[Hallicrafters] SX-28 selectivity fault?

[Richard Knoppow]

----- Original Message ----- 
From: "jeff" <vortex6066 at yahoo.com.au>
To: <hallicrafters at mailman.qth.net>
Sent: Monday, December 13, 2010 11:09 AM
Subject: [Hallicrafters] SX-28 selectivity fault?


> Hi,
> Im in the process of fixing the faults in an SX-28 
> receiver that has been re-built by the previous owner.
> One last thing Im not sure about is the operation of the 
> selectivity switch. As Ive never owned an SX-28 before, I 
> need some advice as to whether the selectivity switch is 
> working normally or is faulty.
>
> On 'broad IF' receiving a strong medium wave station, the 
> meter indicates over S9. When the switch is then set to IF 
> Medium, the S reading drops about 3 S points. When then 
> switched to IF Sharp, drops another 2 points. The audio 
> sounds like the bandwidth is reducing as expected, but am 
> unsure about the drops in signal strength reading. 
> Probably faulty, but thought Id confirm.
> Any advice appreciated,
> regards,
> JEFF
> ZL3JK
> New Zealand.

    How are you going about aligning the IF?  The IF 
alignment procedure in the Hallicrafters handbooks can be a 
bit confusing. The crystal filter used in these RX is an 
early type which varies the frequency with bandwidth. Set it 
up according to the book and leave the signal generator at 
the frequency of the crystal peak.  Then peak the IF cans 
with the IF bandwidth in the minimum non-crystal position. 
You may have to go over this a couple of times. The If is 
slightly under coupled in minimum bandwidth, critically 
coupled in medium bandwidth, and over coupled in maximum 
bandwidth. According to the curves in the handbook the 
center frequency stays about constant. The gain of the 
transformers will be highest in the medium bandwidth 
position and slightly lower in the narrower and wider 
positions, but not by very much. I would tune all this using 
a modulated signal with the AVC off and peaking according to 
an meter on the audio output. Adjust the generator for a 
minimum signal that just overcomes the noise so that the RF 
gain can be set as close to maximum as possible. The RF 
stages should be set up the same way. The AVC can slightly 
detune the stages it controls. While this is not usually 
significant for the IF stages you still want them to be 
close to optimum for weak signals so its good practice to 
use a minimum signal to dive them.
    Again, when properly adjusted the carrier level will be 
highest in the medium bandwidth position and very slightly 
lower in the narrower and wider ones, but not by much. Check 
the symmetry of the pass band after aligning. If you have 
access to a sweep generator it will make life a bit easier.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com