[Hallicrafters] SX-25/ SX-24/ S20R band 4 padding problem

[Carl]

That half turn loop is right out of National Radio. I used to be pretty 
good manipulating them when I was in the Service Dept.

Carl
KM1H


----- Original Message ----- 
From: "Jim Liles" <james.liles at comcast.net>
To: <hallicrafters at mailman.qth.net>
Sent: Friday, April 03, 2009 2:19 PM
Subject: [Hallicrafters] SX-25/ SX-24/ S20R band 4 padding problem


> Regarding the band 4 performance issues.  Search the Hal archives and 
> you
> will find a good deal of info that will help with the problem.  The
> Hallicrafters guy's added band 4 to tout the radio as VHF capable.  It 
> was
> an absolute after thought.  The Q of the coil is not the problem.  It 
> is
> primarily a result of total disregard for tracking.  If you measure 
> the
> various tanks from end to end of the band you will find that they will 
> swing
> from 455Kc above the LO and wind up well below --- (Who designed 
> that?!?!?)
> .  The second problem is the LO on band 4 is totally unstable.  It 
> results
> in a warble in a CW note and infinite mixer products.  If you put a 
> spectrum
> analyzer on it, rework is the only fix.  The tracking issue can be 
> solved by
> installing a shorted turn that can be manipulated and an extension of 
> a half
> turn at the connector so that you can decrease and increase 
> inductance.  You
> can place a grid dip probe next to the coils to verify adjustment ---  
> there
> is just enough room. The SX-25 is far in a way more vulnerable and 
> affected
> by the problem than the SX-24 because it uses 2 R.F. stages ---  
> exponential
> bad tracking.  I have an SX-25 that we modified to correct the 
> tracking
> using the shorted turn method but have not yet redesigned the LO. 
> Last
> thought --- The LO lives below the signal on band 4.  Kindest regards 
> Jim
> K9AXN
>
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