Purchased a NC-303 today

Discussion in 'Technical' started by W6MQI, Nov 1, 2015.

  1. W6MQI

    W6MQI Member

    First post here just recently came across AM North American forums looks to be a good resource. Anyway I picked up a working NC-303 today from the original owner for a decent price. The unit seems to be in good shape nice a clean underneath no indication hammy hands gettin jiggy inside, and it included to 100kc cal. One thing I noticed there's no 4H4, but a 6V6 instead should I leave this in place or go solid state? I will also be replacing all the electrolytic caps, and some of the other time bombs inside. I understand from reading these are nice receivers I plane to use it with my Johnson viking II, and Ranger. Here's a few pictures

    20151101_172043.jpg 20151101_172432 - Copy.jpg
    20151101_172540.jpg 20151101_172648.jpg
     
  2. W5HRO

    W5HRO Administrator

    If you can find a 4H4 then replace the 6V6, but the 6V6 was a temporary workaround and National released a service bulletin in regards to that (FNS-41) because even back then 4H4's were getting scarce. If you can find a 4H4 be sure to replace the series dropping resistor (R63) that goes to it because they usually all drifted in value a bit.

    Replace all of the electrolytics including all of those white tubular caps. Those leak like crazy and I found that out when restoring mine. Go through the posts in the topic below and the last page is where I replaced those with the new yellow ones. That is a must do.

    NC-303 Receiver Project
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    Attached Files:

  3. W6MQI

    W6MQI Member

    Well that's a lot of information to filter through I have a feeling the 303 will be on the bench for a while sprucing things up. I noticed you changed a couple of caps to enhance SSB operation I might try that and see how it sounds. My primary modes of operation will be CW, and AM no plans to use it for SSB as right now. If you said in the text I missed it, where did you purchase those yellow caps from (Just radios) maybe? About a year or so ago I rebuilt a Hallicrafters SX-28 pre-war model it took me a little over a month to complete, the 303 looks be a much easier unit to work on sort of looking forward to the restoration.

    Dave
     
  4. W5HRO

    W5HRO Administrator

    I think I got those from Antique Electronic Supply, but there are other places that sell them. They used to be made by IC (Illinois Capacitor), but I think they are made over in China now.

    Oh yeah, change C55 from 33pf to 5pF. That will eliminate 95% of the distortion when on SSB. There was also another mod if used on CW which I didn't do where you swap two resistors on that CWO/PD tube stage. I will look for that when I get home tonight.

    You can also change the CWO/PD tube to a 6BY6, but you have to do the opposite by increasing the value of C55 making it larger than the stock 33pF. I didn’t even try it on mine because a soon as I popped in the 6BY6 the gain was way too low. This is just a guess, but a 50pF cap might work and be the right value if using a 6BY6 there.

    You don't have to do the filament regulation mod like I did, it was sort of extreme and I went through a few different things until I figured out the best way to do it. It will make the receiver much more stable doing it though.
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  5. W5HRO

    W5HRO Administrator

    Ok, here is that additional information
    1. Change C55 from 33 to 5 pf.
    2. Change R46 from 120 to 270 ohms.
    3. Interchange resistors R72 and R73 (e.g., make R72 100K and R73 470K).
     
  6. W6MQI

    W6MQI Member

    That would be great if can locate the CW mod.
    Just put together the cap order list not sure yet how far I will go with resistor replacement if I find some wild voltage measurements I'll investigate further then adjust accordingly. Just hate to start ripping resistors the darn rig is so clean under the hood all the solder points are all nicely painted with factory red nobody has touched this radio since it was manufactured it's rather unbelievable. One other question you mentioned using WD-40 to clean tube sockets is this correct? I've never heard of this before I have always used a dab of deoxit on the end of a tooth pick, and spun the tooth pick in each pin hole.

    Dave
     
  7. W6MQI

    W6MQI Member

    Oops,
    We must have posted at the same time didn't see this one.

    Thanks Dave
     
  8. W5HRO

    W5HRO Administrator

    Oh, switching R72 and R73 was a mod in one of the old ham magazines (1959-61) and it improved the 6AL5 CW noise limiter. I forget what changing R46 does, but it connects to the CWO/PD oscillator and also improves something. All 3 items I listed came from the same magazine article so all 3 steps need to be done together. It improves SSB and CW reception both.
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  9. W5HRO

    W5HRO Administrator

    If you haven't done so already don't forget to get the most important cap of them all. I thought I had a pic of this in my NC-303 topic, but it turned out to be in another one. I probably got this at Antique Electronic Supply, but I'm not positive. It will save the power transformer nevertheless.

    main_cap.jpg
     
  10. W6MQI

    W6MQI Member

    I was looking at this the can cap only. https://hayseedhamfest.com/collections/national/products/nc-303-re-cap-kit. Was even thinking of opening up the one in the radio and try to replace the innards myself.
     
  11. W5HRO

    W5HRO Administrator

    Actually the one I posted is way better quality: http://www.tubesandmore.com/products/C-EC40X4-350

    In the end it and the yellow caps would cost you just a little bit more total, but the can cap in that cheap set wouldn't be near as good. Those yellow caps are cheap so maybe close to $50 or just a little more for everything total.
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  12. Mikeinkcmo

    Mikeinkcmo Member

    Congratulations on your new toy. I bought a 303 here last July but haven't so much as turned it on yet. I bought it to compare performance with my old trusty NC300. It's in the queue, but have to get it up and running first.

    In contrast to the general direction of this thread, I don't change out components unless they are actually defective. Electrolytics get "burnt in" for 24 hours before leakage is verified. I use "current sources" so no danger of trashed bench gear because of a bad performer.
     
  13. W5HRO

    W5HRO Administrator

    Generally after you do enough of these old receivers and gain experience with particular models you will discover that's a really bad idea or approach.

    Not changing all of the electrolytics along with those white tubular paper caps in the National receivers will only lead to problems resulting in you having to slide them out of the cabinet every few months trying to figure out what is wrong and that’s if it doesn’t damage something first like the power transformer, resistors and/or IF can windings like when those white tubular caps become a dead short and start passing DC. It’s better to just do it right the first time and eliminate the need for any of that later.

    I wasn’t going to change out those white tubular caps in mine and Carl was giving me shit about it. Then later that same night the one in the CWO/PD circuit finally shorted to where I was seeing the full DC voltage across it. It’s actually the 3rd NC-303 I’ve owned so when I pulled the cover off the other one I still have someone had already replaced that same cap so apparently it was a common problem in the NC-300’s and NC-303’s both.

    The last time I turned on both of my HR0-50 and HRO-60 receivers I noticed things were not quite right anymore so I checked a few of those same tubular caps and found they are leaking badly now too so I need to recap both of those receivers before using them again.

    The first NC-303 I had started humming bad one night after using it for about a month and the main high voltage can cap had started to leak. Luckily I noticed it in time before it took out the power transformer. I happened to walk to that end of the house and heard it just in time. If I had been outside it probably would have been too late and shorted the HV to ground completely.
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  14. W6MQI

    W6MQI Member

    I have to agree it doesn't hurt to replace all the paper, and electrolytic caps. One time I was working on a Johnson Ranger had everything working fine after several weeks of work. when.... KA BOOM one of those black and yellow caps blew up tin foil, and paper every where. The cap was across the meter well one can guess what happen to the meter it was kaput I mean done! Luckily I was able to find anther meter on Ebay I learned my lesson the hard way. One thing I hope I don't need to do is replace a bunch of resistors so hopefully things will work out.

    Dave
     
  15. W5HRO

    W5HRO Administrator

    That NC-300 cap replacement kit you posted is the perfect example because those are the exact caps that need to be replaced in the NC-300 and NC303 both if they still have the original factory caps in place. From looking at your photos it is still factory original. Even if you power it up and it works those caps WILL be leaking to a certain extent and some worse than others. You just won't know it until the really bad ones start shorting out one-by-one.

    Those recap kits exist because because others figured out that those are the bad ones a long time ago. The kit doesn't contain any of the ceramic disc caps because those usually don't go bad in those receivers.
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  16. Mikeinkcmo

    Mikeinkcmo Member

    This has been my SOP for 50+ years, and yes there is some risk, and occasionally you have to lift the hood on one thing or another, but never had any serious damage thus far. Guess I've just been lucky.

    I don't leave radios running unattended.
     
  17. W5HRO

    W5HRO Administrator

    I can guarantee you that if you take an old stock NC-300 or NC-303 that still has those old white tubular paper caps like Dave's has and unsolder or clip the end of each one and connect a HV cap checker and check for leakage that several of them if not all WILL be leaking and some worse than others. The receiver may still work, but with degraded performance because of those leaky caps and you just won’t know it.

    Some of the older National receivers like the HRO’s that have those caps with the wax coatings that get really hot and often drip some of that wax after being turned on for the first time in years are getting hot because they are leaking badly. My HRO-50 started having problems last winter and I checked a few of those caps and the leakage was absolutely horrible. The receiver still worked like that for a long time, but I should have replaced all of them in the very beginning. My HRO-60 has the same problem as well.
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  18. W6MQI

    W6MQI Member

    I don't either especially since I've had two caps blow up in the Ranger though every cap has been replaced now I still don't trust it. Kinda like a weapon always assume it loaded! Had an M60 discharge next to me one time I've never been the same since.
     
  19. Mikeinkcmo

    Mikeinkcmo Member

    Don't forget the cap on the plate of the rectifier. Those eat power transformers in pretty short order.
     
  20. W6MQI

    W6MQI Member

    I recapped the 303 last night and everything seems to be working as per manual except WWV cal. When the cal switch is switched to WWV, dial set to WWV position on 40m I hear nothing no WWV to zero beat too. Also noticed some distortion in SSB mode added a 100pf cap to pin7, V8 seems to have fixed the distortion, but I'll look into Brian's mods for SSB next. Still need to go through and do tube voltage checks, and an alignment all in all not a bad receiver think I'll keep it.

    Forgot to mention didn't do any mods for current regulation yet, but gotta say the receiver is very stable in stock config.

    Dave.
     
  21. W5HRO

    W5HRO Administrator

    You need the XCU 303 calibrator for the WWV cal to work. I found mine not too long ago, but they are almost impossible to find today. It has the place to connect the wire (ANT) for receiving WWV.

    IMG_0647.jpg

    Yes, those mods I posted will correct 95% of the SSB distortion. Mainly changing C55 from 33pF to 5pF will do it.
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  22. W6MQI

    W6MQI Member

    Ah yes section 1.15 clearly states at the end of the paragraph WWV reception is only possible with the XCU 303 well that sucks.
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    After searching for T8 for a few minutes then referring to the schematic T8 is actually T6 nice........ How many more typo's are there?

    In the alignment chart fifth column from the left (Adjust to receive test signal) I assume they mean zero beat the cap or coil to the sig gen signal?
     
  23. W5HRO

    W5HRO Administrator

    That's when you have the 70-ohm or close enough resistor in series with the hot lead of the coax connected to the SO-239 connector if yours has it installed. I made a coax jumper with the series resistor in it and all shielded so I could just screw that into the connector from the generator. I think I used a 68-ohm resistor. Back then most coax antennas like dipoles were considered to be 70 to 72-ohms which they actually are so that’s where that came from, but a 51-ohm resistor there may work just fine too. 70-ohms is a non-standard value though so 68-ohms is close enough.

    In those steps on the chart you adjust the vcap and coils for maximum S-meter reading. It’s not a zero-beat thing.

    The XCU-303 calibrators do pop up on eBay on very rare occasions, but be prepared to pay big dollars for it if one does pop up again. What you could do is modify the XCU-300 you have and add the extra tube on top and make a XCU-303 out of it. Just follow the schematic and parts list.
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  24. W6MQI

    W6MQI Member

    Getting the idea the 80 kc 2nd I.F. is a rather critical alignment if one doesn't find the exact freq to align things too. My first three attempts left CW signals in the "S" selectivity position sounding raspy or rough around the edges. Finally hit a freq were things were acceptable, but not 100% pure just an ever so slight flutter or roughness on the signals. Is what I'm hearing do to the voltage regulation issues possible AC getting through? or what you mentioned earlier about improving CW reception?

     
  25. W5HRO

    W5HRO Administrator

    Yeah, first you need to do that 3 step mod before doing the alignment. That CWO/PD stage is being WAY overdriven with the stock components, mainly C55, the 33pF. If you don't have a 5pF cap just use two 10pF in series, but clip the leads really short where they solder together.
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