[Hallicrafters] SX-100 Mark 1A

[Jim Liles]

Hi Roy:
Your friend has acquired a fine radio.  Regarding the 50KC sub-chassis, 
there are no paper caps on that sub-chassis and you can measure all 
resistance values from the tube and crystal sockets --- no particular reason 
to remove it unless it is failing.  A recommendation for the Mark 1A is to 
bring it to Mark 2A level.  There are only 6 changes as I recall which make 
the radio sound and operate on sideband like a product detector has been 
installed.  You can run the RF gain full out and not have to deal with 
distortion.  A couple of the alterations are, the BFO is tapped on the plate 
side rather than the grid side of the BFO oscillator.  Next, they changed 
the AM/CW switch from a single pole to double pole in order to implement 
slow AGC on CW/SSB which enables the RF to be left on 10 for SSB.  Next, 
This is a good one, align the 50KC IF system at 50.75KC not 50.50KC.  This 
accomplishes two things; It reduces the presence of the unwanted sideband 
and reduces the reverse feedthru of the BFO oscillator from the detector/V7 
to T6 to T5 to the AVC detector --- this reduces the affect the BFO has on 
the S-Meter.  There are a few resistor changes but I'll need to review the 
schematics as they escape me --- too early in the morning.  Kindest regards 
Jim K9AXN

Original message:

> Halli Folks,
>
> A friend has gotten from his uncle an SX-100 Mark 1A.  I've seen them 
> discussed
> on the list with high praise.
>
> Most likely I'll get to replace caps and do an alignment on the thing.  I 
> will
> want to do a good job since this is both nostalgic for my friend (he 
> lusted
> over them in catalogs) and from a family member who was the original 
> owner.
>
> I can see from the BAMA manual that this is a fine receiver, in some ways 
> like
> the SX-101A.  I also see that it has a 50KC IF module that's bolted to the
> chassis with some 10-12 leads to manage.  (I just finished an SX-88 and 
> had to
> remove/replace the IF module.)
>
> Any tips or gotcha's on this receiver?
>
> Roy,
> K1LKY
>
> Roy Morgan
> 13033 Downey Mill Road
> Lovettsville VA 20180