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

[Jim Liles]

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