[Hallicrafters] SX-28 Dial Tracking

[Rodger]

Joe,

As a starting point, make sure that the main tuning and bandspread 
capacitors are properly indexed to their respective dials so that 
maximum capacitance occurs at the correct dial setting (it should be 
noted in the manual).  Then make sure that you have the bandspread dial 
set for the "set" (usually minimum capacitance) point before beginning 
calibration.  Check to see if the end plates on the tuning caps have 
been severely "knifed".  Most oscillator sections will have 1 or both 
outer plates segmented so that they can be bent slightly to improve 
tracking but the bends should be minimal and if the plates show signs of 
being severely adjusted I would bend them back close to flat for the 
initial setup.  Also make sure that the rotor plates are centered 
between the stator plates, if not you can do this by adjusting the 
thrust bearing on the end of the capacitor.

Then I strongly suggest using an external receiver to listen to the HFO 
signal of the SX-28 as you are tracking the oscillator.  One of the 
small shortwave sets with memories is excellent for this purpose and I 
use a Grundig YB-400 as my bench test receiver.  The HFO should 
oscillate at a frequency equal to the dial frequency plus the IF 
frequency (around 455-465 Khz. for the SX-28 depending upon the exact 
frequency of the IF crystal filter).  I would have to check the manual 
but they may use low side tracking on the highest frequency range (the 
oscillator is equal to the dial frequency minus the IF frequency).  By 
listening for the HFO signal in an external receiver you won't run into 
the problem of aligning on an image and it is faster to switch memories 
in an external receiver than to crank a generator back and forth in 
frequency.

I have found that for receivers that have received the "magic 
screwdriver" treatment a good starting point is to set the trimmer cap 
to midrange and then adjust the coil so that calibration is correct 
somewhere near the center of the frequency range.  If the tracking is 
way off due to previous owner's magic screwdriver then I have found that 
it is faster to get the unit tracking properly using the center point 
plus and minus 20% as your initial setup.  For example if the frequency 
range is from 5 to 15 megahertz then use 10 as the center and adjust the 
inductor at 8 megahertz (-20% of center) and the capacitor at 12 Mhz. 
(+20 % of center).  There is nothing set in stone about these magic 
numbers but if the tracking is really poorly adjusted it is much faster 
to get it set close using tuning points that are fairly close together 
and then fine tune it for perfection using the points called for in the 
alignment instructions.

Sorry for an answer that is probably too long, I just finished my 
lecture for tomorrow and it seems I am still in "college prof" mode.

73, Rodger WQ9E

Joe Connor wrote:
> I need some help. I picked up an SX-28 parts unit. It's too nice to part out so I'm going to restore it. The S-meter is missing but I'll worry about that later. I replaced the 'lytics and it's working on all bands. BCB (Band 1) is outstanding. The other bands are not so hot. I'm starting the slow process of replacing all paper caps and out-of-spec resistors. 
>
> My initial problem is dial tracking. I get the sense a prior owner had a magic screwdriver. We all know what that means.
>
> On Band 3, CHU comes in where it is supposed to but WWV comes in at 5.8 mHz. I can't adjust the trimmers to bring it to where it is supposed to be.
>
> On Band 4, WWV comes in at 10.5. mHz. Again, i can't adjust the trimmers to bring it down.
>
> Any ideas? It plays so well on BCB that I suspect it will be a great set once I get the RF alignment right. 
>
>
> Joe Connor
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