[R-390] OT: Problem with HP-606A linearity

[2002tii]

Barry wrote:

>Someone bent the fins outward on the tuning capaitor in an apparent attempt
>to correct the linearity (or, perhaps more accurately, alignment with the
>dial markings); however, from what I could tell, this was the wrong thing to
>do.  Since the frequency was too high at the "bad" points, then according to
>the LC formula, there isn't enough C (or perhaps not enough L?) at that
>position.  At any rate, decreasing C didn't seem right and I straightened
>the fins.
>
>      *     *     *
>
>Looking at the capacitor, it appears it is a custom-made job that was
>designed for a particular capacitance "curve".  The edges of the rotors are
>not round but have flats along the way.  I presume this is to "shape" the
>capacitor to agree with "curve" so as to align with the dial markings (I
>hope that made sense).
>
>At any rate, it appears there isn't enough C at or near the midpoint of the
>rotation so the output frequency doesn't match the dial all along its
>travel.
>
>Is there a common-knowledge way to get around this?  While I can always
>ignore the dial markings and use the frequency counter, I'd like it to be
>within spec if possible.

You are correct, the shape of the cap fins was selected to give 
approximate linearity.  Typically, the outer fins are segmented 
radially so they can be adjusted segment by segment for closer 
tracking.  Reducing the spacing increases the capacitance and 
vice-versa.  The first thing I would do is what you did -- straighten 
the outward-bent fins.  It sounds to me as if the variable cap is not 
adding capacitance fast enough as it enters mesh from the high end of 
the band, so try increasing this either by (i) bending the outer 
plates inward starting at the end where they start to mesh, or (ii) 
if you have rotation to spare, change the mechanics to give slightly 
more mesh when the dial is at the top limit.  In either case, you 
would then adjust the inductor and trimmer cap.  Remember to adjust 
the inductor for the low end, and the trimmer cap for the high end.

Do you have a manual for the 606?  I don't, but some other HP 
oscillator manuals give detailed instructions for linearizing the 
dials, so I'd check the 606 manual.

Best regards,

Don