[Hallicrafters] SX-100 MarkII

[David C. Hallam]

Jim,

I have a Mark2 so all of the changes outlined for Mark1 are incorporated 
in mine.  My manual says to align the IF at 50.50KC. My original concern 
was "Are there any frequency determining components in the IF circuit 
that need to be changed to retune to 50.75KC?"  I gather the answer is 
NO, just retune the IF transformers.

David
KW4DH

On 1/2/2013 1:52 PM, hallicrafterssr2000 at k9axn.com wrote:
> 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
>

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