Alignment procedures are covered in the RA-117 manufacturer's manual. Presented here are some additional comments and hints gained from the experience of rebuilding several RA-117s.
The photo at right shows a unit tuned to WWV at 10MHz.
Broadly speaking, there are two major aspects to realigning the RA-117, that of dial/frequency calibration and that of aligning the various RF/IF filters. The first consideration to be made is that most of the alignment may well be just fine - even after extensive component replacement and rebuilding - if the primary resonance components in the various sections have not been altered.
There are, of course, two main aspects to frequency/dial calibration:
RF and IF filters:
|Aligning the 1st VFO|
The 1st VFO can be calibrated in place by pulling V4 and feeding a frequency counter from pin 1.
|Aligning the 2nd VFO|
While it is reasonably easy to use the crystal calibrator to set the 2nd VFO dial pointer at any 100KHz point along the film scale, part of the fun of the RA-117 is achieving a high degree of linearity across the entire dial from 0 to 1000 KHz. If one is lucky, this will not have been upset since the unit left the factory.
The graphs show the improvement achieved on three units. After the initial rebuild and reassembly of the units, and some realignment to just get the frequencies reasonably correct, the tracking was as shown with the red lines. Unit N0115 was not too bad except at the end points, with an overall deviation of less than 5 KHz. Unit N0714 had a gradual error over the range, but less than 3.5 KHz overall deviation. The blue lines show the results after some vane tweaking and repositioning of the rotor: much better in both units, in N0714 an overall deviation of about 1.5 KHz, so less than ± 1 KHz or within one scale mark on either side of target. Unit N0144 had quite of bit of error, but the error was quite linear. Minimal adjustment of the proportion of base capacitance brought the error down to less than ± 1 KHz. No vane tweaking was required.
In aligning the 2nd VFO there are three objectives to be achieved:
The variables that one has to play with to achieve the above objectives:
Some comments on technique:
|Aligning the Crystal Calibrator|
While doing an instrument recalibration of the calibrator it was found that peaking the coils with the instruments may not bring the unit to function. The coils may still be far enough off to prohibit the mixing loop from automatically starting.
A test lead on the plate or grid of the tubes to pull the frequency may set the unit into or out of operation. Fine adjustment of the coils may be necessary to find the operating area for the unit.
The manual does not describe exactly how to align C108 (balance/null on the 37.5 MHz amp).
During alignment of one unit, a slight whistle was noticed when the antenna attenuator was switched in.
Trimming C108 under these conditions reduced the whistle.
Racal RA-17 & RA-117