[Hallicrafters] SX-28A Alignment

[Bill Feldmann]

Tom,

On both my SX-28's the crystals were lower than 455kc.  One was about 
451.5kc and the other around 452.8kc.  With time they most likely age or 
were not right on frequency to start with.

These were the SX-28's in my article in the Feb issue of ER.  However first 
do a rough alignment at 455kc so you have enough IF gain to find the peak if 
the radio is way out of alignment like after a major recapping or rebuild.

Both of mine aligned fine when the final alignment was done to the crystal 
frequency.  Just be sure you are finding the frequency in the crystal sharp 
selectivity.  Not being able to find a peak in this selectivity most likely 
means you have a bad crystal or shorted phasing cap from dust and dirt or 
bent plates.  Also a contaminated crystal from dirt or oil will have a poor 
peak.  I have had good luck pulling the crystal holder, disassembling it, 
and cleaning the crystal and shoes in the holder in alcohol followed by 
detergent in water and rinsing.  I saved crystals in a 75A1 and two BC-348's 
that way.

After proper alignment the bandpass should be around 4kc at + or - 6db down 
in sharp non-crystal selectivity.  6 to 8 kc in mid and 12 to 15 kc in wide. 
There should be a flat bandpass in wide with around 3db peaks on each end. 
If the peaks are not equal and the bandpass is not flat you can flatten it 
out or even the peaks by reducing the higher peak using the last IF 
transformer with a little stagger tuning.  I had to do this on one of mine.

In crystal wide selectivity you should be able to null carriers a little off 
the center IF frequency using the phasing control.  You should have a good 
peak in mid crystal and a very sharp peak in sharp crystal.

If the alignment is to the crystal and a little off 455kc, + or - 5kc, the 
radio will work just fine and never know the difference after the master 
oscillator and RF are aligned properly.  Just follow the alignment chart in 
your manual for the RF.

Bill N6PY

----- Original Message ----- 
From: "Dream Lobo" <DREAMLOBO at MSN.COM>
To: "hallicrafters" <hallicrafters at mailman.qth.net>
Sent: Monday, March 07, 2005 9:18 AM
Subject: [Hallicrafters] SX-28A Alignment


> Greetings
> I am currently restoring two SX-28A's and I situation came up with the 
> first one which is finished minus the alignment. The IF frequency of 455 
> khz is what the manual says to set the signal generator at when beginning 
> the alignment which I did, but, the receiver seems to actually want a 
> frequency quite a bit removed from that for max gain. I have seen this 
> before in other receivers but the difference was slight so I used the 
> slightly different frequency as the one I set the signal generator on. 
> Only a few hundred cycles or so. In the case of this SX-28A, max gain 
> seems to be around the frequency 451 khz which is a much larger difference 
> that I have encountered before. I did the first part of the alignment 
> using frequency 455 khz but I am thinking that the gain doesn't seem to be 
> what it should be. The question is, have any of you encountered the same 
> type of situation and if so, what did you do about it. I am think that 
> maybe I should use the frequency that seems to be the actual frequency 
> that the crystal filter wants to see. Any thoughts on this matter would be 
> most welcome. Also, I have need of a band switching knob for an HT-37. If 
> anybody has one they can part with please let me know.
>
> Regards & 73
> Ae8o
> Tom
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