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Restoring a RACAL RA-237B Low Frequency Converter – Gerry O'Hara
I purchased and restored a RACAL
RA-117 a couple of years ago.
This radio has been working very
well since the restoration, but I
found that it only received signals
down to around 750kHz, below
which I receive mainly spurii and
noise – not too surprising, as the
receiver is only designed to receive
down to 1MHz at full spec and
below this with 'reduced
performance'. I knew that a low
frequency (LF) converter was
available for this receiver,
designated the 'RA-237B' and
decided to look-out for one of
these to mate with my receiver.
Finding an RA-237B
which meant I would still be incurring some shipping cost (but not much). I won the auction and
secured the unit for a reasonable cost. On arrival, the unit was well-packed and in pretty good
shape, with a full tube complement and, thankfully, a copy of the RA-237B manual (which is not
readily available for free download on the web).
The RA-237 v the RA-137 v the RA-37 and the 'B'
The RA-237 unit is a modification of the RA-137 unit which is
designed for the RA-17 receiver, the primary difference
between the two models is the addition of a 2MHz crystal notch filter to help remove remnants
of any 2MHz local oscillator signal due to any imbalance in the balanced mixer in the RA-237,
I had been keeping
an eye on Ebay
and other potential
sources for one of
these (fairly
scarce) units over
the past couple of
years and finally
found one for sale
on Ebay - 'locally'
here in British
Columbia (actually
in the Okanagan),
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Summary of Contents for Racal Instruments RA-237B
- Page 1 On arrival, the unit was well-packed and in pretty good shape, with a full tube complement and, thankfully, a copy of the RA-237B manual (which is not readily available for free download on the web).
- Page 2 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.
- Page 3 RACAL RA-237B Gerry O’Hara 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.
- Page 4 RACAL RA-237B Gerry O’Hara balanced mixer, though a hole could be drilled for this purpose also (however, its adjustment is very broad and makes little difference). Restoration Work As noted above, the unit was in reasonable shape on receipt from the...
- Page 5 Connecting entails a couple of coax links (1MHz output from the RA-117 and output of the RA- 237B to the 2 to 3MHz ‘RF in’ socket on the RA-117), plus HT and AGC lines. Switching the RA-237B on and the unit to LF, I heard a little hiss but other than that …. not a ‘sausage’!
- Page 6 Gerry O’Hara After a lot of fiddling with the controls, I took the RA-237B back to the workshop and ran a coax cable from my RA-117 (located at the other end of the basement) to deliver the 1MHz signal that it needs for the harmonic generator (it needs a couple of volts at 1MHz to provide enough ‘drive’...
- Page 7 I was incorporating some mods per the RA-237B manual – this particular one being the insertion of a coupling (DC blocking) capacitor from the anode of this tube to the input of the 1MHz low-pass filter (photo, above) – I had made an error in the wiring resulting in no HT being delivered to the RF tube (oops!) –...
- Page 8 500kHz was possible (though it picks up my signal generator ok right down to 10kHz). Conclusion So, was it worth spending $110 on the RA-237B plus $20 on replacement parts? – I think it certainly was, if only for the entertainment value of getting it working, however, being able to...
- Page 9 ADDENDUM: RACAL RA-237B Low Frequency Converter, Serial Number 179, Restoration and Schematic Notes - Gerry O’Hara, VE7GUH, August, 2011 Background: Racal RA-117 Serial Number ‘N8’ was one of a batch of 3 donated to the SPARC Museum, Coquitlam, BC in 2007. The Museum already owned two RA-117s, acquired in the late-1990’s from the Canadian Navy on scrapping a destroyer.
- Page 10 RACAL RA-237B Gerry O’Hara 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).
- Page 11 RACAL RA-237B Gerry O’Hara - All carbon composition resistors were replaced with 5% tolerance 1W metal film parts, except the two 220 ohm ‘High Stab’ parts in the balanced mixer screen grid circuit (replaced with 2% 1W metal film parts) and 39 ohm ‘High Stab’ part in the cathode-follower cathode circuit (replaced with a 5% 1W metal film part).
- Page 12 RACAL RA-237B Gerry O’Hara switch); -Several discrepancies were noted between the schematic and S/N179: A 0.01uF capacitor (C61) is included in the schematic between the anode of the RF stage and the 1MHz LP filter (absent in Serial No. 179). This change is interpreted as a protection of the 1MHz BP filter inductors in the event of a capacitor failure in the filter sections;...