[Hammarlund] HQ-129-X Question

[Kenneth G. Gordon]

On 29 Nov 2010 at 15:12, Bob Macklin wrote:

> On any single band there are only 3 frequency determining components.
> The tuning capacitors which should not change with age. Especially the
> Hammarlund variables. The inductor which may change a little with time
> and the trimmer.

In many receivers (and transmitters) there are also parallel padding 
capacitors, usually fixed micas.

> The only one that has an effect on the linearity between the end
> points is the main variable.

Well, James Millen, among others, might argue with you about that. :-) 

I think linearity, with regard to the main-tuning cap, has more to do with the 
SHAPE of the variable capacitor plates, although the relationship between 
the main-tuning cap, the inductor, and the padding caps comes into play too.

According to a couple of articles I have read on the subject, one by Millen, 
another by Langford-Smith in "The Radiotron Designers Handbook", linearity 
in a capacitor-tuned circuit is VERY difficult to achieve as there are, quite 
simply, too many variables involved. It often isn't economically reasonable to 
go to the max.

> And I doubt it has changed a measurable
> amount.

You're absolutely right, of course.
 
> I am having similar problems with my NC-109. I have bands B and D very
> close. Close enough for a receiver with a string drive.
> 
> But I cannot get band C close to the manual. I can make it correct at 5MHz
> ( the bottom) and 7MHz but not at 14MHz (the top).

To my mind, since it is right on on bands B and D, and not on band C, then 
the tuning cap cannot be the problem. Therefore, it has to be something in 
the circuit unique to band C. 

Possibly a shorted turn in the inductor?

A fixed mica padding cap gone bad on ya?

Powdered iron in the inductor (if there is any) gone weird on ya?

All of which, although very rare, I have run into at least once in my almost 56 
years in electronics.

> I am going to try and get it close between 5MHz and 10MHz and ignore
> the portion above 10MHz.

Well, if you are anything like me, it will still keep bugging you, yeasting 
around in the back of your mind, sometimes for years, until you figure it out 
and get it fixed. ;-)

Ken W7EKB