RA218 ISB Adapterand Fine Tune Unit

October 2012: Much can be found on the Internet relating to the RA17’s (or RA117’s) inability to resolve SSB. I think this is actually an unfair criticism since, as I have previously said, the type is more than capable of producing comfortable resolved SSB provided care is taken NOT to overload the detector. This will undoubtedly require judicious use of the input attenuator and IF gain control. Also, bear in mind that a 50 year old receiver with deteriorating resistors and capacitors is NOT going work as well as when it left the factory.

Racal made several Sideband adapters. This is the RA218 Independent Sideband Adapter and Fine Tune Unit which was specifically intended for use with the RA117 and was primarily intended for resolving RTTY (or RATT) as part of a system with the designation CJK. The RA218 can also be used with the RA17 or RA17L, however in that circumstance the Fine Tune facility is not used for reasons which will become clear later. This RA218 came to me in a very sad state. I had been warned that it was not complete but that it probably only needed a few valves. True, there were several valves missing; a couple of EK90s and a couple of 12AT7s. Also missing were the two oil-filled 600 ohm output transformers which had been replaced with small output transformers for driving loudspeakers. However the most crucial part that was missing was the 18KHz crystal.

As can be seen from the schematic on the right, the 18KHz crystal, X4 is by no means conventional. For a start 18KHz is literally audio frequency, and anyone who knows anything about quartz crystals will know that a crystal cut to resonate at ten times that frequency will be large. In the RA218, crystals X1, X2 and X3 are all in the region of 118KHz and are all about 50mm high. So to cut one for 18KHz would just be impractical. In fact I even doubt if it would be possible.

However, looking at the schematic on the right it would appear that X4 is NOT actually a single element, but two pieces of quartz cut in such a way that  their resonant frequencies are actually separated by 18KHz. If this is the case then it is indeed a very cunning solution.

Obtaining the missing 600 ohm transformers was relatively simple since they are identical to the smaller of the two fitted in the RA17 series. But, it goes without saying that obtaining a genuine 18KHz crystal was going to be a ‘non-starter’. Deciding that I was not going to be beaten, I set about looking for a simple low-cost solution to the problem.

What I came up with is both simple … and appropriately cunning too …

August 2015:

A good friend of mine recently obtained an RA218 which he asked me to check over. This one did have the rare 18KHz crystal! My theory about the dual quartz element proved to be correct as can be seen in the photographs on the right.