[Test-Equipment] Re: HP606A linearity

[Mark Shaum]

>
>    I recently obtained one of these guys at a local sale. I
> worked for -hp- for many years and must have repaired dozens
> of them but its been perhaps 40 years since I've worked on
> one and I can't remember certain things.
>    This one has had plate bending done to the tuning
> capacitor. In particular the plates at the high frequency
> end have been bent to lower the capacitance. I can't
> remember if this was a normal factory method. Usually plate
> bending on any multiple band device is a no-no and I suspect
> the plates should be straight on this one. Does anyone know
> for certain?

Bending the plates of the tuning capacitor (knifing it is usually/was 
called) is pretty common to reach the desired linearity.  Usually certain 
rotor plates are slotted to allow bending the rotor "tabs" over various 
ranges of the capacitor travel.

My own 606A has a linearity issue, in that I can align the end points but 
the middle of every frequency range reads high.  I didn't spend much time 
investigating, thinking there may be a mechanical issue with the gearing but 
it's possible knifing the tuning cap will be necessary to bring the unit 
back into specs.  Not a big deal as I used a frequency counter to display 
the output frequency anyway.

BTW as it is capable of 100% modulation the 606A and B make fine low power 
AM exciters on 160 and 75 meters.  There is a bit of carrier frequency 
"warble" with modulation present on the 606A that gets worse with increased 
frequency.  This was addressed by the added buffer stage in the 606B.

"Rig here OM is a HP606A driving a SB-220; to about 15 carrier watts output"

Mark K9TR