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National NC-2-40CS AVC




 
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Author Topic: National NC-2-40CS AVC  (Read 2456 times)
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k7mdo
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« on: January 22, 2019, 11:42:19 PM »

Working on the restoration of an old National from right around WWII.  As received the set was untouched and had sat for a great many years in an unheated storage shed at the Oregon Coast (not a good spot for that).  It didn't really seem to degrade it too much and the chassis is not corroded.  I decided to "bring it back to life" as I couldn't find anyone interested in collecting the behemoth (84 lbs with no cabinet).  This was a receiver used as an aviation base station as far as I can determine and carries the serial number "21" which puts it pretty early.  It was obviously rack mounted and I doubt it ever had a cabinet.

Now the "interesting" problem.  When I first brought it up on a variac just to see if it made HV and filament, it not only didn't smoke or spray capacitor goo all over, it played well...  I didn't stress it too long but did note that the "S" meter read about 3 units as a bottom reading, no input.... seemed OK and it rose on strong stations.... 

Now, after re-capping it (a job of no small effort and a big supply of 0.1 caps), and replacement of a few higher wattage resistors, etc, the "S" meter now sits at about what one would expect as "S-9" with no input at the antenna.  I measured the AVC voltage and found it to be around 8 VDC and it definitely varies up a little with a really strong station but....  it is simply too much AVC....  MVC and CW work fine but of course the "S" meter is not in circuit then... and returns to "0". 


I attached the circuit diagram and I have poked around quite a bit but am hesitant to simply insert a voltage divider of some kind to correct the meter offset. I would like to figure out why it idles at S9 with no signal....


Any thoughts?....

73, Tom


* National receiver.JPG (3947.5 KB, 1731x1155 - viewed 248 times.)
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MikeKE0ZUinkcmo
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« Reply #1 on: January 23, 2019, 07:48:24 AM »

I've never seen the variant you have, but the schematic seems to be the same as mine.   The 240D is a very nice receiver, and I've HAD MINE RESTORED for over a year now and have enjoyed it very much.

 Have you measured the voltages as called out in the chart on page 9/10?   Of particular interest are the voltages for the AVC circuit, and the "Audio Cathode".

Have you verified you installed C-35 correctly?   C-35's "+" lead should go to ground.

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Mike KE0ZU

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k7mdo
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« Reply #2 on: January 23, 2019, 10:01:14 AM »

Mike, thanks for the ideas.  No, I don't have a manual exactly....  Just the circuit diagram..  But yes, caps are correct.  Interestingly, of the 20 or so 0.1's I replaced (all original) not one was out of tolerance....  But I did find numerous resistors up to 25% high in value.

There had definitely been years ago repairs that had particularly poor solder work done, I corrected those.

The one thing that also may need to be removed and tested is the "limiter" potentiometer. It does provide some kind of noise reduction, but not linearly.  And, it affects to some degree the "S" meter offset I am experiencing.  But not much. It is hard to access so I haven't worked on it yet.

The set is extremely sensitive and stable even for SSB listening.....  AM audio is quite good.... Dial calibration is good as well for that era. It has 200-400 kc band rather than AM "talk" nonsense radio😀.

Will figure it out and let you know the outcome, 73, Tom

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k7mdo
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« Reply #3 on: January 23, 2019, 11:44:33 PM »

Acquired the remainder of the manual and started the process of verifying voltages...  So far the main difference I see are all voltages too high.  This set has 110 or 220  or battery operation capability ....  I tried reducing the input too 100 vac but found no appreciable difference....  But now am wondering if the plate transformer or the back panel jumpers are correct... Should have looked there first....  Probably more to follow.  But Mineral Washington AM ham fair and get together calls. 

Tom
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MikeKE0ZUinkcmo
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« Reply #4 on: January 24, 2019, 06:31:41 AM »

What type of meter are you using?   I believe the manual suggests 1K to 5K per Volt types (VOMs) so modern meters will read high.

If the voltages are in the 5 to 10% range even at 117VAC, I wouldn't worry much.
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Mike KE0ZU

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k7mdo
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« Reply #5 on: January 24, 2019, 09:48:09 AM »

Yes, probably the Fluke is too "good".

Mike I am going to take another look at C-35...  When I replaced the filter caps I remember looking real "close" at the original circuit and thinking it was odd
the way it was wired... 

I will tear into it today....  😥

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k7mdo
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« Reply #6 on: January 24, 2019, 09:59:25 PM »

OK, sure enough C-35 was "-" grounded instead of "+" grounded....  now the bad news is that it only made things worse... i.e. the AVC voltage is riding at 12 now instead of 8 and now pegs the "S" meter when AVC is engaged. 

I admit there must be some additional failure either in the wiring or component values...  for now the set works just fine as long as I don't require AVC functionality.

I studied moare today on the manual and the voltages... I even tried backing the variac down so that the plate voltages were actually in line with the published values in the manual.  That turned out to be about 85 VAC on the variac to achieve correct plate voltages... oddly, other than having to increase the RF gain a little, the reception was unaltered... but the AVC voltage remains unacceptably high. 

Will have to shelve the set for two weeks as the XYL has me off to Cuba and the associated islands on a cruise.... but will tackle this on return. (assuming we can get through TSA)


Mike, if for some reason you have the shrouds off your set let me know what the AVC voltage runs on a "normal" healthy receiver.

73, Tom
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MikeKE0ZUinkcmo
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« Reply #7 on: January 25, 2019, 06:08:23 AM »

Did you put in a new cap?   Do you now have -45 or so Volts on the cathode of the audio amp tubes and about -60Volts on the B- common?
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Mike KE0ZU

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k7mdo
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« Reply #8 on: January 25, 2019, 08:35:38 AM »

Mike, I can check tonight...  I am off to "WINTERFEST" ham fair today...  Might be be tubes, etc that I need..... 😁
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k7mdo
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« Reply #9 on: January 25, 2019, 08:20:09 PM »

As Dr. Pangloss says "The best of all possible outcomes!", at the WINTERFEST ham fair today I found another NC-2-40CS.....  relatively unmolested for only $25.  I had originally only been hoping to find some tubes... now I have a "parts-pony".


Unfortunately, this set is NOT an exact duplicate, mine has a factory installed 8 ohm speaker option on the back and I already see a few little differences....  I don't know if this one will play (kind of doubt it at first glance) but it provides me with a circuit example and some nice spares!


I can see the original filter caps were replaced some time long ago so will assume for now correctly and compare to mine.


I can't work on this for 10 days but will report progress (and voltages) at first opportunity.


Thanks again, 73, Tom


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k7mdo
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« Reply #10 on: February 07, 2019, 06:52:06 PM »

I'm back from wife's "impressed" vacation....  working on the National receiver again.

Mike, I am really puzzled by the voltages I find.

Second detector (6SL7)
Grid       -32
plate      189
Cathode -28

A.V.C. (6V6)
Grid        -23
Screen     0
Plate       -20
Cathode  -23

These voltages are WAY out of line and yet, the Limiter works, the radio plays extremely well as long as I don't flip the switch to AVC.... the audio sounds VERY nice, particularly on AM.  The CW oscillator functions very well and allows an easy tune of SSB signals.  The set is very stable.... etc....  I replaced both the AVC and Second Detector/limiter tubes thinking I had a shorted or bad tube with no effect.


When first acquitred the AVC worked though as I said the "zero signal" point on the meter was at about S-3... now the AVC voltage after some recapping has escalated so that the meter pegs when AVC is turned on and the gain of the set reduces the signal throughput to near zero.

The only thing I remember is when I originally acquired the radio, the Phone Jack was a little loose in the front panel and I found that the little screws that held the wafer stack in place were both missing their nuts so the thing was about to fall apart.  I traced the wires and found nothing at that time out of the ordinary and went ahead and put a couple of nuts on the wafer stack to keep it all together....  I have compared it to the "parts pony" I acquired and it seems to be wired OK.


Any ideas?, Tom




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k7mdo
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« Reply #11 on: February 08, 2019, 07:52:35 PM »

Eureka.....  think I have the problem solved.  The noise limiter potentiometer turned out to have an "open".  It is a 1 watt 10k pot (wire wound) and it shunts the plate of the AVC tube to ground... when it is OK.  without the shunt the plate voltage of the AVC tube rises and applies bias to the grids of the IF tubes effectively shutting them off when the AVC was selected.

Thanks Mike for the encouragement...  the set is pretty remarkable for the age and unless one can't get tubes in another 100 years or so it will still be playing!

73, Tom

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MikeKE0ZUinkcmo
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« Reply #12 on: February 17, 2019, 12:41:17 AM »

Sorry for the delay in getting back to you.   Glad to see you found the problem and the receiver is working well.  These are some impressive receivers considering their age.
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Mike KE0ZU

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k7mdo
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« Reply #13 on: February 17, 2019, 09:14:39 AM »

While at the Salem ham fair yesterday I found an original manual for a NC-2-40D. I purchased it and see quite a few differences, particularly in the audio output area.

The CS (mine) has three selections including 8 ohm, 500 ohm, and 20,000 ohm while the D only appears to have 20,000 ohm for speaker connection shown on the circuit diagram.  However, the set I purchased for "parts" has both 500 ohm and 20,000 ohm but no 8 ohm connection.

Seems there were a lot of variations?I

Nevertheless the push pull 6V6's really put out a pleasant bunch of audio.

73,thanks again, Tom

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k7mdo
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« Reply #14 on: August 16, 2019, 12:00:10 AM »

OK folks, here is the "finally" finished product....  I was able to find a cabinet with the correct height to accommodate the normally rack mounted NC-240CS that has been under restoration now for close to one year.  I was able to bring it to life thanks to a lot of advice here on the forum.  The cabinet is some stray computer one from the last few years and had the correct height for the front panel.  The cabinet was white which didn't match the front of the radio so I painted it a camouflaged color from WWII.  In my junk I had a pair of handles and some heavy duty feet that got applied to the cabinet first.  Quite satisfactory.

Plays great, stable after only a few minutes, tubes are easy to find and are apparently not sought after by the audio folks. Those push pull 6V6's are hard to beat.

This particular model has three audio outputs 8/500/20,000 ohms which I have not seen on any others I have come across.

Thanks again to all for help.

73, Tom


* NC-240 small.jpg (143.75 KB, 612x408 - viewed 113 times.)
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MikeKE0ZUinkcmo
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« Reply #15 on: August 19, 2019, 10:03:24 PM »

Nice looking radio, great to see the finished project!!!

Whats next?
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Mike KE0ZU

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KA0HCP
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« Reply #16 on: August 22, 2019, 02:03:55 PM »

Very nice.  It is exciting reading about some of the venerable Nationals being restored.  I've got an unmolested 1935 HRO awaiting restoration.
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New callsign KA0HCP, ex-KB4QAA.  Relocated to Kansas in April 2019.
k7mdo
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« Reply #17 on: August 22, 2019, 10:49:38 PM »

It seems like I should have kept a running legend of the radios I have "brought back to life".  Currently I am well into a pair of Hammarlund HQ-150's.  They are both restorable and I have them playing but the "cosmetics" seem to be the difficult part for the radios I seem to come across.  It seems all the families of the hams that bought nice radios in the '40's and '50's seemed to think that storing them in unheated garages here in the northwest of Oregon would "age" them somehow.  It sure does!

Currently the NC-240CS is still sitting in the "driver's seat" of the radio room as it is so surprisingly functional.  I added a 10" internal speaker to that cabinet the other day and am happy to have a "all in one" set!  Pretty sure it now weighs 100 lbs.!


Mike, on the other side of the room sit two HT-37 transmitters that have been gone through... I have a friend that wants one but once working well, they are hard to part with!  Great AM rigs.  Have them paired with my NC-303 which went on line about a year ago. (a fantastic performer)

My other project that continues to be on the front burner is my Gates BC1g that I have now got to the stage of me having to "prune" the final pi network to achieve 75 meter resonance.  I am researching it to see the most favorable technique....  simple "pi" or something more complex.  I have a lot to learn.

In the meantime, hopefully, I don't find another ailing rig on the doorstep for a while!

73, Tom




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