[Hallicrafters] SX-42 alignment - msg. to Al Parker

[k6uuz at juno.com]

Hi Skip et al;

I guess that you are ready to grasp at straws. Let me offer a few. I
hesitated to jump in sooner as there are those smarter than me on the
reflector and what I have to offer is pretty basic, but then, that is
what gets us sometimes. I have aligned hundreds of radios and in so doing
have come across some pretty weird things. Just recently I had one that
would not calibrate, similar to your situation. The low end would come in
but not the high end. What tipped me off was that 600 kc came in two
places on the low end. Checking it physically found that the tuning cap
was off about 30 degrees. so when tuning down it would close then start
to open again. Setting it correctly corrected the problem and it aligned
beautify.  Another case was where the kit builder set the tuning cap 180
degrees off and crossed the dial cord to compensate. when applying a 600
kc signal it appeared to come in at 600 kc as shown on the dial but was
really the second harmonic of 1200 kc, as the tuning cap was nearly open.
Setting it correctly cured that one too. If you have not already, check
for physical problems. i.e. Tuning cap fully closed for the low end and
opens correctly as you tune up. Some tuning caps are not symmetrical;
that is the plates are not a true 1/2 circle but are cam shaped to try to
make the dial more linear. Normally the low frequencies are stretched out
and the high frequencies are crowded together. Most caps open from the
bottom. Check that the dial cord is installed correctly. Check that the
oscillator is on the correct side of the carrier. Probably higher than
the carrier. You should find the carrier at two places on the dial; 2 X
the I.F. frequency apart. For a 455 kc I.F. this is 910 kc. If you have
high side injection, the lower one is correct. If low side injection, the
upper one is correct. Select the correct one and tune the antenna, RF and
mixer adjustments to peak on this one. The other one (the image) should
be much weaker. All tuning adjustments should be near their center of
range, that is in the middle 1/3 rd. The fact that your high end
adjustments have small effect makes it suspicious that you have excess
capacity, like the tuning cap nearly closed when it should be open. Good
luck.

Ed Richards K6UUZ

On Sun, 27 Jul 2003 17:00:34 -0700 Waldo Magnuson <magnuson at mac.com>
writes:
> Hi Al,
>     I'm rapidly getting totally frustrated with the SX-42 alignment.
>     I first replaced C48 with a new silver mica of the same value 
> (470 
> pf).  No changes to speak of.  Both the old and new caps measured 
> about 
> the same value (I have a PACO and a Heathkit cap. tester and check 
> one 
> against the other).  I couldn't measure any leakage.
>     I next substituted a 330 pf in place for C48.  I could still not 
> 
> tune to 1500 kc., 600 kc still tunes with the slug about 1 turn in.  
> 
> With the dial set at 1500 kc it takes an osc. setting of from 1200 
> to 
> 1380 kc to get any modulation output at the speaker terminals.  It's 
> 
> like C48 and C47 are being swamped by some other capacitance.  I 
> monitor osc. freq. with a HP freq. meter so I am confident of the 
> frequencies.
>     I removed and inspected the slug of S-36 and it looks perfect.
>     However someone (in the past) has replaced the cathode bypass 
> cap. 
> C38 with a 510 pf instead of a .01 uf.  It's tied to pin 4 of 7F8 
> mixer.  I don't see how this could change the tuning of T23 but I'll 
> 
> change it - very tough to get at.
>     Al, I really appreciate your thinking about my 42 and the help.  
> 
> Thanks.
> Skip Magnuson
> 
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF **for assistance**
> dfischer at usol.com
> ----
> Hallicrafters Collectors International: http://www.w9wze.org
> ----
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> 


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