[Hallicrafters] SX-28 selectivity fault?

[Jeff]

Thanks Richard,
I only did a rough peaking of the IF, and Ive since found that the wiring of 
T2 secondary is incorrect (some wires transposed), as compared to a junk set 
I also have.
I appreciate the advice re procedure, as the manual is a bit confusing as 
you mentioned.
regards from down-under,
JEFF
----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: "jeff" <vortex6066 at yahoo.com.au>; <hallicrafters at mailman.qth.net>
Sent: Tuesday, December 14, 2010 8:55 AM
Subject: Re: [Hallicrafters] SX-28 selectivity fault?


>
> ----- 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