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Old 29th Dec 2003, 9:51 am   #1
jsel84
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Default Halicrafters S38 finding 455freq. in IF coil

I have been working two Halicrafters S38 units .
they are both 38S units. As I'm going more into to them I can see they are slighty different.
I have the manual and one unit is part for part. It is also tuning up fine and is working well.
The other I noticed a tuning con. is not there. I am almost positive it was made that way instead of two like the other there is only one.
If you know of this unit its C5 & 6.
The other is working but I am have a problem tuning in the 455 in the IF coils. On the other unit when I tune the Sig gen to 455 and bring the output to the IF coil I can fine tune the coils.
On the problem unit it doesn't react the same.
I can tune in stations on all bands but can't tune the IF coils at 455. I can't hear the the tone from the SG when tuned to 455.
I hope this make sence to some one
thanks for the help
Jim

Last edited by Station X; 28th Dec 2004 at 7:09 pm. Reason: Import
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Old 29th Dec 2003, 2:04 pm   #2
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Default Re: Hallicrafters 38S finding 455freq. in IF coil

Jim.

Do you mean Halicrafters S38?

You will find several Hallicrafters Websites and Discussion Groups on the net where you may be able to find more specialist help with your problems.

There were several variations of the S38, which may account for the "missing" capacitor.

As far as the IF problem goes have you tried sweeping the SG either side of 455 KHz to see if you can get a response somewhere? There ought to be a response, as you say that you can tune stations on all bands, which indicates that the IF Amp is working. If the IF seems way off frequency it is possible that the IF frequency was changed at sometime to say 470 KHz. If it is only slightly off, then maybe the set needs realignment.

Graham

Last edited by Station X; 28th Dec 2004 at 7:11 pm. Reason: Import
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Old 30th Dec 2003, 11:46 pm   #3
jsel84
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Default Re: Halicrafters S38 finding 455freq. in IF coil

The unit is pulling in stations . I tride sweeping a bit above 455.
how far off can it be.What would be the best way to bring it back to 455? I did go the the sit but couldn't find any vari. on the 38S.
thanks for any advise
jim
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Old 31st Dec 2003, 5:34 am   #4
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Jim,
I have several S38's and none have a trim capacitor for C6. It could very well be that someone removed the fixed cap and tried a trimmer in that position in hopes of peaking the gain. I don't think that would work, the tuning would be too broad. Just change the wiring back to the original circuit where C6 is a 2700pf fixed cap.
It would be best to follow the aligment proceedure in the manual where you connect the sig generator to the fixed plates of the rear section of the tuning cap. If you cannot peak the IF transformer trim caps, then you have a fault in the IF amp. Check that the IF amp valve is a good one and that the valve socket voltages are correct.
Try connecting the sig gen to the control grid of the IF valve pin 4 through a .01 ufd cap. If you can tune the last IF transformer now, the the IF valve is working and the last IF transformer is OK. If so, move the sig generator to pin 8 of the 12SA7 and see if the first IF transformer will tune to 455 kcs, if not, the transformer or the 12SA7 is bad or its socket voltages are not correct. All this time, the ground of the sig generator is connected to the chassis through a .01 ufd cap.
Caution. the chassis of the S38 is connected to one side of the mains so I would advise you to use an isolating transformer while doing these tests.
During the IF alignment, the S38 bandswitch should be set to band 1 (Medium Waves). John.
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Old 31st Dec 2003, 6:12 pm   #5
jsel84
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Hi John
Thanks for the tips I will get on this.
I have a correction to to my problem, its not C5 & 6 but but C10 and 11. It's the the two trimmers directly behind the output trannie. On the Alignment ins. it's numbers 5 & 6. On my first 38S those trimmers are the but on the problem unit only the two holes are there and one trimmer is under the chassie in the same place. A friend said they may have done away with the two for production cost??.
Back to your advise, you said
"Try connecting the sig gen to the control grid of the IF valve pin 4 through a .01 ufd cap" what tube are you refering to or is the the 12SK7 (IF amplifier) also?
The voltages are correct but will check again. I also replaced the 12SA7.
When tuning to 455 can I just use a .01 cap directly to the fixed plates of the rear section of the tuning cap?
Oh thanks for the caution I have had that thrill, once was enough
I always double check with my volt meter first.
thanks and regards,
jim

Last edited by Station X; 28th Dec 2004 at 7:15 pm. Reason: Import
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Old 31st Dec 2003, 9:55 pm   #6
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Jim, I noticed on another S38 schematic that adjustment A10 was called adjustment 6 and A13 was called adjustment 5.
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Old 1st Jan 2004, 12:43 am   #7
jsel84
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Hi john
I took your advise and I was able to tune in 455 when I put the SG output to pin 4 on the 12SK7 tube.
I used the .01 cap on the hot and ground of the SG.
That worked great.
When I put the SG on pin 8 there was only a faint sound of 455 from the SG the trimmers didn't do to much.
I rechecked all voltages and all was on the money.
I also changed the 12SA7 tube, no change.
When I decided to re cal the unit from the Alignment Ins.
I was able to tune in 455 on the first IF and also retune the second IF to it highest peak.
What does this tell you. I'll go for a total alignment after I hear back .
thanks
jim
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Old 1st Jan 2004, 4:41 am   #8
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Jim, it seems that the first IF transformer is defective in some way. Check the resistance of the primary and secondary windings - they should be 5 or 6 ohms. If zero ohms, then the associated trimmer cap has a internal short. If very high resistance, like infinity. then you have an open circuit in the coil winding. Often the break is where the fine coil wire connects to the trimmer cap lug and you have a chance to resolder it with the aid of a piece of fine tinned copper wire to bridge the gap. If the transformer is beyond repair then it is quite easy to replace it with just about any other transformer in that IF range. Even a transformer designed for say 465 kcs can be retuned to 455. John.
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Old 1st Jan 2004, 8:47 am   #9
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Default Re: Halicrafters S38 finding 455freq. in IF coil

I tested the first IF coil and got 23 ohms on each pair .
I removed the IF coil from the radio and checked for any open wires. I checked the Black and green wires 23 ohms, red and Blue wires 24 ohms. My meter was set to ohms at 200 ohms.
Its strange, how can the coil be bad if when I put the OP leed of the sig gen directly to the fixed plates on the tuning cap I can get a reaction and tune the first IF coil to a peak. But when I put the OP leed of the sig gen on pin 8 of the 12SK7 tube I only get a faint sound of the sig gen and get a very little reaction when turning the screws on the first IF cap. I also tried this one the other S38 and got the same reaction, what happens is when I put the OP leed on pin 8 the radio sort of loads down a bit when the leed touches pin 8. Who knows
thanks for any clues
jim

Last edited by Station X; 28th Dec 2004 at 7:18 pm. Reason: Import
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Old 1st Jan 2004, 4:02 pm   #10
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Default Re: Halicrafters S38 finding 455freq. in IF coil

It is possible that there is damp in the winding, which will cause a "damping" effect on the coil, making the tuning very broad (the moisture alters the Q of the circuit).

If you have already removed the coil, try leaving it somewhere warm (like the airing cupboard), with the screening can removed, for a few days, to bake out the moisture.

If this cures the problem, you may have to consider drying out the other RF and IF inductors.

Jim.

Last edited by Station X; 28th Dec 2004 at 7:19 pm. Reason: Import
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Old 1st Jan 2004, 4:48 pm   #11
jsel84
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Thanks for the reply but why would both units have the same reaction?
Where the radio is its very dry. Each radio was in different locations in the house.
Does the Ohms tell you that some thing could be wrong?
jim
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Old 1st Jan 2004, 5:27 pm   #12
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Jim, one possibility is that there is a short or low resistance from pin 8 to earth on the 12SA7. The tuning condenser plates touching each other and shorting out might be one kind of fault. John.
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Old 1st Jan 2004, 6:47 pm   #13
jsel84
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Default Re: Halicrafters S38 finding 455freq. in IF co

Hi john
Are saying pin 8 on the 12SA7 tube not the 12SK7?
Is it correct to check the second IF can on pin four of the 12SK7
then for the first IF check pin 8 on the same tube (12SK7).
Or should I be checking the first IF can on pin 8 of the 12SA7.
Pin 8 of the 12SA7 goes to a lug on the tuning cap.
Thanks for any advise on this
Jim
BTW happy new year
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Old 1st Jan 2004, 10:16 pm   #14
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Hi Jim, Yes pin 8 of the 12SA7.
From your measurements it appears that the first IF transformer is OK. The suspicion now is that there is a short from pin 8 of the 12SA7 to earth, the most likely place is in the tuning cap where one of the moving plates may be touching the fixed plates. This will short out your sig gen voltage. John. All the best in 2004 Jim!
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Old 2nd Jan 2004, 7:19 am   #15
jsel84
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Hi john
If I got this correct, I am first checking the second IF can by putting the Sig gen OP on pin 4 of the 12SK7, tuning 455 to a peak.
Then put the sig gen OP on pin 8 of the 12SA7 and tuning 455 to a peak.
If this correct I can do that.
All works great if that is correct.
The understanding I thought was to put the sig gen on pin 8 of the 12SK7 not the 12SA7.
Does this now sound correct?
I have no problem tuning up the radio now
Jim
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Old 3rd Jan 2004, 5:51 pm   #16
JHGibson
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Yes Jim, for the final tune up of the IF transformers, the sig gen goes to pin 8 of the 12SA7 which is the signal grid, G3. This is what the manual means when it says to connect the sig gen to the fixed plates of the rear section of the tuning cap since the rear section is connected to pin 8 of the 12SA7.
Do I understand that you have done this and the radio is working now?
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Old 3rd Jan 2004, 8:23 pm   #17
jsel84
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Hi John,
Yes, I have done this and the Radio is working great.
I may try your hint about taking the wires off of pin 8 eight and tuning the first IF can just for the experence.
I will keep all notes on this for other projects.

I would like to thank you very much for helping me with this project.
I guess you can tell I am not that versed, but willing to learn by the use of this great site.
Many thanks to you and all that helped
Jim
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Old 3rd Jan 2004, 10:04 pm   #18
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Default Re: Halicrafters S38 finding 455freq. in IF coil

That's great Jim. Let me sign off with an interesting note about the S38. On band 4 only (15 to 32 Mcs), the local oscillator runs lower in freq than the sig freq. This has the advantage of doubling the conversion gain and improves the image response (but only with a 12SA7 type frquency changer).
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Old 4th Jan 2004, 5:34 am   #19
jsel84
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Hi John
Not to sure if I understand "This has the advantage of doubling the conversion gain and improves the image response (but only with a 12SA7 type frquency changer)".
Doesn't the S38 use the 12SA7?
Regards
Jim

Last edited by Station X; 28th Dec 2004 at 7:22 pm. Reason: Import
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Old 5th Jan 2004, 3:13 am   #20
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Default Re: Halicrafters S38 finding 455freq. in IF coil

Hallicrafters chose the 12SA7 for the frequency changer in the S38 because of those useful characteristics above 15 mcs that I described. You can find a detailed description why, in the "Radiotron Designer's Handbook" 4th edition by author Langford-Smith in chapter 25.2

Last edited by Station X; 28th Dec 2004 at 7:23 pm. Reason: Import
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