RACAL RA-237B
Gerry O’Hara
7
soldered all the tube base connections –
problem solved. I then set-up the harmonic
generator and balanced mixer - that all went
well per the manual, then I checked the unit’s
RF sensitivity – it was really poor. However,
I found that if I touched the signal input grid
of the mixer (pin 1 of V3), I could receive
signals (but, as to be expected, rather weak). I
changed-out the 6BA6 RF amplifier tube – no
change. On checking the circuit I realized that
when 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!) – that corrected,
the RF sensitivity seemed acceptable. 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, but felt
that I should double-check these with the
RA-237B connected to the RA-117 – they
were very close (it is very important to
align these with the dust covers in place).
The manual describes checks at higher and
lower supply voltages, but as this unit was
only being used at a standard 117v AC
supply, I did not undertake these. The
500kHz (photo, left) and 1Mhz low-pass
filters seemed to be working ok, with only
minor adjustments being needed per the
manual. I found the pre-selector tuning a
little out on the highest band only and this
was soon corrected with a tweak of the
trimmer. Job done.
Installation
My RA-117 is installed in a large steel table-
top case that has a secondary cover mounted
on 1” standoffs to allow cooling even if the
cabinet is covered. I found some 3.5”
standoffs and the RA-237B now sits neatly
