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Author Topic: Hallicrafters SX-62A can it be converted from narrow band FM to wide band?  (Read 4441 times)
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Scott SWL
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« on: October 15, 2021, 09:54:58 AM »

Hi,
I have a 2 Hallicrafters SX-62 and I want to make them into my ultimate listening radio, I don't really like the way the narrow band FM works on the broadcast band FM, and I was wondering what is different in the radio that needs to be changed.
if it's just a matter of changing some caps in the IF's or discriminator I would consider doing it.
Currently the local repeater is 29.625 FM, but my receiver picks it up fine on AM with slope detection, so I don't think I would lose much in converting it if I can.
Pic from the web


* sx62a.png (419.11 KB, 780x500 - viewed 249 times.)
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WD5JKO
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« Reply #1 on: October 15, 2021, 02:09:19 PM »



Scott,

  keep in mind that commercial broadcast FM spaces the channels every 200Khz, and the deviation is +/- 75Khz. This means the SX-62 needs to have an IF bandwidth of 150Khz. On 10 FM, above 29 Mhz, the deviation can vary, but is usually around +/- 7.5 Khz.

I know nothing about the SX-62, so I will leave the topic there.

Jim
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WD4DMZ
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« Reply #2 on: October 15, 2021, 04:38:07 PM »

I have two of these radios, a 62 and a 62a, and they sound great with the matching R42 reflex speakers. Where did you read that they are narrow band FM?

Rich
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Scott SWL
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« Reply #3 on: October 15, 2021, 05:59:49 PM »

It's labeled FM, and no where in the manual does it make say wide band. Also when tuning you have 2 or 3 peak tuning areas on the same station   My SX-71 is labeled NBFM on the selector, and acts the same on the lower FM frequencies that are narrow band.
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WD4DMZ
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« Reply #4 on: October 15, 2021, 06:44:19 PM »

Modern stereo FM is multiplexed so that is likely what you are seeing. Both of mine do peak in two different places. Also, I have two SX42 radios that preceded the SX62 and they also find two peaks. On all four of the radios one peak does sound better than the other. But, with the matching speaker even my audiophile son is amazed. The only issue is that they are not stereos. Occasionally I will turn two of them on at once and tune each one to a different peak. Then I have stereo sound with great base that can rattle the windows.

Rich
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Scott SWL
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« Reply #5 on: October 15, 2021, 07:39:58 PM »

Do you need to turn the sensitivity down to get the best sound? Mine seems to saturate  at a point after "8" on the sensitivity dial on the FM bands
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« Reply #6 on: October 15, 2021, 09:03:11 PM »

I believe this receiver is wideband FM and was one of the few to cover the old and new FM bands.

"The first decade of FM broadcasting, starting in 1936 to 1946, occurred in the 42 - 50 MHz band.  The frequency band was changed to the current 88 to 108 MHz band in 1946.  FM stations operating in the old frequency band were required to move to the higher frequencies and most did so before 1948.

Many initial call signs in the FM band were assigned in the following manner:
      an initial "W" or "K", depending on whether the station location was east or west of the Mississippi River,
      followed by two numbers, and
      one or two letters usually representing the community of license, for example "PH" for Philadelphia.
Like K 47 SL (now KSFI, Salt Lake City, UT).
"

https://www.fcc.gov/media/radio/fm-radio-1936-to-1947

http://theradiohistorian.org/fm/fm.html

http://www.wa2ise.com/radios/fm45.html

I did hear one of these one time at a friends house as his dad had one of these. The audio was mono but with a good set of speakers the audio was superb. With two stages of RF amplification it is very sensitive.

I hope you get it tuned up and working.

https://bama.edebris.com/download/hallicra/sx62a/094-901437C_Hallicrafters_SX-62A_MK_2A_Operating_And_Service_Instructions_Aug1961.pdf

AC0OB
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Scott SWL
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« Reply #7 on: October 16, 2021, 04:11:04 AM »

I still have some issues with it, I have recapped everything except for the IF's. I did the discriminator already.
Maybe I need to do the caps in the IF's, and try alignments again.
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WD4DMZ
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« Reply #8 on: October 16, 2021, 11:32:53 AM »

Realignment is a bit touchy. Be careful with the tuning slugs. On one of my SX42 radios I had it near perfect but could not resist making one more tweak and a slug cracked.

The sensitivity control on one of them has an issue as you mentioned. Probably corroded wire. I just back it off a bit.

Good luck.

Rich
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Scott SWL
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« Reply #9 on: October 16, 2021, 11:50:52 AM »

When I bought my Sx-71 it had a broken slug in the converter, I found out because it was almost deaf, and when I tried to align it on its side it started working well, and when I put it upright it stopped working, it was a pain, but I crazy glued it together, and swapped it with the bottom slug, so if it breaks again, gravity will keep it in place.
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« Reply #10 on: October 16, 2021, 02:49:00 PM »


  It is wideband FM.   It should sound just fine on the FM Broadcast band.  Yes you absolutely should change all the caps in the IF and Front end. Some of them are very difficult to get to. There are also several silver mica caps inside the discriminator transformer that need to be changed as well. (or the few SX-42's and 62's I have done needed them) There will probably be a few resistors that have gone high as well.  The low value resistors in the front end, that I'm sure acted as parasitic supressors, were well out out tolerance. They are 15-330 ohm in series with various band coils.  Some installed values do not necessarily match the schematic.  These may be change orders along the way or maybe some fixes for oscillations on the production lines.
    If you just peak the FM IF transformers, you will not get very good sounding FM.  If there was every a receiver that requires a FM sweep generator and scope for alignment, this one is it.  Once you have aligned it correctly, try this method.  Switch to AM first, and tune for minimum recovered audio.  Then flip to FM. You should have the signal centered in the passband at that point and it should sound great.  It is not the most sensitive FM receiver on the planet, but it does work well.  It is a very cool old boat anchor.

  Good luck!

  Steve
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Scott SWL
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« Reply #11 on: October 16, 2021, 06:52:07 PM »

I love the look of the receiver and this winter it will get the IFs recapped and the 62B's AFC circuit added, all of the caps have been done, as well as most of the out of spec resistors.
I have never done the sweep method of alignment, I have a FM Stereo / mono signal generator that has a sweep function, I need to figure out how to use.
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Scott SWL
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« Reply #12 on: October 19, 2021, 03:43:36 AM »

Realignment is a bit touchy. Be careful with the tuning slugs. On one of my SX42 radios I had it near perfect but could not resist making one more tweak and a slug cracked.

The sensitivity control on one of them has an issue as you mentioned. Probably corroded wire. I just back it off a bit.

Good luck.

Rich
Did you replace / repair the slug? How do you get the end caps out of the bracket?
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