National NC-303 Receiver

[w5hro]

I just acquired this National NC 303 receiver from eBay, see pic below. This is the one receiver I was never able to find years ago and it now completes my longtime receiver wish list. I temporally hooked about a 20’ piece of wire to the hot ant terminal just to see what it would do. I am amazed at how much sensitivity the receiver has and how stable it is on frequency while listening to slopbuckets via the SSB mode, etc. It doesn’t drift at all, amazing

I have searched around and can’t find any service bulletins or modifications anyone has done to one of these receivers. I did find one or two articles on the NC 300, but not the 303.

Does anyone have any info or service bulletins? The serial number is 524 1587. It also has wrinkle finish paint along with the XCU-303 xtal calibrator installed.

[w3jn]

No mods out there probably because (a) they are pretty rare and (b) it doesn't really need any.

As I recall the product detector in the one I had was much better than the one in my present NC-300.  If you think the product detector isn't doing a good job of providing a good low-distortion audio output, I found that the IF injection to the product detector was way too high in the NC-300.  I solved this with a capacitive divider at the IF input of the product detector - a 100pf/1000pf divider IIRC.  If yours sounds good the way it is, leave 'er alone!

[W3GMS]

Congratulations on picking up what appears to be a very clean and good working NC-303.  I have had an NC-303 for about 35 years and I really enjoy operating the receiver.  It works well on all modes and I am sure you will continue to enjoy the performance that the receiver offers.  I will look to see it I have any mod sheets but I don't think I do.
Enjoy!
Joe, W3GMS

[nq5t]

Does anyone have any info or service bulletins? The serial number is 524 1587. It also has wrinkle finish paint along with the XCU-303 xtal calibrator installed.

I dug through my files and found a 1998 email from John Sehring regarding a 1960 Hints and Kinks piece from CQ magazine.  The email indicates the "hint" was from the National factory.

Change C55 from 33 to 5 pf. 
Change R46 from 120 to 270 ohms. 
Interchange resistors R72 and R73 (e.g., make R72 100K and R73 470K).

I have not made these changes to my own 303.  It DOES need a better 9.6 Mhz mixer image trap than the useless one (L18,C86) provided by National.  Great radio, though!

[KM1H]

If you want to push the envelope there are 2 mods to consider.

1. 6GM6 RF amp to improve sensitivity 20M and up. Follow the tube manual spec for voltages, cathode bias, etc. This overcomes some of the 6BA7 mixer noise. Most noticable if you like to play with AM on 10M.

2. Change the 6BA7 to a 7360 or 6ES8 Pullen or standard dual triode mixer. The Pullen gives the best NF and overload performance followed by the 7360 and 6ES8 in that order.

With the 6GM6 and Pullen it will be as good as any R-390A or SS whizbang for digging into the noise. The same mods work on the NC-300 and neither requires any socket changes or metalwork.

If all you use it on is 75M AM leave it alone except for some AGC mods and maybe cooling down the IF gain if its overloading on a real antenna.

I did the 6GM6/7360 change to my 75A4 back in 1965 and still use it slaved to a TS-940. Best radio I own for "DX digability".

Carl
KM1H

[KM1H]

Take it to any automotive paint supplier and have them computer scan it. Chances are it will come up with an already stored match. If not it will tell them the mixing formula.

You can buy a quart and spray yourself or have them put in rattle cans (expensive). Heath SB amps came up with an Alfa Romeo match for me.

I wouldnt go to a box store, the water based crap wont resist scratches well.

Carl
KM1H

[Jim, W5JO]

I may try the automotive paint supplier and have them computer scan like you said, but what about the texture? I came across some articles about creating wrinkle finish, but I don't think is going to be all that easy. What did you guys add to the paint at National to create the texture, do you remember?

Probably the best way is to use any wrinkle finish then overspray with a color to get the shade you want.  If you try to computer match a textured paint, the computer will not give you a good match because of all the shadows in the texture.

[KM1H]

I dont know how National did that and even if the colors were exact between production runs. The HRO-60 certainly wasnt consistent.

Rustoleum has a textured paint in rattle cans only in several colors. Ive used it along with a top coat for a microphone base and Im about to try it on a piece of obscure test equipment.

Carl
KM1H