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 Post subject: Re: Hallicrafters S(X)-11 Number ??
PostPosted: Jun Mon 07, 2021 7:53 pm 
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We can be sure the Hallicrafters engineers had a reason. Was it that they did not want to modify the front panels on radios that may have been built in one small batch as a test run? Was it because putting it on the front panel makes the IF oscillate? No way to know. Dachis mentions that the last company to buy the Hallicrafters name destroyed all their old records. We'd love to have that information even though it would not necessarily answer this particular question! I suppose I could try to find some photos of how the wiring to the front panel on the SX-16 and -17 was done. Might be some clues if I could find any.

On the other hand I built my long planned audio load box with a 5k to 8 Ohm transformer included this weekend. As I was hooking that to the radio I inspected the IF wiring and saw that I could easily move the two runs much farther apart. It was literally just push them farther apart easy! So while testing the audio box I cranked the RF gain to max and spun the crystal phasing knob slowly through several turns without getting any oscillations. So maybe that fixed it, maybe the oscillations will return as they have done in the past.


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 Post subject: Re: Hallicrafters S(X)-11 Number ??
PostPosted: Jun Sun 20, 2021 10:41 pm 
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Most of the work is done, almost down to the point where I have to decide to either call it good enough or pursue the holy grail of even better.

I did wrap the IF wiring that runs to the front panel switch with copper tape and grounded that. My stability problems are far worse when a signal generator is attached for some reason. It seemed a little better with the copper tape but certainly not fixed. I could try putting some further shielding at the switch. I don't know if Hallicrafters used shielded wires on the later models or any shielding at the switch.

But I left the copper tape in place and put the bottom plate on. A considerable amount of retuning was needed, as you would expect. Maybe a little more stable but again not completely stable.

Over the air however it seems quite stable. It is working quite well given that I don't have a factory fresh, "perfect", SX-11 or even someone else's restoration to compare it with. Leaning towards calling it good for now.

I made some 10dB SNR sensitivity measurements just a bit ago and I got:


25MHz 15uV
11MHz 3uV
4.5MHz 3uV
1.9MHz 2uV
900kHz 4uV

That is not horrible and I don't have any data that I can find that says it should be better. Certainly modern gear could do better and later tube gear did better. But maybe state of the art at the time?

While doing that I finally put together my 5k to 8 Ohm audio interface that also incudes 8 and 4 Ohm loads and headphone jacks with a volume control. Now I can use the radio with my 8 Ohm Heathkit speaker and modern don't clamp your head in a vise headphones. Yay! As I mentioned in the beginning the radio had come with a matching transformer soldered to the top of the power transformer. The top of the cabinet had been modified to mount a speaker. The speaker was long gone and the solder job on the matching transformer was right on the edge of letting go and allowing the transformer to rattle around inside the radio and creating havoc. It is a wonder indeed that UPS did not manage to do that to my radio but it arrived intact and now the transformer is safely caged and earning its keep.

I thought more about the BFO coupling and researched some later Hallicrafters schematics. My SX-43 still uses a gimmick. I think it predates the common use of SSB on the amateur bands however. The later and holy grail of Hallicrafters collectors everywhere, the SX-88, was made at the beginning of SSB adoption in the Ham bands and it bumps the coupling cap up from a gimmick to 68pF which is more like the ~300pF Jim Rozen used on his 11 and which I found to be a good choice on mine too. However, the 88 is a 20 tube monster and one of those tubes is used as a buffer amp for the BFO signal so maybe not a good design to imitate on the more common Hallicrafters radios.

The BFO circuit of the SX-96, another early SSB era design, is shown below. Gone is the gimmick, the coupling cap (C93) is now 300 pF. So, I am convinced that the light coupling we see in the earlier Hallicrafters designs was chosen because it worked well for CW. If you are having trouble with SSB on one of them you might want to try a real cap. 300pF may work well on all of them but maybe not. Putting a real cap on my S-38 BFO circuit is now officially on my list of things to do some day.


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 Post subject: Re: Hallicrafters S(X)-11 Number ??
PostPosted: Jun Mon 21, 2021 5:20 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 1936
Location: Peekskill, NY
The sx-11 does have a variable BFO injection, something the more modern sets don't have I think.

And yes, I do wind up with the injection level higher up when doing SSB on that set, and lower for
good CW reception. The SSB reception on that set, for 160, 80, and 40, is really stable.


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 Post subject: Re: Hallicrafters S(X)-11 Number ??
PostPosted: Jun Mon 21, 2021 10:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Mt. Airy, Maryland
Quote:
The BFO circuit of the SX-96, another early SSB era design, is shown below. Gone is the gimmick, the coupling cap (C93) is now 300 pF. So, I am convinced that the light coupling we see in the earlier Hallicrafters designs was chosen because it worked well for CW. If you are having trouble with SSB on one of them you might want to try a real cap. 300pF may work well on all of them but maybe not. Putting a real cap on my S-38 BFO circuit is now officially on my list of things to do some day.


Apples and oranges - the SX-96 has a 50 KC IF whereas the SX_11 has a 465 KHz IF. The capacitive reactance in the 96 is probably a bit less hence more BFO injection, but you really can't compare the two.

You are correct - the reason they did this was so the BFO did not overwhelm the detector/AVC and reduce sensitivity. 300pF coupling cap will probably do just that. 20-50 pF at 455 KHz is probably a much better balance.


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 Post subject: Re: Hallicrafters S(X)-11 Number ??
PostPosted: Jun Mon 21, 2021 11:40 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 1936
Location: Peekskill, NY
"so the BFO did not overwhelm the detector/AVC and reduce sensitivity..."

When retrofittin that sx-11, I stepped the value up till it seemed correct. With the injection control all the way up, yes it does close the eye tube a bit.


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 Post subject: Re: Hallicrafters S(X)-11 Number ??
PostPosted: Jun Tue 22, 2021 4:33 am 
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I did not account for the difference in IFs but the ratio in capacitance is much larger than 10, the ratio in frequency, so it still represents an increase in the coupling. The SX-11 does have variable BFO injection as well as variable RF/IF gain which radios like the S-38 lack so 300 pF may be too much on those radios. But I have never copied more than a word or two of SSB on my 38 so I think it is worth the effort to try a larger coupling cap to see if I can get some SSB copy. It is never going to be as good as radios with injection/sensitivity controls, I know that, but the difference currently is vast so maybe I can reduce that to merely large!

Never say never however. Someone posted an S-38 project on the Facebook Hallicrafters group today. It has five knobs, an S-meter, an isolation transformer, and probably a kitchen sink or two. That radio might do well on SSB but it goes way beyond a "restoration"!


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 Post subject: Re: Hallicrafters S(X)-11 Number ??
PostPosted: Jul Fri 23, 2021 9:09 pm 
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Joined: Feb Fri 13, 2009 5:09 am
Posts: 736
Location: Santa Clara, CA
There is another SX-11 that just appeared on eBay, item number 174838671982, and it seems to also not have the IF Bandwidth switch on the back apron. So, there is at least one other one like mine out there in the world! It looks pretty clean, and has had some capacitors replaced, but the power cord is cut off, and the power transformer, BFO can and eye-tube bracket (???) are painted red .


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