RACAL RA-237B
Gerry O’Hara
6
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’
for the 2MHz harmonic generator and my signal genny only gives around 100mV or so). With
my bench HT supply hooked-up to it (the RA-237B has its own filament supply) and the AGC
line shorted to ground, I coupled it to my Collins 75A-2 tuning the 160M amateur band (1.5 to
2.5MHz coverage on the Collins, the segment 2.01 to 2.50MHz effectively covering 10kHz
through 500kHz using the RA-237B). This set-up had the RA-237B running on the bench so I
could undertake some diagnostic tests.
I soon found out that there was a problem in the harmonic generator (photos, below) – however,
the first thing I noticed was an error in the tube pin labelling in the alignment procedure in the
manual (I found several errors in the manual, including incorrect tube pin references) – the
manual was obviously
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 7),
although the schematic had
been amended to reflect this
change – so no chance of
measuring much 2MHz signal
on pin 6 (Grid 2) per the manual instructions! Measuring on the correct pin of V3, however,
there was still nowhere near enough 2MHz signal at the balanced mixer. While checking
voltages on the harmonic generator tube (a 6AU6) I noted that the screen voltage was around 85v
(it should be 55v) – then I noticed an error in the schematic: the values of R19 and R20
providing the screen potential to the 6AU6 had been transposed (I checked against the RA-137
schematic to make sure) - this error was giving the higher than expected screen voltage. I
swapped the resistors and that fixed the voltage discrepancy, but no change in 2MHz output. I
then swapped-out the 6AU6 – again, no change. After double-checking all connections in the
harmonic generator section, I figured it had to be an infamous 'RACAL dry joint' on the tube
base (all other connections had been freshly-soldered in my re-build) – sure enough, when I the
waggled the 6AU6 the output jumped 10-fold – giving plenty of 2MHz drive to the mixer! I re-
