[Hallicrafters] SX-100 MarkII

Wed Jan 2 13:52:00 EST 2013

Good morning David:

There is a very good reason for the change to 50.75Kc.  All of the SX-100 
series should be changed to 50.75Kc and the circuit changes that accompany 
the I.F. frequency change should be included.

The difference between the Mark1A and Mark2 series is the Mark2 behaves as 
though it has a product detector.  You can run the RF gain at 10 or close to 
it without distortion with CW and SSB --- the Mark1A not a chance.

The changes that were made include the following:

1.  The BFO tap was moved from the grid to plate side of the BFO tuned 
circuit increasing the BFO drive.

2.  The AM/CW,SSB switch was changed to a two pole switch adding capacity 
which significantly extending the AVC hold time in CW/SSB mode.

3.  The AVC bias was changed from +4.5 volts to 0 volts enabling the AVC to 
build at a substantially lower signal level.

4.The I.F. center was moved from 50.5Kc to 50.75Kc.  This was required for a 
couple of reasons.  Before the frequency change, (Mark1A) when you turned 
the BFO on, the S-Meter would move upward significantly showing signal.  The 
amount of rise was contingent on how well you aligned the 50.5Kc I.F. 
system.  What actually was happening is the BFO was doing a reverse track 
through V7 --- T6 --- T5 --- then back to the AVC detector.  To mitigate the 
problem they aligned the 50.5K I.F. system to 50.75Kc thus filtering the 
carrier/ BFO signal from reverse track.  This was an absolute necessity as 
the BFO signal was increased by moving the tap point as in note 1 above and 
the AVC was set to occur at a substantially lower level note 3 above.

The performance difference between the Mark1A and Mark 2 is remarkable to 
say the least.  It is a truly impressive instrument ------  Just don’t shake 
the table.

Here is a copy of Bob Krassa's AC0JL recent post regarding the changes.  He 
is currently working with a Mark2A

--------------------------------------------------Bob Krassa 
AC0JL ----------------------------------------------------

I am about to work on a 2A.  Was having a lot of trouble reading the 
schematics available either on the web or from manual copy sellers, so I 
obtained an original 1A schematic which I enlarged by 200 % and then 
compared carefully with the semi-legible 2A schematic that is available on 
the web.   The differences that I found were:

Changes from Mark 1A to Mark 2:

C93 BFO injection moved from 6SC7 pin 3 grid to pin 2 plate
C111 100mmf added across R64 on V8 audio amp grid resistor
C112 0.5 mfd added on AVC line to CW/SSB switch
C113 0.047 at V13 typo corrected was mislabelled as C101 on  previous 
schematics, no change in circuit.
C114 0.01mfd 1400V added on AC plug
CW/SSb switch changed from single to double pole
R46 and R47 deleted and V7 AVC cathode now grounded (R46 and R47 were a 
voltage divider so the cathode would  be positive 4.5 volts

Further Changes from Mark 2 to Mark 2A:

IF changed from 50.5 to 50.75 kc
Calibrator changed to plug-in type and
CR1 1N295 diode added
R77 changed to 220K

I have several extra prints of the 1A schematic that I will provide for $20 
each  (shipping included) anywhere in CONUS, AK or Hawaii.  Contact me off 
group if you want one.  I can also provide copies of my revised but legible 
2A schematic for the same price, but of course no accuracy guarantees and it 
may take a week to get additional copies made!  Size of the prints is about 
46 inches by 20 inches.

73s Bob Krassa AC0JL

----------------------------------------------------------------------------------------------------------------

Kindest regards Jim K9AXN

-----Original Message----- 
From: David C. Hallam
Sent: Wednesday, January 02, 2013 10:16 AM
To: Hallicrafters at mailman.qth.net
Subject: [Hallicrafters] SX-100 MarkII

I am starting to refurbish a SX-100 Mark II.  Since there are no
significant circuit changes between the SX-100 Mark II and the Mark IIA,
is there any reason why the IF of the Mark II can not, or should not, be
retuned from 50.50KC to 50.75KC of the Mark 2A.

Some place I seem to remember reading the reason for the change. Can
anyone tell me why?

David
KW4DH

-- 
A democratic government is the only one in which those who vote for a tax 
can escape the obligation to pay it.
Alexis de Tocqueville

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