[Heathkit] HW 100 CXR NULL

[Bob McGraw - K4TAX]

The difference in the "sound" between LSB and USB is typically from two 
sources: (a) the BFO oscillator frequency is not positioned equally when 
switching between LSB and USB on the slope of the SSB filter.  (b) the 
response or ripple across the filter is not the same from the low side to 
the high side as compared to the high side to the low side in viewing the 
shape of the bandpass.

Typically there is some ripple across the top or bandpass section as 
measured from the center freq to the 3 dB points on the slope of the filter. 
Inexpensive or poorly designed and sometimes defective filters are typically 
worse in this regard.  Very good filters have a uniform shape and a very 
smooth top as well as steep skirts and little to no ripple across the base 
band.

The carrier null typically is adjusted for the best value when switching 
between LSB and USB.  A compromise between the two is usually a must.

As to the pot, make sure the little metal case is grounded, clean the pot, 
exercise is a few times.  As to what is best to clean it with, your "witches 
brew" is about as good as any.  Everybody has their favorite. Mine is "what 
works best for me on that specific pot".

That 0.5V P-P value may be more noise and very little carrier.  A selective 
voltmeter would be a nice item to have just to make sure while making this 
adjustment.

73
Bob, K4TAX
Creating the glow in the sky from LOTSARF


----- Original Message ----- 
From: "Dick KF4NS" <kf4nsradio at verizon.net>
To: <HEATH at LISTSERV.TEMPE.GOV>; <heathkit at mailman.qth.net>
Sent: Thursday, July 12, 2007 5:56 PM
Subject: [Heathkit] HW 100 CXR NULL


> Just finished a couple mods and removing a couple, so decided to realign 
> the rig before I box it up. When I got to the CXR NULL, I tried to do it 
> with my VTVM but with my old eyes I found that I could see things better 
> with the scope.
>
> On the LSB null capacitor, I am reading 0.5V P-P and it was easy to 
> adjust. When I got to the USB null pot the trouble started. I was only 
> able to get it down to 1.4V P-P and that was after lots of frustration due 
> to errratic readings. I have heard about the nightmares concerning that 
> pot but never had a problem since I built the rig in '70. I cleaned it 
> good with contact cleaner and then added some D5 but it still is difficult 
> to adjust and the lowest reading did not change. Heath gave no indication 
> of what to consider a satisfactory reading other than to look for a 
> comparable reading for USB/LSB.
>
> Any ideas or comments?
>
> By the way, the most recent mod really did inprove the audio. I did the 
> balance of the BFO that Mark WBYJKR SK, recommended. Now there is no 
> difference in the sound between USB and LSB and the repositioning of the 
> filter center frequency makes the sideband signals sound so much better. 
> Sure do miss having Mark around since he had a world of knowledge about 
> the HW/SB rigs. Hope he is up there teaching some angels how to tweak 
> their old heathkits.
> 73, Dick KF4NS
> St Petersburg, FL 33714 USA
> Keep The Glow!
> _______________________________________________
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