background image

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 
seller, and testing some 
of the passive 
components revealed that 
they were in reasonable 
shape – much better than 
those that were in my 

RA-117!  Most resistors 
tested within tolerance 
and the capacitors were 
mostly serviceable (the 
electrolytics were not 
that great).  However, as 
I had completely 
replaced all paper and 
electrolytic (and some 
silver mica) capacitors in 
my RA-117, I decided to 
do the same for the RA-
237B to promote long-
term reliability (photos, 
left and at top of 
following page).  All 
tubular ceramic caps 
tested ok and were left in 
place.  While doing so, I 

noticed some discrepancies between the schematic in the manual and the unit on my bench: these 
changes were apparently modifications made to improve performance and reliability, including 
inserting a blocking capacitor between the RF stage anode and the 1MHz low-pass filter (this I 
suspect to avoid disaster if one of the capacitors in the filter shorted, as this would burn out the 
preceding inductors), some changes in the harmonic generator (damping the anode tuned circuit), 
and including a 1pF coupling capacitor in the bandpass filter.  
 

Summary of Contents for RA-237B

Page 1: ...ntial 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 which meant I would st...

Page 2: ...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 bloc...

Page 3: ...file 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...

Page 4: ...c and some silver mica capacitors in my RA 117 I decided to do the same for the RA 237B to promote long term reliability photos left and at top of following page All tubular ceramic caps tested ok and...

Page 5: ...perly and to allow set up and test however my RA 117 is located at one end of the basement and I don t have enough room in my miniscule workshop at the other end of the basement for it in its rather b...

Page 6: ...y originally written for the UK tube complement which uses 6F33 tubes in the balanced mixer Grid 3 is pin 6 and they forgot to change the text to suit the 6AS6 s fitted in the US version Grid 3 is pin...

Page 7: ...table I then re connected the RA 237B to the RA 117 and it was soon pulling in stations and lots of QRM from DC to 980kHz phew I had aligned the harmonic generator and balanced mixer in the workshop b...

Page 8: ...however sensitivity is a little lacking compared with the RA 117 in the segment of the spectrum covered by both 750khz to 1MHz though seems to be to specification Unfortunately I do not have a real L...

Page 9: ...cosmetic condition some with missing and or broken tag strips capacitors removed broken knob skirts missing decals etc The two best shape receivers were to be retained for the Museum s collection N8 w...

Page 10: ...he 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 receivin...

Page 11: ...checked and found to be 500mA Tube screens were cleaned inside with acetone to remove brown residues scratches touched up with black permanent marker The knobs were cleaned with soapy water cut with...

Page 12: ...n the schematic between the two sections of the 2MHz bandpass filter after the harmonic generator absent in Serial No 179 This change is interpreted as improving the characteristics of the bandpass fi...

Reviews: