RA137A LF Converter

August 2015: Last month I picked up a couple of very sorry looking RA137As. Both were incomplete and looked as if they had been stored in a chicken shed! Although both were missing the rectangular tuning bezel, a quick glance over them convinced me that I had a good chance of  getting a working converter from the two ‘wrecks’. Using what I took to be the date codes, one was assembled in January 1963 and the other in June 1964. The older of the two was definitely in a worse state. A large part of the Perspex tuning cursor was missing and the rotating ‘drum’ was seized. An interesting point here is that this unit used an unusual drive cord arrangement for turning the drum where it looks like the drum is spring-loaded and a cord is used to ‘wind it back’. Despite it’s shocking condition the front panel on this unit was the better of the two so I decided to swap the panels over.

Having decided to use the newer of the two RA137As as the basis for my re-build I removed the top and bottom covers from the RF compartment. The photograph on the left is the 1MHz LP filter on the output of the RF amplifier. Note the loose ferrite. What has happened here is a result of deterioration of the threaded plastic former which also doubles as the means of securing the inductor to the board. The plastic has snapped where the nylon (or plastic) screw held it down. This situation applied to all but one of the cores in this picture and another two in the Pre-Selector compartment. Luckily I was able to salvage enough parts from the ‘donor’ unit to secure all the inductors.

Unlike the RA17, the RA137 is not complex. It consists of an RF amplifier with a switchable preselector, a double balanced mixer and a cathode-follower output stage. The 2MHz Local Oscillator is derived from the 1MHz output from the RA17 by way of a simple harmonic generator. See the image below.

RA137 functional block diagram

Despite it’s simplicity, the RA137A proved to be an annoyingly awkward unit to work on … largely due to the fact that the valves are mounted end-on to the double sided tag boards. Consequently, the underside of each socket is partially obscured by the lip of the enclosure.