Racal SA.97 Wobbulator page 7

Some final comments and observations …

I have been asked how I would compare the SA.97 to the Samuel & Hutton CT-501. The answer is simple. There is no real comparison since the SA.97 is essentially un-calibrated. There isn’t a single  graduated control on the front panel and the display bears no graticule. Given my extensive array of  test equipment, it would be possible for me to apply signals at known frequencies and levels and then draw lines on the Perspex CRT cover. Maybe this was how the display was calibrated, but it would need to be done every time the SA.97 was switched on and that is simply not practical and is likely to be the reason why I believe Racal themselves opted for the CT-501 in production.

I thought it would be interesting to describe the controls on the SA.97. The Brilliance and Focus controls are self explanatory.  The Sweep Generator has a single potentiometer control labelled ‘Sweep Centre’. As one would expect, this can be used to adjust the sweep such that the output is symmetrically centred on 40MHz. The pot shaft is slotted so it may be that in practice there was no knob on it. There is nothing behind the blanking cap.


The Lin/Log Amplifier module has a switch for selecting Linear or Logarithmic gain and an adjustable gain control. The latter only appears to be effective when ‘Lin’ is selected.


Along the bottom we have the X-Y controls. On the left there is the TimeBase control offering three different sweep rates. Next is the X-Shift which allows the user to move the entire trace left or right. Next is the X-Gain control which is ‘presumably’ used for setting the trace width to the diameter of the CRT.

Then we have the Y-Gain control which as its name suggests, sets the vertical gain and thus the height of any displayed trace. The Y-Shift control allows the trace to be positioned anywhere in the Y-plane. The Sweep - Width control allows the user to ‘zoom’ in on a portion of the trace by limiting the range of the VCO

control signal. For this it is necessary to make appropriate adjustment to the Sweep Centre control on the Sweep Generator. As you can see there is a distinct lack of annotation other than control labels.


I have to admit that I find the SA.97 a rather odd piece of ‘Production Test Equipment’. I myself am ex-Racal and spent many years at what was then part of Racal Defence, latterly designing and building ‘Special Test Equipment’. Calibration was of paramount importance. So why the SA.97 is little more than a relative display is baffling to me? Something else which is worth pointing out is that this SA.97 is not as old as one would first assume. The early ‘bat-wing’ knobs as fitted on the RA17 MK1 might make you think that this is a relic from the 1950s. I initially thought this was the case and that the TimeBase knob, which is the same style as fitted to the RA1772, was only there because the original knob had been missing. However I now believe that the knob is original. As can be seen from the preceding pages, ALL the quarter-watt resistors and nearly all the half-watt resistors are from the mid-late 1960s. The three tall electrolytic capacitors in the PSU are all dated May 1968; which is much later than I had expected. The RA1217 which was manufactured in the mid 1960s actually employs the same switch knobs as the RA1772, albiet a different colour. I doubt very much that this SA.97 was ever refurbished by Racal. Hence why I think the knob is original. So why did Racal go to the trouble of manufacturing this particular SA.97 in the late 1960s when infinitely more accurate equipment was available ‘off the shelf’?   

18 Nov. 2015:

I am indebted to Andy Jackson, G8JAC for identifying the end user as the RAF

by deciphering the legend on the left thus …

Section 10S is electronic test equipment. However, there was an interim period when the "last seven" digits of the Nato Stock Number were used, prefixed by the old stores catalogue section title. Now, electronic test equipment is NSN Class group 6625.

We know that it's British so the Nation code will be 99. Put all that together and we get: 6625-99-580-9271.

Put that into Google and Bingo! You will get cross-references to Racal Communications Systems Generator, Sweep, Type SA97.

So, your Wobbulator is (a) a Nato coded item and (b) it was used by the RAF.