NC-303 & NC-300 small knob

[K6JEK]

I've been looking to get an NC-303 to fool around with.  I'm having no luck and am thinking of settling for the much more common NC-300 with an eye to doing the K0EOO mods described in ER about 100 years ago.  I notiice that 100% of the NC-303's and 1% of the NC-300's have a little knob to the lower left of the main tuning.  Most NC-300's just have a plug there.  But I saw a picture of one that had a knob just like an NC-303.  Is it a little vernier? Was it an option in the NC-300?  Do you suppose I could fabricate the part in a machine shop?

Jon

[W3NP]

Yes, it's a little vernier that was added to the 303 to make it a little easier to tune SSB, since the 303 was introduced as SSB was growing in popularity. To my knowledge it was not an option on the 300, but was probably purchased from National and added to existing 300's. It is just a little push in/pull out shaft with a felt washer on the inside that rides against the main tuning knob skirt. You could probably HB one up if you felt you needed it. I have both receivers here and never use the vernier on the 303.

If I remember correctly, there was a reason for the hole and plug on the 300....was it for the bandswitch shaft to slide out?? I don't feel like taking one apart so does anyone remember? 

[The Slab Bacon]

Yeppir! you have to pop out the plug to pull out the bandswitch shaft. I had to do it to mine.

[ve6pg]

..SAME DEAL HERE..DONT HAVE A 303,BUT 'SEEN A FEW. AS FAR AS AM GOES,I LOVE MY '300. SSB IS TERRIBLE,AS FAR AS DETECTION GOES. JUST A BFO I SUPPOSE.THE 303 MUST HAVE A PRODUCT DETECTOR,IT CANT BE ANY WORSE...TIM..SK..

[The Slab Bacon]

The 300 also has a product detector, but it is a piss poor design. (I have a 300)
It is a self oscilating BFO/product detector using, I believe a 6BE6. The BFO section is too wimpy for strong incoming SSB signals. There are 2 cures (mods) for this. One is to build a seperate BFO that will provide more injection. the other was in an old magazine (I built this one and plugged it into the accessory socket) that uses an amplified AGC circuit with a voltage doubler that knocks the incoming SSB signal down to a level more suited to the wimpy BFO. The old school rule of thumb for "distortion free" SSB reception was that the BFO injection voltage had to be 10x the SSB signal voltage.
                                                   73 de KB3AHE

[nq5t]

much more common NC-300 with an eye to doing the K0EOO mods described in ER about 100 years ago. 

I put the ER mods in an NC-300, and the results, while better than the stock radio, were not all that great.

The NC-303 SSB receive is much better out of the box, and after I got one, the NC-300 was passed on to someone else.  The '303 is a pretty nice radio overall, although it suffers like the NC-300 from SWBC image problems wreaking havoc on 20M especially (at least in my location).  The little trap circuit in the front end that's there to mitigate the problem is relatively ineffective.  Still, I like the radio a lot, mostly because of that beautiful dial :-)

Grant/NQ5T

[w3jn]

One way to decrease product dicktector distortion is to decrease the coupling to the input of the prod detector.  Use a smaller series capacitor, in other words.  This has a direct effect on the recovered audio, but it's better than listening to distortion, IMHO.

Another thing to look at is the output filtering of the prod detector.  Since 50 KC is fairly close to audio, it's not all that easy to filter out the residual RF - the 50 KC RF will tend to intermodulate again with the audio in the first audio toob and create distorion.  A pi-net LPF might be in order, and may help matters.

Last, a VERY common problem with product detecvtors of this vintage is too slow an attack time; IE the signal builds too strongly/too quickly for the AGC to take action on it to eliminate overdriving the detector.

All in all self-excited converters just plain suck.