Racal SA.97 Wobbulator

November 2015:

Here’s a tip; If you want your precious RA17, RA17L, or RA117 to continue to give you excellent service, don’t twiddle with anything behind the front panel!

Actually, the same can be said for ALL multi-band HF receivers. However, in the case of the RA17 series, one minor careless tweak could leave your receiver seriously compromised. It’s not that the receiver is fiendishly complicated either, which it isn’t. Yes it contains around 25 valves. But to the trained eye, it is nothing more than a series of relatively uncomplicated building blocks connected together in such a way as to satisfy a very clever arithmetic function. The product of which is a rock steady receiver requiring no complicated band-changing switches.

A crucial element in making this possible is the 40MHz band-pass filter on the output of the 1st mixer. This is an 8-element over-coupled network giving a bandwidth of 40MHz +/- 650KHz. Unlike the 37.5MHz band-pass filter, itself consisting of 8 loose-coupled elements, which can actually be aligned simply by adjusting for maximum deflection on the front panel meter, the 40MHz filter can ONLY be aligned using specialised equipment.

So although RACAL, along with Dr. Trevor Wadley, had created a receiver that was arguably easier to physically manufacture than others of the day, it on the other hand posed a serious challenge when it came to first-time alignment. So they designed the SA.97 Visual Alignment Unit (to give it its official Racal name) specifically for this purpose.

The SA.97 combined the ‘oscillator’ and ‘oscilloscope’ into one self-contained instrument which was by necessity, easy to operate.

I first saw this SA97 when visiting a friend about two years ago. It was sitting gathering dust on a shelf surrounded by other gems of Racal history. At the time, I made the comment that I would very much like to get my hands on it, to get it working, purely out of curiosity since I use the DG8SAQ VNWA for aligning the filters. So I was very pleasantly surprised  when my friend gifted it to me last month (October 2015). I was forewarned that it didn’t work, but that didn’t deter me. The brilliance control had little or no effect and the amount of Y-shift was limited to the upper half of the screen. At least the CRT worked! Thus I had something to start with. But I had absolutely no documentation on it other than a very brief reference to it in a magazine article. No circuit diagrams or any indication as to how it achieved its purpose!

If you want information on anything, simply type it into Google and you will find what you are looking for. Well, I’m afraid that’s a myth. A search on the Internet for ‘SA97 wobbulator’ came up with little more than several documentation references and a couple of photographs. Ironically the photographs in question were of the same SA.97 that I am in possession of. How weird is that? It therefore goes without saying that the SA.97 is a very rare piece of Racal hardware indeed by virtue of the fact that it was what is termed ‘in-house test equipment’ and was, I suspect, eventually superceded by the Samuel and Hutton CT-501.

A popular on-line encyclopaedia defines a wobbulator as ‘an electronic device primarily used for the alignment of receiver or transmitter intermediate frequency strips. It is usually used in conjunction with an oscilloscope, to enable a visual representation of a receiver's passband to be seen, hence simplifying alignment; it was used to tune early consumer AM radios. The term "wobbulator" is a portmanteau of wobble and oscillator.’