National NC-300 AGC

[w5rkl]

Good afternoon everyone,

I'm new to AMfone but not new to AM mode. I admit I haven't been very active lately, other things seem to get more
attention. My question is

Can the NC-300 AGC circuit be modified by itself without performing the other NC-300 modifications that are currently on the
internet? I only want to improve the AGC circuit. Any suggestions? I've downloaded and read the many NC-300 mods from
the internet and only want to fix the NC-300 AGC at the moment. Maybe I'll do the others later.

Thanks and 73's,

Mike
W5RKL

[WA1LGQ]

        Hi Mike. I don't know of any AGC mods except for the quickie that I did on mine. I hope some others surface. I noticed quite a bit of distortion on some AM signals, especially those with lots of low end. I did not think that it was coming from the audio amp. All I did was add some C to the agc circuit and it cleaned up nicely. What a difference. I did not document the change, and I would have to open it up to see exactly what I did, but if my memory is correct the change was to C67, at the junction of the two 1 meg resistors. Just bump it up to .1 or larger and with a beefing up of the audio amp it sounds much much better..........Larry

[kb3rdt]

I know this is an old post but mine sounds muddy wonder if i did audio amp beef up now kinda muddy wonder if that fix it or do i need to look deeper if anyone else done any mods to get better responce let me know thanks Carl KB3RDT

[KM1H]

Id start with the mods issued by National as there were several over its production life. And do it after you replace all the paper caps and out of tolerance resistors. The problem with so many hammy hambone mods is they had no idea about fixing the radio first.

http://bama.edebris.com/manuals/national/nc300fsn/

Carl

[kb3rdt]

i done all that so far i open up the amp to get more responce then i bent the pins on i f switch i like more info cause s meter bangs off the end since i change the resistors that went out of value still same muddy i thought that would help   

                                                                                                  KB3RDT

[Jim, W5JO]

Somewhere out there is a mod to add a diode, resistor and change the AGC cap then wire that to the mode switch.  There is an unused terming on that switch that is activated when the switch is in the SSB position and the mod will make it fast attack/slow decay resulting in better sounding SSB.  The change of the cap will also affect the AM postion.  If you have a muddy sound, then something is wrong with the radio.  As Carl said, diagnose the radio first and get it receiving properly, then look at any mods.

[The Slab Bacon]

Somewhere out there is a mod to add a diode, resistor and change the AGC cap then wire that to the mode switch.  There is an unused terming on that switch that is activated when the switch is in the SSB position and the mod will make it fast attack/slow decay resulting in better sounding SSB.  The change of the cap will also affect the AM postion.  If you have a muddy sound, then something is wrong with the radio.  As Carl said, diagnose the radio first and get it receiving properly, then look at any mods.

I did that to mine!! It uses a cathode follower into a voltage doubler IIRC. It is the bees knees for slopbucket reception. I built mine in a minibox and plugged it into the converter socket. that way leaving the receiver original and unmolested. IIRC it also coupled into the AVC through a couple of steering diodes. I gave the docs for the mod to the fellow that bought my NC-300 on saturday. Maybe he'll scan them in and post them here.  

[kb3rdt]

There is a few mods done to mine it has a neg feedback and beefed up the caps for hi-fi maybe need to change the c63 back to what it was

[kb3rdt]

well guys found the muddy sound i f  was all messed up a  friend of mine helped me fix it but i was on 160 other nite tuneing in slopbuckets and he bout knoked off my seat big twang went i said what was that then he sounded like crap first thought i had the rf gain too high so i backed it nope still the same times i wonder if it's my ears or some guys like sounding like crap i like some info on nc-300 mods people have done 

[kb3rdt]

ok guys thanks for the help on nc-300 it was my i-f causing my muddy sound the if contacts where bent so 2 i-f where not working  but now it sounds good until a strong signal then it gets destorted but can back off rf gain but can't hear the other guy as well untill i turn it back up i wondering if you guys ever did any mods to fix this! turning it up and down all the time it was fine untill i replace all the resistors that went out of value but it was bad until i did this had run rf gain all way up and now it's good untill i get overloaded if you guys got any mods that would be a help!