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RACAL RA-237B
RACAL realized that the receiver was not really useable 'barefoot' below 1MHz and therefore
produced a 'Low Frequency Converter' specifically for this purpose: the RA137 for the RA-17 and
the RA-237 for the RA-117 (the 'B' suffix denoting the North American version – using US tube
types and having different connectors). The designs of these two LF units differs in that the RA237
includes a 2MHz crystal notch filter in the output circuit.
The RA-237 essentially comprises a pentode RF amplifier with switchable filters for each band
covered from 10kHz through to 980kHz. A wideband option is provided, with or without a 500kHz
low-pass filter. The 1Mhz crystal oscillator signal from the RA-117 is fed to a harmonic generator,
the 2MHz harmonic of which is selected and fed to a double-balanced mixer, along with the signal
from the RF amplifier (via a 1MHz LP filter). Output from the mixer at 2 to 3 MHz is fed to a
cathode-follower stage via a 2MHz (crystal) notch filter to remove remnant 2MHz signal that may be
present (this should be cancelled-out in the mixer action, but slight imbalance will leave a trace).
Output from the LF converter is fed to the RA-117 2 – 3 MHz tuneable IF stage, with the red
markings on the dial being used (these are reversed to the normal scale). The RA-117 also feeds
AGC bias to the LF converter RF stage. HT is provided to the LF converter by the RA-117, this
being switched in when operation below 980kHz is selected (and simultaneously removed from the
front end of the RA-117). Thus when the receiver/LF converter is receiving signals below 980kHz,
the RA-117 front-end and Wadley Loop circuits are non-operational, thus avoiding all the spurious
responses generated at LF when using the receiver below 1MHz 'barefoot'. The receiver topology is
still triple-conversion: RF amplifier at signal frequency, double-balanced mixer to 2-3 MHz, tuneable
2-3MHz IF, mixer to 1.6MHz, mixer to 100kHz, detector and AF stages.
LF Converter S/N 179
This unit was obtained through an Ebay auction in July, 2011, ex-Vernon, BC. The unit was in
reasonable cosmetic condition on arrival, noting:
- One of the Paxolin safety cover over the power supply compartment was cracked;
- Some minor re-work was noted in the pre-selector unit (two silver mica capacitors
replaced;
- The front panel had minor abrasions and scratches;
- All controls functioned mechanically;
- Dial lights and tube heaters worked when powered-on (the RA-237 power supply
only provides heater power for the unit. HT is derived from the RA-117);
-No HT was applied to the unit – decided to undertake refurbishment first.
General:
- Chassis thoroughly cleaned;
- All paper and electrolytic capacitors replaced. Paper ones were replaced with
polyethylene or polypropylene film types). Two silver micas (56pf and
68pf) were also replaced in the pre-selector unit - these had been changed-out at some time in
the past, subbed by an 82pf silver mica and a 68pf low-voltage ceramic respectively;
2
Gerry O'Hara
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