[Test-Equipment] Re: HP606A linearity

[Richard Knoppow]

----- Original Message ----- 
From: "Mark Shaum" <k9tr at dtnspeed.net>
To: <test-equipment at mailman.qth.net>
Sent: Saturday, June 21, 2008 7:52 AM
Subject: [Test-Equipment] Re: HP606A linearity


> >
>>    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

     I am skeptical about plate bending on multiple band 
devices like the 606. My problem is that even though I must 
have worked on dozens of them for -hp- I don't remember if 
this was done at the factory. The usual technique with other 
generators was slipping the dial to get the tuning range on 
the right part of the capacitor. I straightened the bent 
plates on this one as best I could and tried the dial 
slipping technique. So far its brought the calibration much 
closer. This is work in progress so I don't know yet whether 
this will fix the calibration.
     I suggest you try this trick on your generator. The 
dial is held by a clamp which is accessible by removing the 
main tuning knob. The direction to move the dial depends on 
whether the mid-frequencies are too low or too high.
     Nearly all large variable air capacitors have slots cut 
in the outer plates. Its very tempting to bend these plates 
but I know for a fact that doing so will get you in trouble 
for some equipment, for instance SP-600 receivers.
     The high quality modulation of the 606 is one of its 
great virtues along with constant output. Mine is about 
perfect other than the calibration problem.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com