NC300

[KM1H]

Dont think Id ever own a radio that looks like Miss Piggy got loose in it; I hate those friggin things. Reminds me of the typical schlock TV/Radio shop quality (lack of) of old.

A huge problem with Collins was they didnt know anything about ham requirement AGC or gain equalization in the 75A series. The first thing I did to my A4 in 65 was install 7360's in both mixers to eliminate the overload problems and a 6GM6 RF amp with just enough gain to establish around a 6dB NF on 10M using a HB 5722 noise generator. The front end circuit is quite lossy so the only easy cure is better tubes.

The 6BA7 noise is still less than a 6BE6.

I havent made it to mod time in the Nearfest A3 yet but it is a candidate for a 6ES8 Pullen first mixer for starters. Right now its only been recapped and aligned. Then I'll take it from there but it will be primarily an AM radio (it came with the AM filter) wheras the A4 is strictly for CW and a little SSB.

Todd Ive no idea how many 300's I did at National, several dozen for a minimum and I always found it an easy radio to work on BUT we werent dropping panels yet in the 60's. The manual alignment works well. You can also play with the IF gain in the cathode of V-4 as it was a factory selected value. This gives you some control over front end overload as well as changing V-3 to a 6BY6 and V-1 to a 6GM6 using the same procedure I did above to the A4. The 300 I have here has a noise floor well below antenna noise on 10M so it hears anything any other good radio will.

Also check all those damn screen, AGC and audio resistors. Even if the values arent too far off they can still be noisy. This includes the ones in both mixers....dont take anything for granted.

[W1UJR]

Here is info on carbon comp resistors and aging, the author notes "A greatly expanded version of this web page appears in the American  Radio  Relay League's Mar/Apr 2008 QEX magazine."

http://www.cliftonlaboratories.com/carbon_composition_resistors.htm

[W1RKW]

one thing I noticed and I'm glad I did it was replacing the PS electrolytic.  the power transformer seems to run cooler.

ONe of the tubes replaced was the RF amp.  when compared to the new one, the old one was all black inside like a light bulb that had lived longer than expected.  I've never seen a tube do this.  I couldn't see inside it unlike the new one. Anyone ever encountered a tube that had black on the inside of the glass?

[Carl WA1KPD]

" I think the key I've learned in troubleshooting/restoring is to do one segment, then stop and test.
Example, replace the caps in question, then test.

I power it up after every component change.
1. I know if the change worked
2. I am often surprised at how much difference a cap replacement makes

[K5UJ]

I havent made it to mod time in the Nearfest A3 yet but it is a candidate for a 6ES8 Pullen first mixer for starters. Right now its only been recapped and aligned. Then I'll take it from there but it will be primarily an AM radio (it came with the AM filter) wheras the A4 is strictly for CW and a little SSB.

Hi Carl pse excuse the off topic but at some point if you would post your A3 mods for AM after you finish your Nearfest A3 I would very much appreciate it.

Rob

[KM1H]

It might be a bit involved for a post here Rob but I will document it with photos. In the past Id do a lot on the fly and never write things down and then try to rely on a 20-30 year old memory when questions are asked!  These days I might have trouble after 24 hours

Carl

[K5UJ]

It might be a bit involved for a post here Rob but I will document it with photos. In the past Id do a lot on the fly and never write things down and then try to rely on a 20-30 year old memory when questions are asked!  These days I might have trouble after 24 hours

Carl

That would be great Carl tnx--photos would probably be clearer for me to follow anyway if it is not too much trouble; I bet I am not the only A3 owner who would be interested.  We could probably find a place for it--I think there is a receiver topic category on here somewhere.  yes I have to shoot photos sometimes just to see how something goes back together after I take it all apart!

Rob

[W1RKW]

So in a previous post I mentioned that I ordered some replacement resistors and caps for the NC300.  The shipment came yesterday and I inventoried the contents.  All was there but I noticed 2 resistor values in the bunch to be a bit odd.

The majority of the resistors are 1/2 watt types made by Xicon and appear to be the typical size of a 1/2w resistor.  I also ordered a couple 1w and 2w but the values needed were not offered by Xicon so I order the Vishay brand.  The 1w and 2w Vishays are the size of a 1/4w and 1/2w resistor, respectively.  Is this a mistake on Mousers part or are the Vishays able to handle that power in that size package? The labeling states they are 1w and 2w. Never have seen this before.

[Pete, WA2CWA]

Over the years I've bought 2 watt resistors (don't remember the brand) that were the size of the old AB 1/2 resistors. Worked fine.

[KM1H]

The catalog part # on the packing slip should tell you the composition. Id suspect they are a high temp oxide type instead of a carbon film.

With the size difference and low price I stock up on 1W films and use almost everywhere.

Carl

[W1RKW]

I went to the Vishay website and pulled their datasheet.  They are indeed 1watt and 2watt resistors.  So no mistake on Mousers end.

Anyway, replaced some parts and tweaked a few coils and caps.  The receiver is working quite well now.  I'm quitting while I'm ahead 

[W1UJR]

I went to the Vishay website and pulled their datasheet.  They are indeed 1watt and 2watt resistors.  So no mistake on Mousers end.

Anyway, replaced some parts and tweaked a few coils and caps.  The receiver is working quite well now.  I'm quitting while I'm ahead 

Good news!
Hope you enjoy for many years Bob.

I'm going to be on the air this evening with the 30K.

-Bruce

[W1UJR]

I went to the Vishay website and pulled their datasheet.  They are indeed 1watt and 2watt resistors.  So no mistake on Mousers end.

Anyway, replaced some parts and tweaked a few coils and caps.  The receiver is working quite well now.  I'm quitting while I'm ahead  

Good news!
Hope you enjoy for many years Bob.

I'm going to be on the air this evening with the 30K.

BTW, the photos below are where your NC-300 last saw service, at the 2000 Dayton Hamvention AM Special Event Station.

-Bruce

[W1RKW]

Hi Bruce,
Sorry I missed you last night.  We didn't get home until late last night.

Nice pix.  Glad to know the NC300 and Riley H. were in the same room together and the rcvr has some notable history.  It's working quite well. 

I'm home for the next week or so and I plan on being in the radio room.  I finally got the shack completely reassembled after an indoor shower from a broken pipe last summer so it's ready for use.  Even got the work bench clean.  So hope to catch you on the air one afternoon. 

Thanks.
B

[KM1H]

In the 2nd photo it looks like Tim cut a bad one and Riley just got a wiff

[K5UJ]

I was at Dayton in 2000 for the first time and remember seeing that station.  I saw it in the daytime.   I was in the process of getting back on the air after being QRT for about 18 years so I went to the hamvention to shop, tire kick and get product information.  I think when I was at the AM station a 75 m. QSO was in progress.  I remember seeing the Big National Rx and speaker and some great sounding audio booming out of it with selective fading all of which I associated with AM.  Unfortunately for me at the time I viewed AM as a curiosity and was afraid to get any vintage gear for fear I'd wind up with a dust catcher.   So I moved on and wound up with a Ten Tec SSB/CW rig.   Took about nine more years to figure out what I had missed.

[WA1LGQ]

I'm enjoying this thread on NC-300's. It's one of my all time favorite receivers, not for its performance really, though it certainly functions quite well enough for me. There is a nostalgia factor and aesthetics that keeps me in posession of one. Gotta love the slide rule dial, large generous meter, and weighted tuning knob. Whenever I have sold one, it wasn't long when I ended up with another. Once upon a time I came across a Collins 80 kHz mechanical filter and installed it in the thing. Worked very well, too bad I sold it. Anyway Bob, in my current NC-300 after modifiying the audio amp for more low end, etc. I noticed that am signals with good beefy lows had quite a bit of audio distortion in the lows. Something made me look at the AGC circuit and I decided that it needed more C to ground. Sure enough, that was the ticket, huge difference in audio quality. Now after reading this thread, I wonder if there is a bad resistor in the circuit. Next time I get to open up the rx I will have to check it out. I have done alignments on these and also restrung a couple, thats fun. Good luck, have fun Bob. Let me know if you experience the distortion............Larry

[W1RKW]

Hey Larry,
How are you?  Long time.  OK on your NC300s.  I'd be interested in your audio mods. The NC300 I picked up from Bruce sounds pretty good but I think the low end response could be enhanced a tad but otherwise this receiver is pretty darn good.  I may just leave too.  Who knows but let me know when you get the chance. 

HNY
Bob

[WA1LGQ]

Hi Bob, Bruce and I were on 75 the other day and the subject of your rx came up as I have an interest in NC-300's and you got it from him. So he seemed to want to be sure that you did ok with it. Right now mine is in the vast storage facility that you visited once. Once I get my shack cleaned up and make more room, I would like to get the '300 set up. Actually I have space in my main rack right now for it, hmmmm, right on top of the R-390. Ok, maybe it will happen sooner rather than later! But I will bench it first and check out the resistors, and see what I did to the audio amp so I can let you know. Has yours had the cap mod that lowers the IF injection into the detector? Normally the injection is too high and heavily distorts cw and especially ssb signals. The mod basically is just dropping the value of the coupling cap to something like 10pF if I remember correctly. The drawback is that it lowers the output of the detector quite a bit. Still, it is an improvement that you will like.........Larry

[W1RKW]

Hi Larry,
Yes, the NC300 is doing well.  It needed a few parts to get it working optimally.  It's very sensitive now.  The RF amp tube (V1) was toast so it was replaced.  Found a cracked resistor and a couple of others that were more than 30% out so yanked them. Also replaced the PS electrolytic as it leaked a little bit.  As far as I can tell the other capacitors are good. They're close to their labeled values which is amazing for its age.  Other than that the rcvr works beautifully.  I never had a receiver like this before. It's a huge improvement over the SB310 that it replaced.  I really like the tuning dial and the feel of the tuning knob. And for something this old it's pretty stable too.  I've got it sitting next to the Junkston V2 that drives the 813 rig. 

I don't know if this had the capacitor mod.  I'd have to look to see.  I don't think it does though because everything looked stock when I had the chassis open.

[N1KK]

Hello Don,

I just picked one up and after a few bad tubes, all the switch contacts
cleaned, pots cleaned, 2 broken wires I had to solder, Accessory socket
not connected to chassis causing intermittent problem, Cap at audio tube
touching chassis, and dirty audio output tube socket....   I finally got
this thing working.  Besides cleaning, the last thing I have to do is re-string
the band switch string/chain to the frequency wheel drum.

Has anyone done this and how hard is it.  For now I just reach in and move
the drum to the band of choice.

Thanks
Ken
N1KK

Having done two of these I would advise that removing the front panel is probably the last thing one would ever want to do on a NC-300. Even with National's original manual for the rcvr and following their instructions for replacing the dial cord system (which requires the panel to be removed) removing the panel - and then getting it back on and having the dial system still working - is easily worth a whole day of aggravation, a lot of bad language if not more. That said, however, when the NC 300's work properly they are great, probably ranking among the best receivers I have ever owned.

Don, W2DL