RACAL RA-237B
Gerry O’Hara
3
Front-panel controls on the RA-237B are minimal: power on/off, RA-117 range (HF/LF),
attenuator, and pre-selector bandswitch and tuning.
Mechanical Construction
The unit is designed to take up only 3.5” height in 19” rack, either above or below the RA-117
receiver. This low-profile makes for an interesting construction challenge. To achieve this, the
tubes have to be mounted horizontally. This was done by designing a low-profile chassis frame
that has the harmonic generator located to the rear in a screened box with the 6AU6 facing
inwards and the remaining circuitry located at the front of the frame (photo, below), its tubes also
facing inwards:
this arrangement
results in the
tubes receiving
good (vertical)
ventilation and
the passive circuit
elements
remaining cool.
The (filament)
transformer is a
low-profile unit
also mounted at
the rear of the
chassis (photo,
below). Passive
components are
mounted on posts
inserted through
Paxolin
(phenolic) boards,
allowing easy replacement of components. The pre-
selector tuning is by a mechanical arrangement (gears
and dial cord), with the selected band (or ‘wideband’ or
’wideband with filter’) switch rotating a drum with the
scale markings behind a horizontal slot on the front
panel, illuminated by dial lights. All circuitry is
screened with aluminum dust covers. The 500kHz
low-pass filter is located in a small aluminum box
mounted above the main circuitry. The pre-selector
tuned circuits can be accessed by removing an
aluminum cover on the top of the unit – this allows
access to all tuned circuits except the inductors
pertaining to the highest frequency range, access to which necessitates removal of the aluminum
cover on the base of the unit (unless two holes are drilled in the cover for this purpose). The
screening covers allow access to all other adjustments except the anode tuning coil for the
