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

[Barry]

I've been working on an HP-606A for a while and have come up on something I 
don't know how to correct.  The linearity is not good.  The specs say this 
thing should be within 1%, but I'm not getting that.

I could get the endpoints on target, but as the frequency went from lowest 
to highest, there was a gradual increase in the output frequency, peaking at 
a little over 4% at about 3/4 of the way up the dial and then dropping off 
rather sharply as it approached the upper endpoiint.

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.

The linearity is better now (about 3% max.), but it's still on the high 
side.  Again, I assume this indicates eiher not enough C or not enough L at 
these positions.

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.

The trimmer cap for the band I'm working on (530kc to 1800kc) is close to 
mid point (about halfway meshed) so that's a good thing.  I just don't know 
what to do to make this thing "linear" again.

I tried changing the position of the dial for this band by resetting the 
endpoints up or down 50kc.  In other words, 530kc to 1800kc became 500kc to 
1730kc.  This way, 1000kc was really 930kc, etc., but the problem didn't 
improve.  I tried going the other way (580kc to 1850kc) with similar 
results.  I was told there is a "sweet spot" where the dial plate could be 
repositioned and it might correct this problem but rather than move the dial 
physically, moving the endpoints like this is effectively the same thing.

I had thought about bending the fins on both ends of the capacitors, thus 
reducing the capacitance at both end points and effectively increasing the 
capacitance near the middle but this seems a bit of a far-out method.  Are 
there other things that might effect this?  Maybe someone on the list has 
had a similar problem with a 606A and could give me some Pearls of Wisdom?

Thanks guys,

Barry - N4BUQ