RACAL RA-237B
Gerry O’Hara
2
as the RA-117 does not include a tuneable 2 -3MHz bandpass filter. The RA-37 has simpler pre-
selector circuitry and no tuning dial. The ‘B’ suffix designation indicates the unit incorporates
US tube types with some associated minor circuit changes and different RF connectors.
Simplified Circuit Description
The RA-237B takes the 1MHz crystal master oscillator signal from the RA-117 and doubles this
to 2MHz in a harmonic generator using a 6AU6 operating in a non-linear fashion, followed by a
50kHz wide bandpass filter at 2MHz. The clean 2MHz local oscillator signal at around 1v p-p is
fed to a balanced mixer comprising two 6AS6 tubes. The incoming RF signal is amplified by a
6BA6 (its gain controlled by the RA-117 AGC circuit), before being fed through a 1MHz low-
pass filter to the balanced mixer. A switchable attenuator is provided, together with a six band
pre-selector tuned circuit arrangement, plus a wideband (aperiodic) input setting. The latter can
have a 500kHz low-pass filter inserted if desired to block strong signals above 500kHz. The
2MHz local oscillator signal is cancelled-out in the balanced mixer and the product of this and
the incoming signal from the RF amplifier is fed to a 6AU6 acting as a cathode-follower to
provide the low-impedance IF output needed to feed the tuneable 2-3MHz input on the RA-117.
As noted above, a simple 2MHz crystal notch filer is provided between the balanced mixer and
cathode-follower stages. The unit includes a transformer for powering the tube heaters, however,
the HT (around 200v) is derived from the RA-117 – this is necessary to allow simple switching
of the HT to the unit or to the RA-117 front-end circuits. The HT voltage to the balanced mixer
is stabilized at 150v by an OA2 tube. A block diagram of the RA-237B and how it connects into
the RA-117 receiver circuit is provide below for reference.
