Help with Ra117 VFO Calibration

[pnh0001]

I have a Racal RA117 I have been doing up. It's finally all going, but the 2nd VFO (Kc/s) calibration is a bit off.

The VFO tunes 4.6 Mc/s (0 kc/s) to 3.6 Mc/s (1000 kc/s). It's fine at the end points after following the procedure in the manual. In between the calibration follows a sort of sine curve. If you plot error in kc/s versus expected frequency ( 3.6 3.7 3.8...) the VFO is 4 kc/s low at about 4.3 Mc/s and then becomes 22 kc/s (!) high at 3.8 Mc/s.

I have read somewhere you can 'knife' the variable capacitor vanes to correct this problem.

Has anyone done this and can you tell me the best procedure, or is it simply remove cover, bend vanes a bit (guided by my graph for direction), replace cover,switch on, measure and repeat as necessary.

If anyone would like to see what I have been up to with this receiver, I have documented it at http://ra117.blogspot.com. Basically I have replaced just about every component in a complete rebuild - down to bare chassis.

Thanks for any assistance

Peter

[MichaelR]

Hi ,

I would check to see if by "manhandling" the capacitor you have pushed the outer vanes towards the static vanes. If you have, slightly bend them back to what looks an even gap. I would then replot your curve and see what difference it has made. It may take some time but I would be very careful and considered about any adjustments you make, one step at a time until you get the less than 1khz errors.

If there is a locknut and adjuster on the end of the rotor shaft this can be used to adjust the position of the rotor in the static vanes but again be very careful making any changes here and remeasure after every slight adjustment to ensure you can be sure that the whole error plot is going the right way.

I do not know of any other procedure but there may be bigger and better people here who do know

Mike

[MichaelR]

Nice bit of work by the way on the rebuild