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Old 29th Jul 2020, 6:31 pm   #101
mrrstrat
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Default Re: Another Hallicrafters Sky Champion S20.

Yes: the trimmers (for alignment) will bring in the reception much sharper. The pad trimmer does not seem to allow for stations lower on the dial to come in unless I retune the IF trimmers for the new station (like from 1350Khz to a station at 800Khz).

- The set will tune in one or two channels pretty decently once the IF trimmers are manually set to pull them in. Then the alignment trimmers will bring in the station much better.

- Once tuned in, the stations below and above the "center point" I am manually tuned in for will not come in (1350khz comes in good, 800khz station will not come in at all, nor will 1595khz).

- I can inject a signal at the antenna (with modulation) to produce a tone that will juxtapose at the station I can pull in. When I move the signal generator up or down, the tone gets weaker to totally gone the farther I get from my 1350Khz manually tuned IF trimmers (T1 and T2).

- A signal injected into the 6L7 cap does not produce a tone in the set (to properly align the T1 and T2 trimmers to begin the real alignment process).

- Bands 2,3, and 4 produce nothing. I can find a signal injected into the antenna only for band 1 and 2 (3 and 4 are dead).

The path I am searching (in no general order):

* Alignment of the C1/C2/C3 ganged capacitor - checking the wafer switch connections and making sure they are all three working together.

* Scope on the mixer to see if I can discern any activity.

* Finish voltage checking - circuit appears to be good and matches schematic.

* Sanity check: make sure valves are good (check on my calibrated TV-7).

* Check why signal at 6L7 cannot be heard: is there a malfunction in the bandpass that seems to favor 1350Khz?

I think I have eliminated the single point of failure reasons (magic bullet reasons why set is not working). This seems to be systemic meaning parts of the circuit are working and there are a couple of out-of-synch things going on.

The set is on my bench and I'll trickle time into it in-between the other things I need to get done. It naturally bugs me that it is working just enough to suggest it can function - I'll try any high opportunity things anyone throws at me


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Originally Posted by Station X View Post
Even if shortwave were disabled medium wave (AM in the USA) would still have had to work or the set would have been useless.

That means that the IF stages and local oscillator would have to work. We keep coming back to that.

You said early that adjusting the trimmer screws of the new IF transformer improved reception. That points to the IF stages working.
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Old 29th Jul 2020, 6:36 pm   #102
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Default Re: Another Hallicrafters Sky Champion S20.

Can a malfunctioning bandpass exclude all but a narrow range of frequencies sent to it? Trying to rationalize why I get one central frequency point where ability to tune in channels drops off in either direction the farther you get from the manually tuned-in frequency...


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Even if shortwave were disabled medium wave (AM in the USA) would still have had to work or the set would have been useless.

That means that the IF stages and local oscillator would have to work. We keep coming back to that.

You said early that adjusting the trimmer screws of the new IF transformer improved reception. That points to the IF stages working.
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Old 29th Jul 2020, 6:43 pm   #103
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Default Re: Another Hallicrafters Sky Champion S20.

I can't rationalize the behavior of this set at all.

You need to follow the advice given by mss660 in post #98.

GET THE IF STAGES WORKING FIRST
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Old 29th Jul 2020, 7:57 pm   #104
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Default Re: Another Hallicrafters Sky Champion S20.




Quote:
Originally Posted by Station X View Post
I can't rationalize the behavior of this set at all.

You need to follow the advice given by mss660 in post #98.

GET THE IF STAGES WORKING FIRST
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Old 2nd Aug 2020, 4:34 pm   #105
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Default Re: Another Hallicrafters Sky Champion S20.

I did the simple exercise: I used a sig generator (with a frequency counter to verify things) and my scope to see what is going on when I inject a signal and when I use the set (polling the mixer in what it has going on). Also used a second set to detect the radios local oscillator.

I can only conclude after this that the LO is not capable of being at 455 kHz (with set tuned higher than 1800khz as per the alignment instructions) and based on where I find the LO signal on another set my 6L7 IF coil(s) have to not be right (valve works and the components are all replaced in the set). The valve is fine, so I believe the coil in the IF can is at fault - it along with the other coils are the only things not replaced. I did replaced the other IF stage IF can with a good 2nd IF stage HV type that appears to work just fine.

As far as the strange behavior I am seeing: I can also see the LO signal frequency drop off on wither side of the set being tuned to around 1300 kHz. At around 1300 kHz I am seeing the LO at about 550 kHz (and nice sin wave on the scope). The other radio set detects the LO of my S20 at around 1100khz on its (the other radio doing the monitoring) dial.

I cannot preclude that the ganged cap tuner coils are not also jazzed up - from the odd behavior I can observe - so I have bundled this set up to be worked on at a later date when I dig into coils (after I pick up another). This is now a winter project.


Quote:
Originally Posted by Station X View Post
I can't rationalize the behavior of this set at all.

You need to follow the advice given by mss660 in post #98.

GET THE IF STAGES WORKING FIRST
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Old 2nd Aug 2020, 5:26 pm   #106
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Default Re: Another Hallicrafters Sky Champion S20.

There's confusion with terminology here again and I can't help but feel that's part of the problem.

The IF stages should always be tuned to 455kHz no matter what. The Local Oscillator, Aerial Stages and RF Stages will have no affect on this. If fact even if the LO, Aerial and RF stages were ripped out of the set the IF stages should still operate at 455kHz.

The frequency of the Local Oscillator will vary depending on the frequency the set is tuned to. By that I mean the frequency shown on the tuning dial. This applies even if you can't hear a signal from the speaker. On MW/AM the local oscillator frequency should always be 455kHz above the frequency the set is tuned to.

Set tuned to 1300kHz. Local oscillator operates at 1300 + 455kHz = 1755kHz, not 550kHz.

Set tuned to 550kHz. Local Oscillator operates at 550 + 455kHz = 1005 kHz.

As you have a frequency counter you could loosely couple it to the Local Oscillator coils and check what frequency the LO is working at. The same principle applies on all wavebands. LO operates 455kHz above the frequency the set is tuned to.

Set tuned to 3000 kHz. Local Oscillator operates at 3000 + 455 kHz = 3455 kHz.

The IF stages of the set must be working at some frequency or you wouldn't be able to receive anything. I can't understand why sweeping the Signal Generator around 455 kHz, with the modulation switched on, doesn't give some response.
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Old 3rd Aug 2020, 3:16 pm   #107
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Default Re: Another Hallicrafters Sky Champion S20.

I thought I'd better put my money where my mouth is!

Here are some pics showing the Local Oscillator in a Trio 9R-59DS receiver being checked. The earthed side of the counter's input lead was connected to the set's chassis. The signal side of the counter's input lead was simply dangled near to the appropriate Local Oscillator coil. Close enough to give a reliable signal for the counter to latch on to, but far enough away not to detune the coil significantly.

In the pictures in this post the set is tuned to 1300kHz (1.3MHz) the counter is showing a frequency 455kHz above this ie 1755kHz (1.755MHz).
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Old 3rd Aug 2020, 3:19 pm   #108
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Default Re: Another Hallicrafters Sky Champion S20.

In the pictures in this post the set is tuned to 550kHz (0.55MHz) the counter is showing a frequency 455kHz above this ie 1005kHz (1.005MHz).
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Old 3rd Aug 2020, 3:21 pm   #109
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Default Re: Another Hallicrafters Sky Champion S20.

In the pictures in this post the set is tuned to 3000kHz (3.0MHz) the counter is showing a frequency 455kHz above this ie 3455kHz (3.455MHz).
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Old 3rd Aug 2020, 3:23 pm   #110
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Default Re: Another Hallicrafters Sky Champion S20.

If you can carry out similar tests on your SX20 this may enable us to eliminate the set's Local Oscillator as a possible problem.
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Old 3rd Aug 2020, 4:59 pm   #111
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Default Re: Another Hallicrafters Sky Champion S20.

Thanks Graham! Awesome explanation and patience with a new radio guy

I'll repeat the tests with my gear: where are you connecting the frequency counter on your set? I have a HP 5314A and it seems to like a physical connection - I never used one with proximity instead of a physical connection.
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Old 3rd Aug 2020, 5:18 pm   #112
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Default Re: Another Hallicrafters Sky Champion S20.

Like I said in post #108 I just dangled a wire near the appropriate Local Oscillator coils.

You could try a direct connection or scope probe connection to the grid of the 6J5 Local Oscillator, but it may just kill the oscillator. Not permanently of course!
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Old 3rd Aug 2020, 5:34 pm   #113
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Default Re: Another Hallicrafters Sky Champion S20.

More worried about inadvertently killing my scope or frequency counter

Quote:
Originally Posted by Station X View Post
Like I said in post #108 I just dangled a wire near the appropriate Local Oscillator coils.

You could try a direct connection or scope probe connection to the grid of the 6J5 Local Oscillator, but it may just kill the oscillator. Not permanently of course!
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Old 3rd Aug 2020, 5:36 pm   #114
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Default Re: Another Hallicrafters Sky Champion S20.

Indeed. as we say in the UK "They don't like it up 'em!"
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Old 4th Aug 2020, 1:42 pm   #115
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Default Re: Another Hallicrafters Sky Champion S20.

And I did observe an effect where the frequency counter would drop when I jostled connections. I figured I would not be able to really get a direct LO measurement.

I plan to try the indirect method of measurement today.

Quote:
Originally Posted by Station X View Post
Like I said in post #108 I just dangled a wire near the appropriate Local Oscillator coils.

You could try a direct connection or scope probe connection to the grid of the 6J5 Local Oscillator, but it may just kill the oscillator. Not permanently of course!
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Old 4th Aug 2020, 6:57 pm   #116
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Default Re: Another Hallicrafters Sky Champion S20.

Ok - this is what I found. To prevent the frequency counter from fouling the LO, I clipped it to the 6L7 grid wire on the insulation (not touching metal). Touching the IF can trimmer screw showed 550-ish kHz and getting it near the can did nothing. Touching the trimmer screw of the 2nd stage can (next to the first stage) I got around 220 Khz.

The frequency appears to be 1450-1600 as it moves up and down slightly but rapidly when the set was tuned for a 1440 Khz station.

I did notice that when I turn down the dial to lower frequencies the frequency counter dropped and then suddenly dropped to below 250 Khz and then drop off to near zero as I went down the dial. By 800 on the dial the frequency counter showed what I would call ambient signal noise (not quite zero but too weak to measure perhaps).
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Old 4th Aug 2020, 7:40 pm   #117
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Default Re: Another Hallicrafters Sky Champion S20.

The IF Transformer trimmers have absolutely nothing to do with the tuning of the local oscillator. Leave them alone for now.

Does the counter's reading change as you tune the receiver?

As you tune across the AM band from 550kHz to 1500kHz you should see the local oscillator's frequency change as you tune the receiver. Pause tuning for a couple of seconds every turn or so of the tuning know to let the counter catch up.

With the tuning set to 550kHz the counter should read 1005kHz.
With the tuning set to 1500kHz the counter should read 1955kHz.

Your counter's reading of 1443.82kHz?? would be correct if the receiver was tuned to 988.82kHz which is around 300 metres.

Try to produce a table showing dial settings and counter readings. There should be a constant difference of 455kHz across the band.
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Old 4th Aug 2020, 7:49 pm   #118
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Default Re: Another Hallicrafters Sky Champion S20.

Another thought.

The S20 has two 6J5 tubes. One for the Local Oscillator and one for the Beat Frequency Oscillator (BFO) try swopping them over in case one's faulty.

Can your counter pick up the BFO at around 455kHz if it's loosely coupled to the "gimmick capacitor" ie twisted wires?
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Old 4th Aug 2020, 7:50 pm   #119
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Default Re: Another Hallicrafters Sky Champion S20.

The counter looks to be capacitively connected to g1 of the mixer...with the counter still connected as in the photo connect the sig gen to the actual top cap of the mixer and tune the sig gen to 455kHz.

Lawrence.

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Old 4th Aug 2020, 7:53 pm   #120
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Default Re: Another Hallicrafters Sky Champion S20.

Good to have you on board Lawrence!
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