The Racal RA37A

2 minute read

November 2015

The RA37A preceded the RA137A just as the RA17 was the precursor to the RA17L. The differences between the two are electrical as well as physical. The RA37A is only 1U high as opposed to the 137A which is 2U high. Just as the preselector in the RA17 is simpler than that in the 17L, the preslector in the 37A is far simpler than that in the 137A.

RA37A preselector
RA137A preselector

As well as the obviously far simpler circuitry and the lack of a tuning scale, the RA37A preselector ranges differ significantly thus …

1. Wideband
2. 12KHz - 37KHz
3. 37KHz - 110KHz
4. 110KHz - 330KHz
5. 330KHz - 980KHz
1. Wideband
2. Wideband with filter
3. 10KHz - 20KHz
4. 20KHz - 40KHz
5. 40KHz - 90KHz
6. 90KHz - 210KHz
7. 210KHz - 300KHz
8. 300KHz - 980KHz

As well as have only 5 ranges as opposed to the 137A’s 8, the RA37A also tunes ‘backwards’ … i.e. turning the preselector knob clockwise actually decreases the tuned frequency.
The RA37A Harmonic Generator is identical to that in the RA137A.

Left: the 1MHz Low Pass Filter is also identical. The 100nF capacitor is a safety modification designed to protect the inductors in the event of one of the ceramic capacitors going short-circuit.

Upper and Lower sides of the Double Balanced Mixer.

This board and accompanying circuitry: Double Balanced Mixer, Output Amplifier and 150V Stabiliser, is identical to that in the RA137A.

In the RA37A, the top and bottom main covers are essentially identical.