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National NC-270




 
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Author Topic: National NC-270  (Read 930 times)
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n8fvj
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« on: September 14, 2023, 07:30:52 PM »

My main receiver is a Hallicrafters SX-100 broadcast band to 34mHz and my Globe Scout has 6 meters and a Globe V10 VFO with 6 meters. Wanting to use AM on 6 meters I bought a National NC-270 with 6 meters. Not many vintage receivers have 6 meters. An article on QST in 1961 stated NC-270 has a sensitive receiver on 6 meters, but some report now not so good on 6 meters. The radio needs re-alignment to be good on 6 meters. And, changing the 6BZ6 RF tube at 8000gm to a 6GM6 at 14000gm should make the NC-270 sensitive enough on 6 meters.

I will rebuild the power supply and replace out of tolerance resistors and paper caps. I will send out to Technical Specialists in FL (hour drive) for alignment. Any rebuild tips?
Also, what other tube receivers has full coverage of 6 meters?
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Pete, WA2CWA
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« Reply #1 on: September 14, 2023, 09:04:54 PM »

There were a number of HF tube receivers from the late 40's to about the mid 60's that had 6 meter coverage. I can't think of one that was good on 6 meters. Drift was a common problem, mechanical instability of the receiver, while OK on lower frequencies, caused problems above 30 MHz. Good signal to noise ratio on 6 meters was always a problem with many of these receivers. If you look at the NC-270 dial, the most active region of the 6 meter band is probably roughly an inch in length (50.1 MHz to 50.5 MHz).  The rest of the dial coverage is zip for listening entertainment and, since the NC-270 is an amateur receiver, there is no bandspread.

Most 6 meters operators back in the "good old days" opted for low noise 6 meter converters like the 6 meter Ameco CN-50. Your SX-100 is a general coverage receiver so it can be used as a great stable IF amplifier and audio amplifier. The CN-50 is a low noise nuvistor converter and it can be tailored for a number of different low band IF frequencies. 7 to 11 MHz, 14 to 18 MHz, and 28 to 30 MHz. The output of the converter connects to the receiver's antenna input terminals.

I used an Ameco CN-50 for years with my National NC-109.

In 3 years of monitoring 6 meters around 50.4 MHz, even during great Sporadic E band openings, I have heard a total of three AM stations. Non band openings, nothing but static.

6 meters has changed drastically over the last several years. The main mode that has gained prominence is FT-8. Even SSB and CW activity has dwindled. AM seems to be non existent even with a good antenna.

And, as far as antennas, a minimum 3 element rotatable beam should be desirable and it should be as high as possible.   




30 days seems to go fast 
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
n8fvj
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« Reply #2 on: September 15, 2023, 05:54:02 AM »

Pete quote: 'There were a number of HF tube receivers from the late 40's to about the mid 60's that had 6 meter coverage. I can't think of one that was good on 6 meters'.

I do not think a 6 meter receiver has a chance to be good without at least a RF 6BZ6 tube of 8000gm (includes HQ-170, but is deaf on 6 meters needing 150 microvolts signal). The dual RF tube SX-42 with 6AG5s is sensitive. I read a 6GM6 can sub with the 6BZ6 and have 14000gm gain. Some transceivers used a higher gain nuvistor vs 6BZ6 as the Rf amp. The 6BZ6 showed up in the late 1950s. Late 1940s receivers into part of the 1950s is definitely out being deaf on 6 meters with the SX-42 an exception.

Only receivers I know of that have 6 meters is the Hallicrafters SX-42, the HQ-170/180 (deaf on 6), the NC-270 and the National 303 with the rare 6 meter converter installed.
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n8fvj
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« Reply #3 on: September 17, 2023, 02:08:16 PM »

I decided to also buy a NC-155. An eHam reviewer stated it is an improved NC-270 with 1 microvolt vs the NC-270 1.5 microvolt sensitivity. The NC-155 does not have the notch filter and the xtal calibrator was an option. I will keep one, sell the other. Both will get new electrolytic capacitors.
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n8fvj
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« Reply #4 on: September 18, 2023, 11:27:00 AM »

With 245 views and one comment apparently the National NC-270 is not popular in the least with AMers.
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K8DI
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« Reply #5 on: September 18, 2023, 08:08:00 PM »

With 245 views and one comment apparently the National NC-270 is not popular in the least with AMers.
I donít know about popular, but a search of the back issue index for Electric Radio magazine shows five issues with articles about it. I only thought of doing this today because the latest issue arrived in todayís mail.  They like to put a vintage ad on the back, and this month itís an ad for the NC-270Ö.

If you get the magazine maybe you already have them, otherwise they arenít expensive. 

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
n8fvj
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« Reply #6 on: September 18, 2023, 10:24:28 PM »

I ordered two Electric Radio with the NC-270 mods.
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n8fvj
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« Reply #7 on: September 20, 2023, 07:24:44 PM »

Received NC-270 today. DeOxit and new electrolytic caps- works perfect. Old can cap got very hot (lots of leakage). Noise increases a lot on 6 meters when an antenna is attached and an interfering carrier registered S6. The receiver seems sensitive enough and have not installed the higher gain 6GM6 for 6BZ6 RF amp. Should arrive in a few days.
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n8fvj
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« Reply #8 on: September 21, 2023, 05:46:24 PM »

The 6GM6 arrived and has same gain as the 6BZ6. I do not get it.
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K8DI
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« Reply #9 on: September 21, 2023, 06:40:41 PM »

The 6GM6 arrived and has same gain as the 6BZ6. I do not get it.
Gain of a tube is dependent on the gm  or mu, along with both the load resistance and the plate resistance (not the plate resistor but the internal effective tube resistance).  Even if these values were the same between these two types (theyíre not) a change in mu from 8000 to 14000 is only a 4dB gain charge ó a single S unit. The different plate resistance negates part of that. No wonder you donít sense a change ó this tube swap, like many others, is an old wifeís tale, a waste of time.

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
n8fvj
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« Reply #10 on: September 22, 2023, 05:17:02 AM »

NC-270 final thoughts. The NC-270 is a competent dual conversion radio and sensitive using the 6BZ6 RF amp. It needed Deoxit on the band switch and tube sockets, but came alive after one spray and turning the band switch a few times. All pots were quiet. It performs well with AM and SSB is excellent. The NC-270 is a little more noisy compared to my Hallicrafters SX-100. I use an AM COMM noise limiter speaker and the NC-270 is quiet with it. The NC-270 is equipped with a crystal calibrator. Drift is little after 15 minutes warm up and does not drift at all after an hour on SSB at least on the lower bands. Drift would not be noticeable after 15 minutes warm up using AM. The notch filter is overexaggerated per specs and does not meet near 50dB attenuation. It acts like a noise limiter on SSB so I do use it. The ferrite IF filter works, but I do not think the published specs are correct. 3 kHz seems more like 4.5 kHz s and 5 kHz seems like at least 6 kHz. Single side band is a little sharper than 3 kHz, but sounds best on 3 kHz. The NC-270 is a sensitive receiver to 10 meters, little drop off unlike some radios above 14 mHz. 6 meters has a little less sensitivity than 10 meters, but band noise picks a lot when an antenna is connected- a good sign. The low HV is unusual at 145 volts. I use a Sovtek 5Y3GT that has a cathode and adds about 30 volts to the HV. This radio sells for about $110 to $150 average on ebay and for the price I consider it a bargain. The National NC-155 is similar and does not have the notch filter or crystal controlled first IF. Crystal calibrator is standard on the NC-270, optional on the NC-155.

Note- the power transformer operates hot on 120 volt and less on 110 volts. At 160F on 120 volts, it is not an issue like some state and no failures have been noted. I would start having concerns about 180-190F.
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