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Old 5th Apr 2020, 11:04 pm   #41
LyntonP
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Default Re: Hallicrafters Sky Champion S20

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Ideally zero beat should be achieved with the variable cap's plates half meshed.
To get near the production of a usable bfo signal the capacitor in the transformer can has to be completely slackened off.
I note the comments regarding the 250 pf silver mica capacitor on the base of the 6J5.
Still looking for one.
Lynton
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Old 5th Apr 2020, 11:45 pm   #42
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Default Re: Hallicrafters Sky Champion S20

I was referring to the pitch control. If you can vary the BFO above and below 455 kHz you'll be able to resolve LSB and USB signals.
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Old 6th Apr 2020, 9:48 am   #43
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Default Re: Hallicrafters Sky Champion S20

Hi Lynton, I've put a 250pF 350v silver mica in the post. A ceramic would also have been OK. Cheers, Jerry
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Old 6th Apr 2020, 10:22 am   #44
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Default Re: Hallicrafters Sky Champion S20

I've never worked on one of these but make sure that the BFO frequency is supposed to be the IF and not IF/2 like on some receivers.

Lawrence.
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Old 6th Apr 2020, 10:44 am   #45
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Default Re: Hallicrafters Sky Champion S20

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I was referring to the pitch control. If you can vary the BFO above and below 455 kHz you'll be able to resolve LSB and USB signals.
Hi Graham
The effect of the pitch control is very small indeed. I did (after much twiddling) manage to clarify an SSB signal but I think (on good advice) the mica capacitor in the can underneath the 6J5 is the most likely culprit. There is one in the post to me from a fellow forum member (most appreciated).
I think after all the adjustments that have been made I will have to do a re-alignment, and probably reduce the amount of coupling that I added.
Lynton
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Old 6th Apr 2020, 3:09 pm   #46
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Default Re: Hallicrafters Sky Champion S20

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What about the undesignated capacitor in parallel with L1? Presumably it's inside can?
Yes Graham the undesignated capacitor is inside the can. It is the adjustable capacitor used to set the oscillating frequency. It is accessible through a hole in the top of the can like an IF transformer.
Lynton
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Old 6th Apr 2020, 3:15 pm   #47
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Default Re: Hallicrafters Sky Champion S20

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I've never worked on one of these but make sure that the BFO frequency is supposed to be the IF and not IF/2 like on some receivers.
Lawrence do you mean 1/2 of the IF frequency?
To be honest I cannot find any information about the BFO, its operation ,setting, or basic frequency.
Documentation about the S20 is very scant, but there is much more about its successor the S20 R, which I believe was a very good receiver. I would love to have a go at one of them!
Cheers
Lynton
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Old 6th Apr 2020, 3:39 pm   #48
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Default Re: Hallicrafters Sky Champion S20

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Lawrence do you mean 1/2 of the IF frequency?
Yes, 1/2 of the IF, it might or might not be the case with your receiver but some receivers used that trick, the R1155 communications receiver being an example, to prevent the IF locking the BFO.

Lawrence.
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Old 6th Apr 2020, 4:46 pm   #49
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Default Re: Hallicrafters Sky Champion S20

Hi Lawrence been searching for more info but no luck. However the S20R uses an almost identical setup and states that the frequency is 455KHz.
Cheers
Lynton
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Old 6th Apr 2020, 4:52 pm   #50
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Default Re: Hallicrafters Sky Champion S20

That's probably it then.

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Old 7th Apr 2020, 12:50 pm   #51
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Default Re: Hallicrafters Sky Champion S20

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Originally Posted by LyntonP View Post
To be honest I cannot find any information about the BFO, its operation ,setting, or basic frequency.
Documentation about the S20 is very scant, but there is much more about its successor the S20 R, which I believe was a very good receiver. I would love to have a go at one of them!
Cheers
Lynton
The chances are the BFO was intended for CW rather than SSB reception. The tuning technique for SSB reception is to switch off the BFO and tune for maximum Donald Duck effect. Then turn on the BFO and slowly adjust the pitch control until the signal is resolved. If the centre position of the pitch control zero beats with an AM station, then rotating it slightly one way will resolve USB signals and the other way LSB signals.
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Old 7th Apr 2020, 1:33 pm   #52
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Default Re: Hallicrafters Sky Champion S20

I had a Hallicrafters of that era which had a front panel BFO injection level control as standard, I increased the value of the injection coupling capacitance a bit and used the control accordingly which I sometimes found useful.

The level control varied the HT supply to the BFO valve.

Lawrence.
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Old 7th Apr 2020, 5:32 pm   #53
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Default Re: Hallicrafters Sky Champion S20

Quote:
Originally Posted by LyntonP View Post
To be honest I cannot find any information about the BFO, its operation ,setting, or basic frequency.
Documentation about the S20 is very scant, but there is much more about its successor the S20 R, which I believe was a very good receiver. I would love to have a go at one of them!
There is a world of difference between the S20 and the S20R. The S20R has two I.F stages and a different dial drive setup. I have to review the schematic to locate the other differences. The S20R is the last one made before the start of the war.

Dave, USradcoll1.
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Old 7th Apr 2020, 6:35 pm   #54
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Default Re: Hallicrafters Sky Champion S20

Replacement capacitor arrived this morning (thanks Jerry), installed in radio but could not detect any oscillation. Turned out to be my unreliable scope, but after some effort it did display a BFO signal.
Set BFO frequency using scope.
Tuned to a faint AM station and played around with RF and AF gain but no heterodyne.
Re aligned IF (don’t have a frequency counter but can check output of SG using my Trio R600). It was a little out but re aligned ok.
Again no heterodyne. Stayed on faint AM station and slowly adjusted BFO transformer until a heterodyne could be heard.
Tuned around bands until I found some SSB and with careful adjustment could resolve the speech.
What do I draw from all this?
My scope is probably giving false information.
I need a frequency counter to to do this kind of work accurately.
I am probably expecting too much from such an old radio.
What I was pleased about?
All the help and suggestions from forum members, especially Jerry who tested all the valves and provided a new capacitor foc.
The amount of extra knowledge I have gained again from forum members (and the Internet).
The whole process was enjoyable/confusing/frustrating/satisfying in many different ways.
Once again thanks to everyone for all the help.
I look forward to the next radio.
Cheers
Lynton
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Old 7th Apr 2020, 6:44 pm   #55
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Default Re: Hallicrafters Sky Champion S20

A 'scope is really a rather inappropriate tool for adjusting the frequency of this sort of oscillator: for the BFO to work it needs to be within a couple of KHz of the receiver's intermediate-frequency (455KHz) - which means that your 'scope would need its timebase to be accurately settable to better than 0.5%

It was almost certainly never that accurate when manufactured! And even if it was, the timebase-frequency control can't be read to that level of accuracy anyway.

Digital frequency-counters are remarkably cheap these days, and make this sort of work *so* much easier.
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Old 14th Apr 2020, 4:56 pm   #56
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Default Re: Hallicrafters Sky Champion S20

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Originally Posted by Jon_G4MDC View Post
Always wanted one of these. Around age 14 a school friend had one.
His logbook contained much better DX than mine and I'm still jealous!
That's the worst thing to happen. I envied a buddies receiver and now have several Communications receivers from the very basic ones to probably the complex. I've always been a bit Excessive Compulsive.
I'm going to thin out the collection as soon as this foolishness is over.
Dave, USradcoll1
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Old 14th Jul 2020, 4:54 pm   #57
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Default Re: Hallicrafters Sky Champion S20

42 posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=168998
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Old 2nd Jan 2021, 12:50 am   #58
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Default Re: Hallicrafters Sky Champion S20

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Originally Posted by LyntonP View Post
Quote:
Originally Posted by Keith View Post
I'm assuming that you've checked/replaced the anode and grid resistors in the BFO circuit.
Hi Keith
Tried on a weak MW station but with no success.
Managed to get the valve support box loose from the chassis giving access to the valve base of the 6J5. There was 0.01 cap and a 100K resistor (and a mica cap) as shown on the diagram.
I replaced the 0.01 and the 100k (not too far out of spec) put everything back together, but unfortunately no effect.
Strangely the wax cap was a completely different brand to all the other caps I have changed? May have been changed in the past?
Lynton
My S-20 BFO doesn't work either. This thread has been a big help in locating the two nuts to remove to get the can to drop down.

I got the can loose too but don't have enough slack in the wires coming in to it to get it down that far so I can replace the components. Did you disconnect the far end of the wires first?

John Limbach, K8NN

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Old 4th Jan 2021, 4:39 am   #59
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Default Re: Hallicrafters Sky Champion S20

As much help you gave with my S-20 project I would say you have worked on one

Quote:
Originally Posted by ms660 View Post
I've never worked on one of these but make sure that the BFO frequency is supposed to be the IF and not IF/2 like on some receivers.

Lawrence.
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Old 4th Jan 2021, 10:57 am   #60
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Default Re: Hallicrafters Sky Champion S20

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As much help you gave with my S-20 project I would say you have worked on one

Quote:
Originally Posted by ms660 View Post
I've never worked on one of these but make sure that the BFO frequency is supposed to be the IF and not IF/2 like on some receivers.

Lawrence.
I don't get that post

Lawrence.
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