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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Thu 08, 2018 12:13 am 
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AJJ wrote:
specops56 wrote:
I did the complete alignment. It was a total waste of time. No change. Still nothing but FM bleed all across the dial.
Here’s the results of the alignment:
Band 1 - Frequencies aligned properly.
Band 2 - 1.6 mhz at proper place / 4.5 mhz stuck at 4.4 mhz on dial
Band 3 - 4.0 mhz at proper place ? 12.0 mhz stuck at 11.6 mhz on dial
Band 4 - 11.0 mhz at proper place / 28.0 mhz stuck at 28.9 mhz on dial

Just to clarify: you're hearing local FM stations across the dial, on all four bands, as if this set is acting like a FM radio, except audio is distorted. Correct?

Only on band 4, All other bands work fine.

Alignment is usually worthwhile because that's the only way to be sure sensitivity is peaked even if oscillator/dial alignment is already correct. Did the sensitivity of band 4 improve?

Hard to tell since the only signals I'm receiving on that band is the FM. I can't even pick up 15 mhz WWV which is strong on all my other SW radios.

The result of dial alignment caught my attention. Often, even when the dial would not track all the way across, the alignment procedure would end with the dial at the correct place on the high side, not the bottom. But this radio seemed to be the opposite. Hmmmm....

That's what I thought too but this one is an enigma.


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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Thu 08, 2018 12:39 am 
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(Sorry, you quoted be while I was still editing my post)

Got it.

We can break this up into individual components:

- Rule out an external source as FM interference. Take the radio to a neighbor's house or the local coffee shop. If there's something weird the set that has turned it into a FM set, it'll behave the same. If not, then it's probably a still undetermined interference source near your house

- BTW, I have a hunch that oscillator adjustment for Band 4 might be off. Test: Place your National next to the Kenwood. Tune the NC60 to 15mhz (WWV); if working correctly, the oscillator should be radiating at 15.455mhz...you should be able to hear the oscillator on your Kenwood on that freq, S-meter should move. If not, check nearby frequencies with the Kenwood

- The alignment instructions is not detailed and clear. For this radio, I think you need to alternate alignment adjustments between high/low side of the dial several times. When you adjust the upper part of the dial, it would push the lower part of the dial off; and vice versa. For example: on band 4, align at 28mhz, then do 11mhz..then repeat, again, and again....It should converge to the correct tracking after several cycles

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Thu 08, 2018 3:14 am 
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I'll give those a try and get back to you.

Terry

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Sun 11, 2018 7:54 pm 
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Okay, I've ruled out any external source of the FM interference.
A second radio tuned to 15.455 mhz picks up the oscillator when the National is tuned to 16 mhz instead of 15 mhz.
Haven't tried the back and forth alignment yet but other than getting the dial calibration closer, I don't see how that will solve the FM problem. Starting to get really, really frustrated now.

Terry

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Sun 11, 2018 9:26 pm 
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I understand that your interfering station is running 90,000 watts in what looks like a fairly populated area. Are you anywhere near the red dot on this map?

Attachment:
WNOK Transmitter Location.JPG
WNOK Transmitter Location.JPG [ 45.36 KiB | Viewed 386 times ]


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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Sun 11, 2018 10:42 pm 
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I'm a good 3-4 miles outside the coverage area shown on that map. Even if I were closer that would not be the answer for the following reasons:
1. I'm receiving different stations at different points on the dial. Not just one. I'm even getting some type of weather station.
2. This interference does not happen on any of my other SW radios both tube and solid state.

I can only conclude that it has to be a defect in the National. I just can't seem to find it.

Terry

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Sun 11, 2018 11:44 pm 
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I seem to remember reading that the local oscillator has to be tuned 455 kc ABOVE the received frequency on the 3 lower bands and tuned to 455 kc BELOW the received frequency on the top band. Can someone confirm this?
I found this for an NC-109 on the ohio.edu website and it may apply to yours. This alignment method may be conventional practice in other communications receivers as well:

"Band "B" covers 1.6 to 4.7 MHz. While I could properly peak the RF trimmer for band "B", the set refused to perform properly except at the very top of that band. Tracking was also very poor for the part that did work. On a hunch, I checked the local oscillator with my frequency counter and found that the oscillator had been aligned for 455 KHz UNDER the tuning dial indication. On band "A" and "C", the oscillator was 455 KHz higher than the tuning dial.
With band "B" back in operation, I listened to 80 meter SSB and CW for a while. I then tweaked the alignment for band "C" with no problems.
Band "D" was well off the mark. I determined that for proper tracking of band "D", the oscillator had to be 455 KHz under the tuned frequency".

Many Nationals have band 'A' as the top bandswitch position, harkening back the the 'A' coil drawer for the HRO receivers. Some newer ones don't follow this method and band 'A' is the first position (standard broadcast band):
A = 14mc to 30mc (BS 10M)
B = 7mc to 14mc (BS 20M)
C = 3.5mc to 7mc (BS 40M)
D = 1.7mc to 4mc (BS 80M) E = .9mc to 2.0mc
F = .48 to .92mc(or.5mc to 1.0mc )
G = 180kc to 430kc
H = 100kc to 200kc
J = 50kc to 100kc


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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 12:40 am 
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Take a deep breath Terry. At least for some of us, this is part of the fun. After we get our own radios fixed, we come here to stick our noses in other people's problems :)

One thing I like about radios is that there's always a rational reason for any problem. Given enough data and brain power, problems get solved.

The oscillator test gave good data. Knowing other bands work and that FM stations are across the band on "Band 4" is also good data. Having ruled out an external source is good data too. Can you please do a couple of more things:

- Please record the frequencies of the FM stations you're receiving and where it shows up on NC-60's dial. This will give clues to what your oscillator is doing.

- You previously said a 15mhz signal from the signal generator would correctly show up on NC-60's dial at "15". Can you repeat the test and tell us if the same tone show up elsewhere on the dial? Also note if the other tones are stronger or weaker.

Given the timeframe when this radio was designed, I think Band 4's oscillator was mis-aligned by the previous owner (for 15mhz, he placed the oscillator 455khz below instead of above). I'm not finding good documentation on the internet. I'll wait for someone to confirm.

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 1:57 am 
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I didn't realize there were many interfering FM stations...not just the 104.7 station that you mentioned earlier.

In that case, I think harmonics of the local oscillator could be causing the problem. Are you able to look at the local oscillator's waveform using an oscilloscope?

Dave


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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 2:06 am 
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Don't worry. Since everything else is working properly in the radio, based on the data given so far, the issue is probably in the oscillator coil/trimmer. There are no other components that I can see from the schematic.


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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 2:11 am 
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Wally's correct about the LO frequency: on the NC60, it's 455KHz above the signal frequency on MW and the two lower SW bands, but 455KHz below on the top band. So, when tuned to 15MHz, you should hear the NC60's LO at 14545KHz. If not, realign it before going any further.

Rob


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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 2:46 am 
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...this is turning into a rather interesting thread because it's a problem not often encountered here.

12BE6 is used as the converter in some FM sets. So I wouldn't rule out the possibility that the radio was mistakenly aligned on a harmonic frequency and another set of harmonics in the oscillator is mixing with FM stations.

We'll wait for more data from Terry.

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 3:19 am 
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Dave Doughty wrote:
I didn't realize there were many interfering FM stations...not just the 104.7 station that you mentioned earlier.

In that case, I think harmonics of the local oscillator could be causing the problem. Are you able to look at the local oscillator's waveform using an oscilloscope?

Dave

Unfortunately, I don't have an O Scope because I don't know how to use one. I always wanted to learn though. I'm not formally trained in electronics. Everything I know I've taught myself by working on these tube sets the last dozen years or so. I can find and repair most common problems but something like this that requires much more theoretical knowledge is beyond me.

Terry

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 3:22 am 
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AJJ wrote:
Take a deep breath Terry. At least for some of us, this is part of the fun. After we get our own radios fixed, we come here to stick our noses in other people's problems :)

One thing I like about radios is that there's always a rational reason for any problem. Given enough data and brain power, problems get solved.

The oscillator test gave good data. Knowing other bands work and that FM stations are across the band on "Band 4" is also good data. Having ruled out an external source is good data too. Can you please do a couple of more things:

- Please record the frequencies of the FM stations you're receiving and where it shows up on NC-60's dial. This will give clues to what your oscillator is doing.

- You previously said a 15mhz signal from the signal generator would correctly show up on NC-60's dial at "15". Can you repeat the test and tell us if the same tone show up elsewhere on the dial? Also note if the other tones are stronger or weaker.

Given the timeframe when this radio was designed, I think Band 4's oscillator was mis-aligned by the previous owner (for 15mhz, he placed the oscillator 455khz below instead of above). I'm not finding good documentation on the internet. I'll wait for someone to confirm.


I will try these steps tomorrow and report back. Identifying the FM stations will be difficult since they are virtually unintelligible due to the distortion.

Terry

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 3:24 am 
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shinkuukan wrote:
Wally's correct about the LO frequency: on the NC60, it's 455KHz above the signal frequency on MW and the two lower SW bands, but 455KHz below on the top band. So, when tuned to 15MHz, you should hear the NC60's LO at 14545KHz. If not, realign it before going any further.

Rob

Then it must be right since it was coming in at 15455 with the NC60 set at 16000.

Terry

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Mon 12, 2018 5:09 am 
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Quote:
Identifying the FM stations will be difficult since they are virtually unintelligible due to the distortion.


One or two will do. No need to try too hard if it's difficult. (I just checked radio-locator.com) See if WLXC (103.1) appears near 13.3 and WMFX (102.3) near 13.2.

This may be enough information: On the NC-60, find that same FM station (104.7mhz) that you initially reported hearing when dial is at 13.5mhz. This time, (like what you did with the last test) use another radio to see what frequency NC-60's oscillator is at; check above and below 13.5mhz.

This will give evidence whether the harmonics in the oscillator is mixing with the FM broadcasts.

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Fri 16, 2018 6:51 pm 
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Okay, John and I just finished testing and here is what we’ve found:

FM station across the dial:
96.7 @ 11.5
104.7 @ 12.0
104.7 @ 13.5
106.3 @ 15.5
88.1 @ 15.7
2-3 unidentified stations together @ 16.8
89.7 @ 18.2
Garbage @ 20.2
Garbage @ 21.8
96.3 @ 23.6
97.5 @ 25.2
98.2 @ 26.6

Injected signals across dial:
15.0 @ 15.0
20.0 @ 20.0
25.0 @ 25.8
28.0 @ 27.2
30.0 @ 29.3

LO beat frequency:
15.0 on National dial beat at 15.355 on other radio.

So, what does that tell us and where do we go from here?

Terry

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Fri 16, 2018 8:35 pm 
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Have you tried a different converter tube yet?

Once upon a time, I chased my tail for a couple of weeks trying to align an NC33. There were all sorts of screwy things happening on band 4 (highest). The other bands did not track the dial properly, and there were "images" all over the place where they did not mathematically make sense. It also had a null right at 5mhz, completely dead but the LO was running. A new converter tube made all these problems evaporate, and the radio just aligned as it should have.


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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Fri 16, 2018 9:45 pm 
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+1 on what blue_lateral said. I think the converter tube has gone a bit crazy by oscillating at multiple harmonics. I did a couple of calculations:

Since the radio is aligned with the oscillator 455khz below the reading on the dial...
- Dial at 15.0 mhz: oscillator was on 14.545 (1st harmonic). When mixed with over-the-air signal 15.0 - 14.545 = 455khz (IF)
- Dial on about/near 13.5mhz: oscillator was on 104.245 (8th harmonic of 13.025). When mixed with over-the-air signal 104.7 - 104.245 = 455khz (IF)
- Dial on about/near 15.7mhz: oscillator was on 105.845 (7th harmonic of 15.121). When mixed with over-the-air signal 106.3 - 105.845 = 455khz (IF)
- Dial on about/near 26.6mhz: oscillator was on 97.745 (4th harmonic of 24.436). When mixed with over-the-air signal 98.2 - 97.745 = 455khz (IF)
It would take some more testing to completely prove this; I think Band 4's coil and trimmer are ok since they are enabling the tube to oscillate. Only remaining suspect would be the tube.

The converter tube is probably weak and/or antenna trim is out of alignment, so shortwave stations such as WWV is not coming in strong or at all.

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Last edited by AJJ on Mar Fri 16, 2018 10:05 pm, edited 1 time in total.

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 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Fri 16, 2018 9:46 pm 
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I'll give it a try. I'll give anything a try at this point!

Terry

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