NC300

[W1RKW]

Just picked up a nice looking NC300 from Bruce/UJR.  It's a bit hard of hearing but otherwise works good.  Looking for some general and specific advice on receiver restoration and the NC300.  I measured some of the components and most of the resistors have drifted out of tolerance so I'm contemplating replacing them.  I haven't check any caps yet except the PS electrolytic.  The PS electrolytic I replaced because of ooze but I was wondering if any others should be replaced for the sake of doing it.  It has several tubular types.  W3JN suggested replacing the plastic tubulars, just because.  Should I just replace the other tubular types.  These types have what appears to be wax or what looks like hot glue filled ends.  What type of cap should one use to replace the tubular types?  Any other tips on the NC300 appreciated too. TNX

Bob

[N8UH]

Hi Bob,

I just got done with my NC300 a few weeks ago. Here are some notes:

- I replaced the electrolytics in the power supply section with plain old axial lead caps of slightly higher value. Same with the cathode bypass in the audio section. I left the multi-section there for appearance only.

- The ballast tube, a 4H4-C, still working, was pulled and stored away. These things are made of unobtanium. I used the factory recommended substitute, a 6V6.

- I verified that all of the factory service bulletins were in place, and checked them.

- The paper and wax caps were all replaced with 630V NPOs for good measure.

- I went ahead and, at the recommendation of a couple of others, used the schematic from the NC-303 for the audio section. An easy mod that improved audio and reduced hum a bit.

- Finally, I went through and aligned the beast. That was... interesting...  A very frustrating experience, but I got it close enough.

The result? The receiver works great. Very hot front-end, decent audio, stable, and a frequency readout that is fairly trustworthy.

I like it!

[The Slab Bacon]

Bob,
      It seems like National back in the 50s used some really crappy resistors. I have seen more National receivers with open or way out of tolerance resistors than any other brand. Expect a bad resistor to be the culprit.

Mine made all kinds of scratchy noises, it wound up being the screen dropping resistor in the first RF amp. Then it crapped out again, this time totally deaf. It wound up being the 33 ohm resistor between the grid leak / signal input and the grid of the first RF amp (again). No signs of heat or overload, it just went high in value for what seemed like no reason. I have also seen similar problems with 173s, 183s, and 183Ds. It must just be an era related problem

[W1UJR]

Just picked up a nice looking NC300 from Bruce/UJR.  It's a bit hard of hearing but otherwise works good.  Looking for some general and specific advice on receiver restoration and the NC300.  I measured some of the components and most of the resistors have drifted out of tolerance so I'm contemplating replacing them.  I haven't check any caps yet except the PS electrolytic.  The PS electrolytic I replaced because of ooze but I was wondering if any others should be replaced for the sake of doing it.  It has several tubular types.  W3JN suggested replacing the plastic tubulars, just because.  Should I just replace the other tubular types.  These types have what appears to be wax or what looks like hot glue filled ends.  What type of cap should one use to replace the tubular types?  Any other tips on the NC300 appreciated too. TNX

Bob

Hi Bob,

The other fellows have given some very sage advice.

That was receiver that KW1I and I used at at the AM Special Event Station at the 2000 Dayton Hamfester.
I've got to send you a photo of the set there.

Here is my 2 cents...and that's all its worth.

On the wax paper caps, unless you care about it looking original, I'd use orange drops.

Resistors, how far out of range have they drifted, think most are at least 10% tolerance or so.
I'm not usually too picky on those, unless that are really out of range, or used in a critical bias function.
There was a good QST article a few years back about carbon comps drifting with age, wish I saved it,
I've thrown away most of my newer QSTS - anything 1960 on.

Is it equally deaf on all bands, or worse on some?
Check that antenna connection from the terminal strip/BNC connector, make sure its soldered well.

[W1RKW]

Hey Guys,
I haven't replaced anything yet except the PS electrolytic.  The original chassis mounted multi section electrolytic was replaced with a multi section electrolytic but mounted underneath the chassis.  I pulled both and replaced with a stock value that fit nicely on the chassis. So from above it looks stock.  I don't care about what it will look like underneath as long as it will perform to spec.  So yes Bruce, I'll probably use caps that will change the look underneath somewhat probably but that won't be a concern.  Just want it to work OK Fine.

Discovered that there are two versions of the NC300, maybe even a 3rd.  One version uses a CLC pi network in the PS.  The other uses a CRC pi network in the PS. This vintage receiver uses the CLC pi type. I was confused when looking at the skizmatic because the drawing showed a 300ohm 10w resistor between the 2 caps.  This rcvr uses a choke instead.  It threw me off for a bit.

Anyway, measured about 25% of the carbon resistors on the chassis.  I expected some drift considering the rcvr is 60 or so years old. Some of the resistors are beyond 20% but I'm wondering if I should bother with replacing anything that are less than 20%.  What's your opinion?  John/W3JN says check the screen resistors as they can really affect the sensitivity of the set, as can cathode resistors.  So I'll give them a look see.

Any of you's guys remove the front panel on an NC300?  There are a couple of tubular type caps behind the tone control.  I think I can do surgery on those if need be without removing the panel.

Overall I have a good find here. It just needs some tweaking.

Bruce, I tried some of the other bands and the receiver seems equally hard of hearing or maybe worse but I didn't spend to much time so I can 't definitively say that it is the same on all bands.   I looked at the antenna terminals and everything looks good there.  What I need is a decent signal generator to  make a good measurement and determination. 

Anyway, that's where I'm at, at this moment.

[W1UJR]

"John/W3JN says check the screen resistors as they can really affect the sensitivity of the set, as can cathode resistors.  So I'll give them a look see."

John is right on target here, they are the most critical ones.
I'm amazed at some of the crazy off value units I've found the set still plays well.

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.
Where we really get ourselves into trouble is when we shotgun in a lot of parts, and then
find a wiring or component value error - ask me how I know. 

[W1RKW]

Hi Bruce,
Yes, you're spot on.  I plan on taking it one step at a time rather than biting a big chunk off and not having a point of reference. I've been there and done that with a Johnson V2. I ended up in trouble with not having a point of reference.  A receiver is a bit more complicated so a methodology is necessary.

[W3SLK]

Slab Bacon said:

Mine made all kinds of scratchy noises, it wound up being the screen dropping resistor in the first RF amp. Then it crapped out again, this time totally deaf. It wound up being the 33 ohm resistor between the grid leak / signal input and the grid of the first RF amp (again). No signs of heat or overload, it just went high in value for what seemed like no reason. I have also seen similar problems with 173s, 183s, and 183Ds.

Thanks for solving that riddle for me Frank. My 183D zipps and pops all the time, even in stand-by mode.

[KM1H]

I wouldnt waste my money on Orange Drops or Dips, they offer absolutely no extra benefit and look fat and ugly and dont fit right in place of axials. Use the 630 or 400V films where indicated.

Nationals resistor problems coincided with a change of owners in 1947-8 who threw quality out and had purchasing go to the cheapest possible. OTOH I went thru a customers first run 1946 NC-240D and every resistor was well within spec. A few months go by and I had a later run one and the resistors were crap.

Id check ALL resistors, they all have a purpose. You miss ones in the AGC or audio grid and you then start blaming it on the radio.

Another good mod if you are into stability is to crystal control the 2nd conversion oscillator per the NC-303 circuit. It was an automatic upgrade on sets that came back to National for service.

Carl
National Radio 1963-69
Service Tech, Service Mgr, Sr Engr Aide.

[w3jn]

I advised Bob to ignore grid leak resistors if they're within 50% or so... there's more chance in something getting ruined by removing them, than any detriment to performance they might cause.   It was that very issue on a NC-183 (a grid resistor in the AVC that had drifted 30 percent or so) where I busted a lug off a tube socket in the RF section.  I ended up just replacing the socket pin and lug, but sometimes you have no choice but to replace the whole socket.

That's when I quit carefully desoldering and unwrapping the component leads from the socket, and instead started using quiggs or whatever they're called.  Much quicker and leaves the original solder joint unmolested with no risk of damage to other components.

[The Slab Bacon]

Mikey,
         On a 183 or 183d, check the plate load and plate dropping resistors in the audio fuzz inverter, I have found many of them WAY out in left field!!

Johnny,
          I'm with you 100%, getting the original leads out of the socket pins can be a real P.I.T.A., and I have had the same problems with them. I now make my own "Quiggs" and leave the socket untouched if possible! (Unless it is easy to get at)

 

[Don, W2DL]

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

[KM1H]

JN: National octal socket pins under the changed (ultimate cheapskates)1947-8 management are very brittle and I can usually clip a lead and push an opening thru with a solder-aid. I refuse to use quiggs....looks like something a TV shop would do.

In the HRO-50/60 you can always steal pins from the 4H4C socket...Ive done that a few times unwrapping a lead

I havent broken any 7 or 9 pin sockets....yet.... but have used the Solder-Aid many times in tight areas.

[w3jn]

Mikey,
         On a 183 or 183d, check the plate load and plate dropping resistors in the audio fuzz inverter, I have found many of them WAY out in left field!!

Johnny,
          I'm with you 100%, getting the original leads out of the socket pins can be a real P.I.T.A., and I have had the same problems with them. I now make my own "Quiggs" and leave the socket untouched if possible! (Unless it is easy to get at)

 

Yeah, I built a little tool using an x-acto knife shaft, inserted a cut-off paper clip into the end, and drilled a hole to one side of the pin vise part of the knife shaft, so you just stick one end of the new component's lead into the hold and wrap it around the paper clip.  Instant built-in quigg!

[W1RKW]

Thanks for the heads up on the tubes sockets and being careful removing bad components. 

Here's an update on the rcvr.  Replaced the PS electrolytic, cleaned the controls and switches, replaced a few resistors and a couple of tubes with stuff I had on hand.   Placed an order for some replacement parts.

With what I installed the rcvr has come to life a little bit. There was a slight hum prior to the cap replacement.  That appears to be gone.  Audio is quiet.  Also got rid of a small crackle when the ANT was disconnected.

It will need a good alignment but I'll do that after replacing the paper caps.  So made a little progress.  It sounds pretty good so far. 

[The Slab Bacon]

the biggest thing I just dont like about a NC-300 is that hateful, noisy-assed 6BA7 first mixer. Someone should outlay those tubes for HF use!!

[KM1H]

I never considered the 6BA7 that bad as the 300 is quite sensitive on 10M and sure runs circles around a SX-101.  It was designed for FM use but the cost was way higher than alternatives. Collins used it also in the later 75A series in both mixers which is probably why National went with it. Monkey see, etc.

[The Slab Bacon]

Carl,
      I have no problems with the sensitivity. However, the mixer imparts a little noise of it's own into the received signal. Even +40 signals still drag a little noise through the receiver and are not totally "dead" full quieting.   

[KM1H]

Never heard of that. Sounds like some tube has leakage at high signal levels. It could even be a resistor or mica cap.

Its time to use a scope.

[KB5MD]

What might I ask is a "Quigg?"  Been doing radio repairs for years and have never heard of that one.

[W1UJR]

Quigg's - Pigtail spirals formed by winding small gauge wire around a small drill shaft or sewing needle.

Used to facilitate the replacement of components without damaging or unwrapping the lead from the terminal strip.
The lead on the component is cut off, the "quigg" coil is slid over the remaining lead, sometimes crimped down before soldering.
Saves time and often damage to other components and terminals. Done well it does not look that objectionable, and provides a solid
electrical connection.

I wind mine on a very small drill bit, but W3JN has a rather ingenious method, mentioned above, to do this.

I wouldn't say that Art Collins would approve, its not exactly the double wrap method, but it is a field expedient measure.

[W1RKW]

has anyone ever done an alignment on an NC300?  IF so what should I look out for? Is the alignment procedure a good procedure? The instructions in the manual seem straight forward but was wondering if there are any quirks to look out for.  It appears the IF needs some tweaking because when the IF knob is changed I have to adjust the tuning about 2 or 4 KCs to recenter.  Sensitivity seems pretty good against the FT-102.  Other than the IF, I think the ole beast is in good shape.

[Todd, KA1KAQ]

the biggest thing I just dont like about a NC-300 is that hateful, noisy-assed 6BA7 first mixer. Someone should outlay those tubes for HF use!!

If it's any consolation Slab, virtually every post-1 A series receiver that passed through my hands over the years had already had the 6BA7s yanked and replaced with other tubes. In fact, I can only remember one remaining in stock configuration, and I think that was the 75A-2A that now resides in the Vortex. Seems Collins actually changed out a few part way through A-2 production.

And half the 75A-4s have been updated as well. My front line A-4 subs a 12AT7 for V3 and a 6EA8 for V5, both 6BA7s. I still have a stock A-4 that I'll someday go through and be able to compare against, the other having left 10+ years ago. I'd guess as the technology advanced, hams were eager to work the new tubes into the older rigs. Mine has a 6GM6 in V2 as well.

Never owned a 300, though the tuning knob with that big cast skirt sure feels nice to spin. I'm sure you'll have a nice receiver when you're done going through it, Bob. I bet Carl's seen more than a few pass through his hands over the decades.

[Ralph W3GL]

 

   Regarding the subject of  "Quigg's"

   Sprage has or had an item in their product line called  "KWIKETTE"
   that was an exclusive with them... Way back when, they included a
   small packet of 8 or 10 in their 10 pack box of disk caps; at least for
   a while to advertise the product.

   The "KWIKETTE" was the same as the "Quigg" except that it had a    
   coating of fluxed solder.  Used the same way except you did not have
   to apply the solder  when using the device.

   I found several baggies of them in the desk drawer here   ...

[w3jn]

I can definitely see why some might not like to use them - they do look somewhat ugly, but then so does a socket or tie point with a mass of heated flux, etc.  I just find it MUCH easier, and just as effective, to "quigg" the leads of the new component and solder it right in, after clipping the old component's leads as close to its body as possible.

I also found in the few cases where I've re-stuffed old wax caps with new ones that the quiggs can be neatly concealed inside the stuffed cap, if the new cap is short enough and you go to the trouble of re-melting the wax in the ends of the stuffed cap.

I had two versions of the 75A-2.  One had the old sk00l 75A1 front end with the 6AK5 and 6BE6, and another had (factory) a 6BZ6 RF tube and 6BA7 mixer.  I never compared 'em side by side, I sold the one with the new style front end, but Derb has the one with the old style.