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Author Topic: Hallicrafters SX-115 selectivity design eror  (Read 44516 times)
K9AXN
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« Reply #60 on: March 28, 2015, 08:23:39 AM »

Hi Gary,

Thanks for the courteous response!  Makes the vintage radio experience so much more enjoyable.

Have a super weekend and good luck finding the SX-115.

Kindest regards Jim K9AXN
« Last Edit: March 28, 2015, 09:40:52 AM by K9AXN » Logged
K9AXN
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« Reply #61 on: March 28, 2015, 09:37:49 AM »

Good morning Tom,

Have no idea what logic you used to come to your conclusions regarding the shift from 50.500 to 50.750Kc.  

I have written several papers intimately describing the design issues the Hal engineers mitigated when they developed the SX-100 Mark2.  The difference in performance is staggering.  Turns a marginal radio into a keeper.  The papers addressed individual issues and the final, one for CW operators and one for the feedback problem and several others for overload.

I would like to have answered your questions offline for obvious reasons but you insist.

The two reasons for the change are:

1.  To isolate the BFO from the AGC detector.
      
     Symptom:  Turning the BFO on generates AGC indicated by the S-Meter in the 50.500Kc Mark1.
     Does not happen in Mark2 50.750Kc.  Tom, get some skin in the game and test the radios yourself.  Also follow the
     yellow brick path.    


2.  To make the radio more pleasant to operate for CW folks.

The bandpass in the 50.500 radio while in the .5Kc position is from 50.250Kc to 50.750Kc for an audio band width from 250 cycles to 750 cycles.  That means that you can by tuning, select a CW tone from 250 cycles to 750 cycles while remaining within the I.F. band pass.

Now the 50.750Kc version.
The band pass for the .5Kc position is 50.500Kc to 51.000Kc rendering an audio band pass of 500 cycles to 1000 cycles.  
You can tune that range to select the pitch that you desire while remaining within the pass band.  Doesn't that sound like a more useful range and if not simply use the BFO adjustment?

If any of this is unclear just think about it.  I'm out of here.  I'm a 75 year old guy that's planning to do some serious hang gliding in upstate NY around Ellenville in my old 1960's haunts.  

I thank all that expressed courteous complementary OR OPPOSING positions.  Makes a great hobby even greater.

Also thank the folks who audited their SX-115, SX-117 and SX-100 radios to confirm the capacitors used.

Good luck and kindest regards Jim K9AXN
        
« Last Edit: May 11, 2015, 09:23:16 AM by K9AXN » Logged
G3RZP
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« Reply #62 on: March 28, 2015, 09:42:21 AM »

Jim,

The last guy I knew who said he was going hang gliding was in hospital for 4 weeks and off work for 6 months.

Take care!

73

Peter G3RZP
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K9AXN
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« Reply #63 on: March 28, 2015, 09:51:01 AM »

Hey Pete,

I'll be thinking of you while soaring with the feathered bros.  Been gliding forever, beats hanging around the gin mills swapping stories. 

I would like to thank you for your professional courtesies.  Keep em lit don't mean ale.

Have a great weekend!

Kindest regards Jim K9AXN   
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KM1H
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« Reply #64 on: March 28, 2015, 03:02:44 PM »

Hmmm, hang gliding in the Catskills where it is still considered winter, I spent 2 years in that area going to school and working at resorts. They also have several mental health facilities up there these days where old folks can wind down from stress........You do seem to be wound rather tight since you started this thread and never expecting to get a serious challenge to your credibility. Let me know where you will be and I'll have my GF fly us over and check the scenery and conditions, as a former Russian Army piston powered fixed and rotary wing pilot she has plenty of winter weather stick time and her current job here takes her into all sorts of weather.

Last year you disappeared for 6 months or so........

Only someone who hasnt spent any time chasing CW DX would complain about a 250-750 Hz CW tone range. OTOH I suspect the only reason for the change was via customer request as in the 50's CW was a lot different than these days. Tom and I have serious 160 and 80M CW time under our belts digging in the noise and a high pitch doesnt cut it.

You make a lot of claims about papers and past work but you never elaborate nor can I find any mention of you in the various Halli archives.
Lots of people can name drop or claim others work as their own.

I still have very serious doubts about your veracity and accuracy concerning the SX-96, 100, 115, and 117 filtering and components. Also have a SX-101A but havent been into it since it was rebuilt before I got it.

Enjoy your vacation and all this can continue again.....
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AC2EU
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« Reply #65 on: March 28, 2015, 03:13:06 PM »

I'm not far from the Catskills and I wouldn't be hang gliding there on a bet this time of year!
Besides, the weather has been very unsettled here and sudden high velocity winds are common.

Good luck with that...  Shocked   Huh  Roll Eyes
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K9AXN
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« Reply #66 on: March 28, 2015, 05:03:16 PM »

Hi Jim,

Your right, going to have to hold for a while.  Not good to be impatient.  Was up there last year a bit early just love NY upstate, spent many years up there during the 60's.  Joe Pepitone had a Gin mill on a mountain side near kingston.  Used to make bets on who could navigate the winding road leading to his place on a skate board.  Life is good.

Thanks for the note Jim --- Kindest regards Jim K9AXN
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KM1H
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« Reply #67 on: March 28, 2015, 06:07:15 PM »

Quote
Jim thank you so much for the fine information . I have owned three SX-115s . I did the instructed alignment etc but always came away a bit underwhelmed  . They wound up on my over-priced/over-rated list . A drop dead beauty --but !  

Ive owned one for about 5 years and it performs like any other Halli using 50 kHz LC filtering, fine for the casual user and poor for someone used to mechanical and multipole crystal filters. The SX-117 cost reduced model is worse. Halli soon abandoned that scheme and went with crystal filtering.

Did you replace all the various paper caps in plastic and do a thorough realignment of all 3 of those sets? Did you check resistors for tolerance? The manual alignment instructions leaves a lot to be desired as do the Hammarlund HQ-170/180 versions which uses a 60 kHz LC IF with a rather elaborate selectivity scheme.


 
Quote
I have also owned a pair of late model SX-100 and found them to be real treasures and near the top of my favorites list .

Ive had a SX-100 MK2A also for about 5 years and find it rather mechanically unstable for good SSB and CW; the whole thin gauge chassis flexes. The earlier model was even worse

I also have a SX-96 and 117 which are only good for shelf queens.

Quote
 As a result of your information I will give the 115 a whirl when the next one turns up . Good bet it will be a keeper .

If you do a proper overhaul and alignment and stay away from questionable parts changing you will be in for a different viewpoint....but remember its basic limitations as it is not a high performance radio such as a 75S3C or tweaked R4C; a NC-300/303 properly overhauled is a better set in several areas and almost a decade older. My highly modified 75A4 runs circles around the SX-115. One station has a SX-115/HT-32B/NCL-2000 and the other a CE-100V/that 75A4/another NCL-2000 and can select either setup with a single switch for fast comparisons.

Carl

Carl
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KM1H
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« Reply #68 on: March 28, 2015, 06:23:21 PM »

Quote
You fellers didn't slip one by me.  I asked Tom after his first belligerent post if he was your proxy, sounded too much like you --- he didn't answer. 

Anyone in the know would say you are totally clueless but I do respect Toms ability to tie you up in knots and completely ignore your continuous belligerence and cheap shots; a sure sign you have already lost the battle.

Quote
Forgot to add I still have the SR-2000 prototype that I worked with and the original Cyclone 2 upgrade which is actually a Cyclone 3 missing some yet to be finished circuitry.  I'm writing an extensive paper regarding the entire technical history from Genesis to death of the SR-400, SR-400A, and SR-2000.  You are going to be dazzled with the information in that paper.  When I finish I'll send you a note so you folks can critique it for me.

I have less interest in any Halli transceiver than I have in a Collins, Swan, Drake, etc which is ZERO. My only tube era transceiver is a NCX-1000 and that only has two tubes and isnt even close to being ideal for any mode but it does sound great on AM RX/TX and fits well on the bedroom desk. Just enough power and audio punch to work 75 and 40 in the wee hours when I cant get back to sleep.
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W8JI
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« Reply #69 on: March 29, 2015, 08:09:29 AM »


The two reasons for the change are:

1.  To isolate the BFO from the AGC detector.
      
     Symptom:  Turning the BFO on generates AGC indicated by the S-Meter in the 50.500Kc Mark1.
     Does not happen in Mark2 50.750Kc.  Tom, get some skin in the game and test the radios yourself.  Also follow the
     yellow brick path.    

Things work the way they work.

You are proposing that a 250 Hz IF shift on a 50 kHz IF was done to reduce BFO back eed through the two resonant circuits.

That makes no electrical sense at all, for the reasons I gave:

1.) If the IF moves, the BFO also has to move the same amount for the same pitch. The net result is the BFO is in the same exact place in relationship to skirts.

2.) The resonant circuits are wide as a barn door. A 250 Hz move is nothing, even if you changed pitch and did offset the BFO 250 Hz more and suffered with the 250Hz pitch change.


Quote
    I have no idea how you came to the conclusions regarding band passes, center frequencies, or any of your
     narrative.  Please slow down and follow the numbers below.

The bandpass in the 50.500 radio while in the .5Kc position is from 50.250Kc to 50.750Kc for an audio band width from 250 cycles to 750 cycles.  That means that you can by tuning, select a CW tone from 250 cycles to 750 cycles while remaining within the I.F. band pass.

Who would find a CW pitch pleasant in that audio range? A caveman returning from the hunt would drop his dinner and head for the hills knowing whatever it was is much larger the he, and he never knew his girl friend to sound like that.      
 
Now the 50.750Kc version.
The band pass for the .5Kc position is 50.500Kc to 51.000Kc rendering an audio band pass of 500 cycles to 1000 cycles.  
You can tune that range to select the pitch that you desire while remaining within the pass band.  Doesn't that sound like a more useful range?

That makes absolutely no sense at all to me. The BFO is tunable for about a 3-5 kHz range, depending on receiver model in that 50 kHz IF family.

This means, no matter what IF frequency is use, the pitch can vary with a front panel delta of at least 3 kHz. With proper alignment it can go from negative pitch (reverse sideband) to a few kHz picth, including right through zero beat.

The user can get any tone he wants, the IF shift does not change that one bit.

So, understanding the BFO has a ~4kHz adjustment range, they very clearly did NOT change the IF 250 Hz to change the CW pitch. That idea makes no sense at all, just like the BFO back feed reduction makes no sense.

73 Tom
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G3RZP
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« Reply #70 on: March 29, 2015, 09:15:53 AM »

Sturley's book goes into some depth about how changing the coupling affects the centre frequency and symmetry, all depending on which method of changing coupling is considered. The least change in centre frequency and the best symmetry are achieved by coupling variation by mutual inductance i.e. moving the coils relative to each other, as done in various Eddystone receivers and the BC453.
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AC2EU
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« Reply #71 on: March 29, 2015, 10:14:48 AM »

While the SX-115 is a great classic "look", it's technology is still as old as the hills. As another person noted, many of the Hallicrafters are also MECHANICALLY unstable as well as electrically. We can repair/improve the electrical, but the mechanical problems remain.
I've repaired many different Hallicrafters transmitter and receiver models for people, but have no desire to own one myself.
Nostalgia is a strong draw for some folks and if this is what they enjoy, then I say go for it.  

My personal weakness is old military radios which have their own problems as practical rigs...there is just something about the ruggedness and history that I find appealing.
A similar irrationality must afflict Hallicrafters collectors!

« Last Edit: March 29, 2015, 07:52:36 PM by AC2EU » Logged

K9AXN
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« Reply #72 on: March 29, 2015, 10:20:24 AM »

Good morning Gary,

Before I leave this thread I need to finish with some sort of conclusion.  Before you purchase an SX-115 you need to hear the whole story.  

I started this thread because a good friend who is home bound asked me to take a look at his prize SX-115.  He was the original owner and was attached to the radio.  His major complaint was "The selectivity is not as it was; SSB is no longer clear.  He also is not a CW person and may be hearing challenged.  I decided to take a look at it even though I was preparing to take a spring summer break.  

Couldn't do a full regression test so started with the selectivity system.  First look under the radio at the selectivity components found what is considered by designers to be a violation of good practices; Ceramic class 3 Z5U caps used in a tuned circuit carrying circulating current.  This statement will draw the wrath of a few people but I ask them to provide any documentation that supports their assertions.

There was some disagreement regarding what I found and some claims that I believe were unsubstantiated.

Primary was the filter was broad as a barndoor.  By today's standards some think it is but for the old timers it's fine especially for a radio they have come to love representing their roots in radio.  The band width in the SX-115 is wider than other 50Kc Hal radios because ceramic Z5U caps were used contrary to good design practices.  I checked to see exactly what influence they had on the performance of the filter.  They shifted the 1Kc resonant anchor point and lower skirt closer to the 2Kc point than 1Kc --- approximately 1.6Kc.  The other positions were skewed wider as well.  

Is this a catastrophic problem?  To me No, but to some who rely on the stated band widths it is troublesome when switching selectivity positions.  There are things that people say that can skew what other people think.  If your told over and over again your favorite radio is broad as a barn door, you won't try to make it better.  Ever wonder why the SX-115 has the worst reputation regarding band width of all of the 50Kc radios?  Haven't heard anyone harp on the SX-88 or others which have the same filter system!  

Changing those caps did indeed make a difference even though it was minimal to me.  The people who use the .5Kc and 1Kc positions will take exception to minimal.  The old timers that think the world of their radio will see hope that it's short comings can be rectified.

Nick, W9NVN, had the most precise and unambiguous analysis of the SX-115 in his post.

He stated several things that may be correct regarding the SX-115 and he did so with courtesy.  The most profound design error is the 6BA7 mixers followed by the filter.  I thought he was correct but now do not agree.

The 3db noise floor on the radio that I'm working with was -118db in the .5Kc position and approximately -120 on the others; unacceptable.  Since changing the caps including changing the 2.2pf top coupling cap to ceramic plate NPO the 3db point for the .5Kc and 1Kc positions are essentially the same at -125 and I have yet to replace the 2.2pf cap in the 1st I.F.

Nick, do you have the design notes for the 12AT7 change and if so why don't you post them?

Nick, I did and do not work with the radios earlier than the SX-150 except the SX-100 and SX-117.  My primary interests are the SR-400,SR-400A, and SR-2000.  This SX-115 is the first that I've had on the bench.  With that I can't say good or bad for the radios you speak of.  My statement about the Hal engineers was that they were as good and forward thinking as any having so much less financial support to work with.  

Gary, please forward your email address to me would like to speak with you.

I have to disclose that I deliberately added two errors to determine whether the impolite ones read responses.

One was that the shape factor was 2.1/1 and the second 2.2/1.  I also said the SX-115 that I was working with displayed the same shape factor that the charts painted which are approximately 4.4/1 which conflicts with 2.1/1 and 2.2/1.  I repeatedly asked them to look at the chart and received no response.  The shape factor of 2.1/1 and 2.2 /1 are not possible and in conflict with the 4.4/1 defined in the chart.  Had they looked at the chart they would have immediately taken exception with my assertion and we could have moved on knowing that we had a two way conversation.  

The following sweep is a linear presentation of the SX-117 and SX-115 filter.  I did not record the spectrum analyzer screen to finitely detail the exponential skirt shape --- that is a to do.  

SX-117     http://www.k9axn.com/_mgxroot/page_10876.html

SX-115     http://www.k9axn.com/_mgxroot/page_10881.html

Have a super day and Kindest regards Jim K9AXN  
« Last Edit: May 11, 2015, 03:05:44 PM by K9AXN » Logged
K9AXN
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« Reply #73 on: March 29, 2015, 11:28:12 AM »

Just had to clarify the concept.

MY QUOTE:
------------------------------------------------------------------------------------------------------------------------------
I have no idea how you came to the conclusions regarding band passes, center frequencies, or any of your
narrative.  Please slow down and follow the numbers below.

The bandpass in the 50.500 radio while in the .5Kc position is from 50.250Kc to 50.750Kc for an audio band width from 250 cycles to 750 cycles.  That means that you can by tuning, select a CW tone from 250 cycles to 750 cycles while remaining within the I.F. band pass.

Now the 50.750Kc version.
The band pass for the .5Kc position is 50.500Kc to 51.000Kc rendering an audio band pass of 500 cycles to 1000 cycles.  
You can tune that range to select the pitch that you desire while remaining within the pass band.  Doesn't that sound like a more useful range?

Note:  I said without touching the BFO dial.  If you wish to listen beyond the 500 to 1000 cycle range simply tune the BFO.

Tom's QUOTE:
------------------------------------------------------------------------------------------------------------------------------------
That makes absolutely no sense at all to me. The BFO is tunable for about a 3-5 kHz range, depending on receiver model in that 50 kHz IF family.

This means, no matter what IF frequency is use, the pitch can vary with a front panel delta of at least 3 kHz. With proper alignment it can go from negative pitch (reverse sideband) to a few kHz pitch, including right through zero beat.

The user can get any tone he wants, the IF shift does not change that one bit.

So, understanding the BFO has a ~4kHz adjustment range, they very clearly did NOT change the IF 250 Hz to change the CW pitch. That idea makes no sense at all, just like the BFO back feed reduction makes no sense.

73 Tom


Jim's RESPONSE:
-----------------------------------------------------------------------------------------------------------------------------------------
Your perfectly right however there is an operator issue here.

Your way requires the manipulation of the BFO frequency in order to use a pitch above 750 cycles in the 50.5000Kc radio because the natural setting of the BFO is 50.000Kc.  The useable band pass with the 50.500 is from (50.250 to 50.750 Kc).  If you wish to use a higher pitch than 750 cycles you will be required to alter the BFO frequency below 50.000Kc to accommodate the frequencies above the natural band pass which ends at 50.750Kc.  

With the 50.750Kc change the natural bandpass is from 50.500 to 50.100Kc.  This allows you to select a pitch from 500 cycles to 1000 cycles by simply tuning without changing the BFO frequency.

With the 50.750Kc radio you can switch back and fourth between filter, CW, and SSB positions without touching the BFO with no regard to what pitch you selected between 500 and 1000 cycles.

With the 50.500Kc radio you have to intervene with the BFO when changing CW and SSB positions when you have altered the pitch above 750 cycles.

If this is not understandable someone else is going to have to find words to wrap around the concept.

Have a good day Tom --- see ya

Kindest regards Jim K9AXN
« Last Edit: May 11, 2015, 03:11:43 PM by K9AXN » Logged
G3RZP
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« Reply #74 on: March 29, 2015, 11:45:44 AM »

Replacing the 6BA7 is almost certainly to be a good start, although there may be a loss in Q in the input tuned circuits: pentagrid mixers can offer somewhat negative input impedances above 20 MHz because of transit time. From the viewpoint of dynamic range, the 12AU7 is better than the 12AT7: there are some reports that the ECC88/6DJ8 and Special Quality E88CC are even better. They are favourites of the audiophools though and can be hard to get.

The Pullen mixer has a following but nobody has published any results in terms of noise figure and intercept points. Examination of the circuit does not suggest that it is outstanding.

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