Home Up National NC-88 NC-270 SS4H4-C reg




This is a National NC-300.  The dream of many a boy back in 1956.  This is a 13 tube chassis plus one for the crystal calibrator option.  Receive modes include CW, AM and SSB. This unit in for restoration is a fine well taken care of  example. 

All relevant documentation may be found on BAMA.  Documents include the operator's manual and National Service Bulletins that include several important revisions.  Go to his free site to see the detailed documentation. Bc8.gif (9295 bytes)

I printed out the BAMA schematics and the BAMA mods for the NC300.  The mods are numerous. This chassis has two of them.  This can be good or bad depending on the collectors desires.  I would think the factory mods, like replacing an unshielded wire with shielded wire, would be totally collector acceptable. The mod is fixing a design flaw identified by the factory.

I dim bulb (a kin to a variac) tested and opened up the NC300 today to start the restoration.  I was able to receive station but  the dial is off and was unable to clarify SSB.  All tubes tested good save one. The mixer is bad.  I cleaned the chassis with Clorox clean up, a tooth brush and a rag. Now it is nice and shiny.

There is plenty of room under the chassis for new Electrolytics.  Since this restoration will not have restuffed wax/paper caps I see no harm in the under chassis installation.  If a collectors want a "museum" level restoration to show off the underside of the chassis then all new components would be hidden inside original container, wrappers or what have you. Reformation of the electrolytic can is out of the question.  There is dried electrolyte hardened on the vent hole.  In my not so humble opinion - Unless I have a published life expectancy of the capacitor or the dielectric you are asking for a recall or burned out components when that capacitor eventually dries up, shorts or burns up.

There are a lot of paper capacitors underside.  They are dipped in some plastic like coating.  But that does not stop the deterioration of the internal paper.  Here again, I think it is best just to "shot gun" them out.

I have a habit of resistor replacement as well.  This is a bit on the edge of needs to be done or not.  If I find any out of tolerance resistors in this chassis I will go ahead and shot gun these too.  The challenge is not to uses inductive carbon film resistors in high frequency (relative to the resistor) circuits like the oscillator and mixer section.  I found that in radios that receive less than 18 MHz carbon film resistors are ok.  Any thing above that non-inductive resistors must be maintained.

Initial pictures.  More pictures will be posted as the restoration progresses. 


Reserved for front shot. IMG_1001.JPG (842289 bytes) Before cleaning. IMG_1008.JPG (701077 bytes) Clorox Cleanup, Tooth brush and a rag. IMG_1011.JPG (804875 bytes) Clean behind your ears too.  IMG_1012.JPG (762296 bytes)
IMG_1014.JPG (32185 bytes) Date of manufacture on the oscillator shield.  Resistor replacement will be time consuming with all those leads.  Use Carbon comps ONLY before the IF!. IMG_1000.JPG (967331 bytes)  All replacement resistors inside the shielded box will be carbon composition.  The highest frequency after the 2nd converter is 2.295 mhz. Carbon films are OK.  IMG_1002.JPG (774822 bytes) Homebrew 100 khz  crystal oscillator/calibration unit.   IMG_1013.JPG (38816 bytes)It is beat up but it works!.  I will check its stability.  IMG_1003.JPG (660052 bytes) Oscillator Ballast tube.  I will look for a solid state replacement for this ballast tube.  Save this ballast tube for Show and Tell.  It will burn up and it is made of "Unobtainium".
IMG_1049.JPG (109562 bytes) 

The base of the first mixer is obstructed by the trimmer caps. 

IMG_1050.JPG (141580 bytes) The nuts were removed and the screws hotmelt tacked in to the holes.  IMG_1051.JPG (145736 bytes) Now that makes for an easy resistor replacement.  Carbon comps only! IMG_1056.JPG (124753 bytes) Screws stay in place. IMG_1059.JPG (146441 bytes) Nuts are next.  This was a whole lot easier than the oscillator section.  


Second Converter modification at V3 6BE6

Pictured non factory crystal mod is to improve SSB reception.   

There is the National FSN-48 that replaces this coil with a single 2295 crystal.  FSNs incorporated in this restoration (found on BAMA).

This mod is identical to the NC303 schematic relative to the grid and cathode of V3 6BE6 2nd converter. The caps were checked, resistors replaced.  One resistor was not the same value as the NC303.  But that could be what the owner had in stock.  

The whole assembly was excited with an MFJ Antenna Analyzer.  The analyzer's tuning cap is not fine enough to hold on the exact crystal frequency.  But you can see the analogue meters, SWR and Resistance, meters dip towards zero.  That is good enough until power up.

IMG_1041.JPG (176461 bytes) Tell tail signs of a modification.  Clear spaghetti.  National used black lacquered fabric.  See the unsoldered connector on tube socket pin 2.  That probably created some frustration! IMG_1042.JPG (170592 bytes) Surprise!  L6 has been replaced with a crystal.  The NC303 has a crystal in place of this coil. IMG_1043.JPG (129887 bytes) 

Not bad.

IMG_1047.JPG (95677 bytes) This frequency lines up with the appropriate side band for the band.  No USB available on 40 and 80 for digital modes.  Hey! there was no Ham digital in 1956. IMG_1048.JPG (172049 bytes) Installed with color coded wires.  Green for control grid, yellow for cathodes and black for ground.  Yes this is  not the National black spaghetti.  But this is a mod.


There is not much left to replace.  The electrolytic capacitors and carbon composition resistors (I orders a whole kit with about 500 resistor.  Time to bite the bullet and stop ordering model specific at a few at a time) came in from this past week. 


IMG_1069.JPG (539890 bytes) This radio is finished and receiving SSB on 160m.   IMG_1070.JPG (102347 bytes) IMG_1071.JPG (98758 bytes) IMG_1072.JPG (105589 bytes) IMG_1073.JPG (34795 bytes) The needle is a bit fuzzy while swinging to the SSB modulation.
IMG_1074.JPG (81095 bytes) Starting settings to receive SSB IMG_1075.JPG (57103 bytes) Starting setting for SSB.  IMG_1076.JPG (600991 bytes) IMG_1077.JPG (597691 bytes) IMG_1079.JPG (55226 bytes) Seductive Night Shot.

This unit is finished.  I was picking up stations at one point wondering why the band was so dead.  Then I connected the antenna.  Wow this unit is sensitive.   If you turn the RF gain down then advance the audio gain to full clock wise an integral slide switch increases the RF sensitivity to some thing extreme.  Don't forget to return the volume down before you advance the RG gain.  

I you rotate the AF gain to minimum the integral slide switch steps down the RF sensitivity back to what I am used to.  You really need the operations manual ( to properly operate this unit.  It is unique. 

Back to the Solder and Rosin Smoke! 


FSNs incorporated in this restoration (found on BAMA):

FSN-33 Replaced previously installed "revision" coax with properly measured RG-174.
FSN-36 p1 Shielded cable from V2-9 to T3 B+ installed
FSN-36 p2 installed previously
FSN-36 p3 Antenna input coils not modified. This is to better match a 50 ohm coax feed.
FSN-36 p3 modify antenna coils to better match 50 coax NOT done. May risk destroying antenna coils for little gain.
FSN-37 p2 RG-174 coax used in FSN-33 and -36.
FSN-37 p3 changing 1 pf cap to 1/2 pf cap NOT done. 1/2 pf is too close to stray capacitance to make a noticeable performance change.
FSN-41 replace 4H4-c ballast tube wit 6V6. Tube available for install
FSN-48 Second conversion coil replaced previously with 2295 kHz Crystal. Replaced all resistors with FSN specified values.



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