[Hallicrafters] Caps on Selectivity switch was: RE: SX-100 Knobs

[David C. Hallam]

Chris,

I was really considering not doing anything with the capacitors in the 
selectivity control considering the probable difficulty of getting to 
them.  I replaced the other capacitors and off spec resistors in the 
radio and started the alignment procedure.  The 50K Hz IF alignment went 
without any problem.  The 1650 KHz IF alignment was another matter.  It 
seemed that no matter what I did, I could not get the 1650 KHz signal 
centered in the pass band.  I had checked the 1600 KHz oscillator and it 
seemed to be right on spec. I knew some of the resistors in the control 
were out of tolerance because I had measured them.   At that point I 
decided there was nothing to do but to dive in.  I tested all the 
capacitors as I took them out and all tested good for capacitance but 
given the unknown accuracy of my old Sprague TO-4 tester, I can't say 
anything about their exact capacitance.   I didn't check insulation 
resistance.

The alignment going to be proceeding slowly from here.  My work bench is 
in the garage and the weather here in Clearwater has been miserably 
hot.  We have been getting about an inch of rain every day along with 90 
degree temperatures.  My test gear is all HP boat anchor stuff so with 
the SX-100, signal generator, frequency counter, VTVM, and sometimes the 
spectrum analyzer all running, it just gets too hot to work.  If it 
weren't for the big fan I have running, I couldn't stand it at all.  I 
thought about bring everything into the house.  First of all, the 
thought of moving all those boat anchors into the house was more lifting 
and toting than I wanted to do.  More importantly my wife wouldn't have 
any part of having all of this set up across a kitchen counter.

73
David
KW4DH

On 7/9/2013 2:47 AM, Chris Kepus wrote:
> David,
>
> Thanks for the rundown on your component selections and attack on the
> switch.  I had a gut feeling you figured out a way to get it done. Way to
> go!  And thanks for the blow by blow description. After reading it, I
> believe an appropriate characterization of your action is "intrepid".
>
> I got a couple of replies off list suggesting that anyone considering this
> job might well first plot the current selectivity curves.  Their point is
> that these caps are not exposed to B+ and even if there is a slight bit of
> leakage, it might not have that much effect on the radio's selectivity.
> And, if the radio is just going to be used for casual listening or on "Net"
> night, that makes good sense.  OTOH, the Hakko could convert a daunting job
> into a difficult but doable proposition.
>
> Yes, the Hakko makes this type of work so much more pleasurable.  Not to
> mention getting it done faster, too.  Don't know how I got by without one...
> :-)
>
> Let us all know the final results of your efforts.  I am curious if the new
> components have any effect on the radio's selectivity curves, sensitivity,
> etc.
>
> Good job!
>
> 73,
> Chris
> W7JPG
>
>
>

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