RACAL RA-237B
Gerry O’Hara
3
- 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). R28 was replaced with a 30 ohm 5% 1W metal film
part.;
- Several soldered joints on non-replaced parts were cleaned an re-soldered to
mitigate possible dry joints (visual appearance of several joint was
rough/dull/cracked);
- Capacitor holders (black plastic clips) removed to reduce clutter (no longer
needed due to smaller capacitor sizes);
-The psu cover was glued together;
-All wafer switch contacts were cleaned with DeOxit 5% spray and DeOxit ‘Gold’
applied;
-All tube pins were wire-brushed and cleaned with DeOxit, then inserted/re
inserted in their sockets several times to clean;
- The tuning mechanisms were cleaned and moly grease applied sparingly;
-Minor bearing surfaces were lubed with either light machine oil or moly grease;
-Tuning capacitor ball bearings were cleaned and moly grease applied;
-Tuning capacitor contacts were cleaned with DeOxit 5% and DeOxit ‘Gold’
applied;
-The HT fuse was 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 Novus #2 and polished
with Novus #1. Position-markers were touched-up with white Humbrol enamel; -
Minor blemishes on the front panel were touched-up with Humbrol enamel (light
grey #40, gloss);
-The chassis and modules were cleaned with alcohol, lighter fuel and/or acetone;
-All BNC connectors were silver polished.
Notes on S/N 179 Circuit/construction:
-The unit is constructed in low height-profile pseudo-modular fashion, with the harmonic
generator and power supply sections isolated (located at the rear of the chassis) from the
signal circuits. Most components are mounted on metal pillars on Paxolin boards, except the
RF front-end, where the inductors and trimmer capacitors are mounted on a metal sub-
chassis; Screening covers are fitted to the modules;
-Anti-microphonic tube bases are provided the double-balanced mixer tubes;
-Extensive use is made of ferrite pots in the high-Q filter circuit inductors (photos I have of
a later S/N unit has these external ferrite pots replaced by metal-shrouded inductors in both
the pre-selector and 1MHz LP filter);
-The 500kHz LP filter is mounted in a different location (above the screening
cover to the 1MHz BP filter) rather than that shown in the manual (next to the aerial attenuator
