[Hallicrafters] SX-117 article

[James Liles]

Good morning Paul:



Here are the notes that you requested.  Hope they generate some friendly conversation.





SX-117 NOTES



The following observations have been accumulated over a number of years.  This information is from grey matter and when I find my original notes I’ll add and fine tune the content.



Ray Osterwald --- editor ER magazine wrote a super article in the Feb, 2007 issue.  It was a superb work covering the bases regarding the SX-117.  If you Read it first, it will help to connect the dots regarding the following EC levels of the radio.



This will help you to determine the logic level of the SX-117 without turning it over.  Turn it on and find an S9 or so constant carrier.  Tune back and forth watching the S-Meter.  If you tune through the carrier to the point that you cannot hear it but the S-Meter indicates signal, you have at least a “C” level radio.  Now look at the top of the VFO variable capacitor.  If you find a round Ceramic NPO variable capacitor, you have at least the “D” level radio.  The “E” level radio was a change to enhance its performance when used with the HT-44.  To determine whether it is level “E” you will need to turn the radio over and check to see if R7 on V2A pin 7 is wired to ground.  If it is not, It is level “E”, the most current.  I do not know the difference between level “A” and “B” and probably do not care.  What I do know is that part of the change from “B” to “C” was difficult for me to accept.





The SX-117 is a fine instrument that evolved through five iterations “A” through “E”.  The evolution of five run levels would say there were a number of design inadequacies and the “E” level should be the pinnacle.  This is not necessarily so, it depends on what you expect from it.  Do you expect it to perform flawlessly in all modes: AM; SSB; CW?  Or can you accept great SSB and CW mode with mediocre AM?  If you prefer good AM operations, you will have to make some compromises.  The “B” to ”C” upgrade enhanced SSB and CW while degrading AM operations.  If your favorite mode is AM, consider partial regression to the “B” level.  



THE “B” TO “C” upgrade

First, they reduced the 5Kc bandwidth to 4.2Kc, almost unacceptable for AM ops.  This was done by changing R85 and R87 from 470 ohms to 390 ohms and removing C65.  You will see C65 on the “B” level schematic.



Second, they moved the AVC sense from the 2nd 50Kc i.f. amp to the 1st 50Kc i.f. amp.  This change resulted in some strange behavior.  The Notch filter seems to have little if any effect, and the S-Meter will indicate things are there that you cannot hear.  



The Notch filter:  The reason for the Notch filter having little effect on the S-Meter is because it is placed between the 1st and 2nd i.f. amps ---- after the AVC sense!!!  It is working fine, note the audio effect.  I prefer the Notch before the AVC sense so the system is not de-sensed with a notched out signal.  They may have done this to prevent a signal that you wish to notch out from overloading the system.  I have operated the “B” level and not seen this overload effect.  



The S-Meter:  The S-Meter behavior is caused by the move of the AVC sense from the output of the 2nd i.f. stage to the 1st i.f. stage.  The bandwidth at this point is substantially wider than the output of the 2nd i.f. stage.  Now, the AVC/S-Meter see a much wider span of the band pass and respond to signals that will not be present after being filtered/narrowed by the 2nd i.f. stage.



This change required the bias on the AVC detector to be reduced.  You will find a 6.8M resistor, R63 on the cathode bias of V10b.  The “B” level used a 1M resistor.  Ray increased this resistor even further to engage the AVC at a lower level.  This was a good change.  If you regress the AVC sense to the output of the 2nd i.f. amp, you will have to change this resistor to the original 1M ohm.  My preference would to go higher on R63 as Ray did to reduce the bias to about 1.5 to 2 volts.  



AVC timer:  They also changed C107 from .47uf to .22uf.  I would go back to .47      

           















NOT CHANGE RELATED:



The Noise limiter:  The noise limiter in the SX-117 and SX-115 are almost as effective as most modern noise blankers,  --- if it has not been injured.  BTW, the HD6225 diodes can be replaced with 1N459A diodes.  There is no cross reference for the HD6225 but the 1N459A tests to be right on and works well.  This noise limiter is able to mitigate a neighbors security system that uses spread spectrum sweeping from 7.1Mc to 8.3Mc.  Turn it on and it’s gone.



Capacitors:



You will find what look like paper caps on the band pass filter switch, that is, if someone hasn’t changed them already.  THEY ARE NOT PAPER regardless of the parts description.  They never were at any production level.  They are General Instruments Polyester film caps and will be as good today as they were 60 years ago.  If you find Ceramic disc caps on the band switch, remove them and replace them with Polyethylene or Polypropylene film caps.  



There are no paper caps in the SX-117 or HT-44 --- do not change them to Ceramic --- this is a mistake.  If you need reasons to substantiate this note and are willing to review research papers, and test procedures that can be done in any Ham shack for confirmation, look at www.k9axn.com service notes 22, 23, and 24.  If this does not convince you, send a note as to why.  You will find me very courteous and open minded.  These things are too lengthy to add here.  The Hallicrafters and other vendor’s engineers had very definite and specific reasons for using Film rather than Ceramic capacitors in these circuits.  I believe these notes about capacitors will stir a good deal of conversation but read the service notes and research papers before deciding how you will respond. 



One other cap note:  The same goes specifically for the four .01 caps that are in the cathode circuit of the balanced modulator in the HT-44 and any other paper looking caps.



The paper looking General Instruments caps in these radios were selected for a valid reason!  If you replace them, use Film not Ceramic.



I believe the “A” through “E” schematics and engineering change notes have been stored somewhere and I ask that if you know what site, let us know.  If not, I will take them out to be scanned and send a link to make them available.  



Best I can do from Grey matter.  Hope you found it interesting and thought provoking.  If you have any additional information, questions, or find errors, I urge you to post them for all to view.  You will find me totally courteous and open minded with any errors that you find whether real or perceived.  There is more but at the moment it escapes me.



Kindest regards Jim K9AXN