder when purchasing a tuner or receiver as there was initially little point!

Content and pages maintained by: Jim Lesurf
using HTMLEdit and TechWriter on a StrongARM powered RISCOS machine.