Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Sep Wed 19, 2018 11:46 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 58 posts ]  Go to page 1, 2, 3  Next
Author Message
 Post subject: National NC60: highly distorted FM broadcast stations on SW
PostPosted: Feb Wed 21, 2018 10:03 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
This is the same NC60 John and I were having the hum / miswireing problem with for so long. Now we have a new mystery problem. Bands 1-3 are working loud and clear as they should. Band 4, 11-30 mhz, is not picking up any shortwave signal but is picking up FM broadcast frequencies. The FM stations are loudly distorted. They sound like the distortion you get with a strong signal overloading the front end or strong adjacent frequency bleed over. I was able to ID 104.7 mhz coming in around 13.5 mhz on the dial. I can pick up strong stations in that frequency band on my Kenwood R2000, including very strong 15 mhz WWV but none of them are being received by the National. I injected a 15 mhz signal into it and it showed up at 15 mhz on the dial.
I’m afraid my troubleshooting skills are not up to the task of figuring this one out. Any and all help would be greatly appreciated!

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Wed 21, 2018 10:22 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12413
Location: San Jose, CA USA
I see this from time to time even on SW receivers that are basically working fine. I haven't tried very hard to understand exactly what's going on, but I suspect that a harmonic of the oscillator is bringing the signal down into the band of interest, or down to the IF frequency. Happens mainly for extremely strong local FM stations. This isn't necessarily an indication of a problem with the receiver.

_________________
Tom K6VL


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Wed 21, 2018 11:15 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
My house is nowhere near any FM stations or I might have suspected that. That's one of the reasons I'm staring at a brick wall and can't see the answer.

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 12:22 am 
Member

Joined: May Sat 12, 2012 1:33 pm
Posts: 1584
Location: Rochester, NY.
What are you using for an antenna? An AM detector won't receive FM wavforms well at all and will sound 'garbled' as you describe.
The upper band FM reception you hear may be called 'images'. Some receivers are more prone to it than others. Image rejection is a measurable quality in receiver specifications.
Some oscillators 'run out of steam' at the higher frequencies and the radio is considered 'deaf' on the top band.
You might try substituting another 12BE6 converter tube or realigning the receiver cap/coil adjustments. Wire and component lead dress and routing is more critical at the higher frequencies.
It is best to leave the lead dress alone and as the factory left it. Rearranging wiring and components can introduce 'buggy' behavior like oscillations, weak gain, hum, images, instability and other maladies.
For a receiver as simple as the NC60, I wouldn't expect great high frequency performance much above 10 mHz, but you should be able to get to the best it can be.


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 1:01 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
I'm using a slinky dipole stretched across the ceiling of the room. This has been my test antenna since I started this hobby. Just to be sure though, I took it out to my ham shack and hooked it to a different slinky antenna with the same results. When we were wrestling with the other problem we tried substitute tubes, including the 12BE6, with no difference. But, since that was an entirely different problem, I'll try it.
I considered doing the alignment procedure but wanted to wait until I got more info and ideas from you guys.

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 1:13 am 
Member
User avatar

Joined: Dec Wed 25, 2013 7:57 am
Posts: 2655
Location: USA
After reading your initial post, the first thing I thought of was "Did he do an alignment?"

_________________
"I got a bad feeling about this." (Han Solo)


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 2:01 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
I have not done an alignment for a couple reasons:
1. We had just fixed the first problem when I discovered the new problem. I didn't want to confuse thhings by before solving the new problem.
2. I'm not sure I really need to. The other 3 bands are loud and clear with plenty of stations. The dial calibration, even on the ptoblem band, is spot on. I verified that on Band 4 by injecting a modulated rf signal at every 5 mhz dial point from beginning to end. Every signal was where it was supposed to be and not distorted at all.

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 7:31 am 
Member
User avatar

Joined: Dec Wed 25, 2013 7:57 am
Posts: 2655
Location: USA
If everything else is ok but sensitivity of Band 4 appears lower than it should be, you may want to try making minor adjustments to Band 4's antenna coil and trimmer (in the alignment procedure).

http://bama.edebris.com/manuals/national/nc60/


Attachments:
Screen shot 2018-02-21 at 10.26.54 PM.jpg
Screen shot 2018-02-21 at 10.26.54 PM.jpg [ 41.08 KiB | Viewed 789 times ]

_________________
"I got a bad feeling about this." (Han Solo)
Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 5:18 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
I'll give that a try and report back. Thanks!

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 5:38 pm 
Member
User avatar

Joined: Oct Thu 18, 2007 11:34 am
Posts: 3763
Location: Port Orchard, Wa 98366
You can try a wave trap on the antenna connections. Also bad solder and ground connections.
There are online calculators for finding resonance values of inputted frequencies.
Some times just a cap across the antenna to ground is all that is needed.

_________________
Visit our Webpage:
http://antique-radio-lab.forumotion.com/


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 7:14 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
Radiosmoker wrote:
You can try a wave trap on the antenna connections. Also bad solder and ground connections.
There are online calculators for finding resonance values of inputted frequencies.
Some times just a cap across the antenna to ground is all that is needed.

What cap value would you suggest? Just attach it across the antenna / ground terminals on the receiver?

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 8:56 pm 
Member
User avatar

Joined: Jan Fri 06, 2012 8:47 pm
Posts: 6421
specops56 wrote:
Band 4, 11-30 mhz, is not picking up any shortwave signal but is picking up FM broadcast frequencies.
Terry, do you have an FM radio anywhere in your house? Is it turned on?

Curtis Eickerman

_________________
http://curtiseickerman.weebly.com


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 9:47 pm 
Member

Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1128
Location: Champaign IL 61822
Wave traps generally don't work for the FM problem. Its mixing of
a high harmonic of the oscillator with FM signals picked up on the
wiring between the bandswitch and tube. I would suspect the same
problem at high frequencies on the dial. All my SW radios that are not
super well shielded (e.g. R390A) have that problem.

Bad solder joints can of course make it worse.


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 11:19 pm 
Member
User avatar

Joined: Dec Wed 25, 2013 7:57 am
Posts: 2655
Location: USA
I agree with Curtis and dtvmcdonald.

A radio's oscillator can emit signals on a number of frequencies. It's possible your NC60 was picking up another FM radio's oscillator. NC60's lower part of Band 4 is near most FM radio's IF. That other radio doesn't necessarily need to be tuned specifically to that (distorted) FM station you were hearing on the NC60.

_________________
"I got a bad feeling about this." (Han Solo)


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Feb Thu 22, 2018 11:47 pm 
Member
User avatar

Joined: Jan Fri 06, 2012 8:47 pm
Posts: 6421
AJJ wrote:
NC60's lower part of Band 4 is near most FM radio's IF. That other radio doesn't necessarily need to be tuned specifically to that (distorted) FM station you were hearing on the NC60.
Yes, if the FM station is being received by an operating FM radio and you have mixing with the FM radio Local Oscillator you can receive one or more FM stations in the neighborhood of 10.7 on your SW radio. The source is actually the FM radio receiver mixer output and/or IF chain.

I used to hear TV audio around 4.5 MHz being emitted from TV audio IF chains in our home. 4.5 MHz was the standard NTSC audio IF just like 10.7 is the standard FM radio IF.

Curtis Eickerman

_________________
http://curtiseickerman.weebly.com


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Wed 07, 2018 6:03 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
Eickerman wrote:
specops56 wrote:
Band 4, 11-30 mhz, is not picking up any shortwave signal but is picking up FM broadcast frequencies.
Terry, do you have an FM radio anywhere in your house? Is it turned on?

Curtis Eickerman

For some reason I didn't get notified of these posts or I would have responded sooner. No, there was no other radio of any kind playing.

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Wed 07, 2018 6:06 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
AJJ wrote:
I agree with Curtis and dtvmcdonald.

A radio's oscillator can emit signals on a number of frequencies. It's possible your NC60 was picking up another FM radio's oscillator. NC60's lower part of Band 4 is near most FM radio's IF. That other radio doesn't necessarily need to be tuned specifically to that (distorted) FM station you were hearing on the NC60.


There has not been any other radio on at any time when working on this one. Also, it's not just the lower part of the band. It's all the way across.

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Wed 07, 2018 6:08 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
I tried the cap across the antenna terminals and it made no difference. I put a homemade shield on the coil on top of the chassis. It made no difference. I put it on the bottom coil and I’m picking up SW stations, weakly, on the band but still have the FM bleed. Only thing I know to try now is aligning it. I'll be doing that Friday with John and will report back.

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Wed 07, 2018 10:44 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2418
Location: Columbia, SC, USA
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

I'm officially stuck.

Terry

_________________
If you can read and write.. thank a teacher.
If you read and write in english.. thank a VETERAN!


Top
 Profile  
 
 Post subject: Re: National NC60: highly distorted FM broadcast stations on
PostPosted: Mar Thu 08, 2018 12:00 am 
Member
User avatar

Joined: Dec Wed 25, 2013 7:57 am
Posts: 2655
Location: USA
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, only on Band 4, acting like a FM radio, except audio is distorted. Correct?

Band 4 still not receiving SW correctly?

_________________
"I got a bad feeling about this." (Han Solo)


Top
 Profile  
 
Post New Topic Post Reply  [ 58 posts ]  Moderators: Marcc, Norm Leal Go to page 1, 2, 3  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot], Lee Petrie and 9 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB