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 Post subject: National SW-5
PostPosted: Jan Tue 02, 2018 2:56 am 
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I dug into the National SW-5 today and I guess I will have to go with the song by Meatloaf, "Two out of Three ain't Bad" because two of the audio couplers/transformers ain't bad but the audio interstage transformer is. This National S-51 audio interstage couples the type 30 first audio stage plate to the type 31 output stage push pull grids, the primary is open. Tomorrow I will dissect it to assess whether I want to attempt a rewind or if this will get replaced. The other transformers are fine and I gave the output transformer a moderately low voltage hi pot test and it passed so it should be good at its voltage rating. I have run into leakage issues with some of the 1930s era output transformers in the past but this one appears fine.

I might be able to find another National original from a parts unit but I don't think it is worth the price and risk of failure since all of these old interstage transformers which are wound with very fine gauge wire and acidic insulating paper are at high risk for failure. In transmitters I use the Hammond replacement for the original Stancor A-53C but I am going to try to avoid using something new in the SW-5. Other components including the regen control tested fine although I did find a terminal coming loose on the choke off of the detector but that was an easy fix and the wire wasn't damaged.

And I have to restring the dial drive but at least I have recent practice with that task from the little National sets and this one is easy to access unlike a lot of modern radios.

Some photos soon. And a moment of silence for my dearly departed interstage transformer please.

Rodger WQ9E


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 Post subject: Re: National SW-5
PostPosted: Jan Tue 02, 2018 3:03 am 
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*moment of silence*

Sorry for your loss Rodger - and hope for a complete recovery.

Fortunately there is little that is beyond someone who is really determined - and I'd say your record speaks for itself.

Can't wait for the pics!


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 Post subject: Re: National SW-5
PostPosted: Jan Tue 02, 2018 4:41 am 
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Location: Hidden Valley, AZ
Hi Rodger,

Shoot me a photo of the culprit, I have a bunch of "unknown application" interstages.

73DG

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 Post subject: Re: National SW-5
PostPosted: Jan Tue 02, 2018 5:57 pm 
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DG,

Thanks and will do. I will shoot some photos of the disassembled SW-5 later.

This is a case where I will likely tack in a modern replacement to allow additional troubleshooting and testing while I either repair the original or find a good replacement. All things considered if this one had to lose a transformer better the interstage transformer instead of the detector load/reactor of the push/pull output because it is easy to temporarily substitute a transformer and rewinding isn't out of the question since it is a fairly simple 1 to 3 ratio design with center tapped secondary for driving push pull grids.

Rodger WQ9E


Last edited by rsingl on Jan Tue 02, 2018 7:13 pm, edited 1 time in total.

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 Post subject: Re: National SW-5
PostPosted: Jan Tue 02, 2018 6:55 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Ringoes, New Jersey, USA
Hi Rodger-

The SW-5 should be a fun receiver to operate and I'm sure you will have it running in no time. I have an SW-3 that I am looking forward to getting on the bench after a few other projects are completed. There is a lot of interesting reading on the 'net about these "thrill box" receivers. I will be following this thread with interest.

Matt


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 Post subject: Re: National SW-5
PostPosted: Jan Wed 03, 2018 11:27 pm 
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I did some disassembly of the SW-5 today to start restoration. Sorry I didn't take any photos before starting but I promise some when it goes back together. The cabinet, including face plate,will probably get a light over-spray of matching brown after a thorough cleanup. There isn't any significant rust but there is discoloration and staining and I doubt if it cleans up.

One article on the web identifies this as a version produced for the Wholesale Radio Co. (forerunner of Lafayette) because of the different front dial escutcheon and the switch on the upper chassis corner for switching bias for either AC or battery filament tubes but I am not sure if this is really definitive since the case side has the National diamond logo painted on one case side. In any case it is was sold originally using the DC/battery filament tubes and the sockets are marked as such.

I removed the bad interstage transformer and it looks like I will have some potting tar removal to do. There is plenty of room inside the case if I decide to go with a modern transplant, it appears the interior size is about 2x2.5x3" once the potting tar and current bad transformer is gone.

Interestingly the shrinking/split brass issue that creates a nuisance with the S meter mounting with many older Hammarlund and Hallicrafters receivers was helpful here because the 4 brass rivets that hold the bottom shell to the transformer experienced the same cracking and I removed all 4 with a punch in about 30 seconds rather than the expected drilling. With all of the S meter "shell mounts" I have had to replace/repair it was nice for the cracking brass problem to be of benefit for once. Interestingly the cracking was all on the bottom side of the rivet with the top still in perfect shape which I guess was due either to the way they were installed or some reaction between the rivet and the top of the chassis.

The interior wiring is in good shape except for that used for the power connection which was cracking so I removed that and it will be replaced with some good condition but old wire I have on hand.

As you can see there isn't much under the chassis so like the SW-3 it is a very simple receiver and the added output stage adds very few components. Because of the large chassis size it is even easier to work on than the SW-3.

Rodger WQ9E


Attachments:
SW-5 chassis top.JPG
SW-5 chassis top.JPG [ 196.03 KiB | Viewed 843 times ]
SW-5 chssis bottom.JPG
SW-5 chssis bottom.JPG [ 190.34 KiB | Viewed 843 times ]
transformer.JPG
transformer.JPG [ 155.58 KiB | Viewed 843 times ]
transformer inside.JPG
transformer inside.JPG [ 138.73 KiB | Viewed 843 times ]
Front panel.JPG
Front panel.JPG [ 168.03 KiB | Viewed 843 times ]
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 Post subject: Re: National SW-5
PostPosted: Jan Thu 04, 2018 1:49 am 
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I am starting to really like working on this SW-5. Not only did the bottom cover of the bad interstage transformer come off easily thanks to the cracked brass rivets but when I went back down after dinner to take a close look at the potted inside it was clear that the potting material has shrunk with age. Gently squeezing the sides of the the transformer case broke loose the remaining adhesion between the potting tar with its paper matrix and the outer shell allowing the tar and defective transformer to cleanly slide out. I think this one is just going to get a new 1 to 3 interstage transformer tucked into the old shell and I think I have one with my other transformer stock since I ordered several while Antique Electronic supply was still selling their own low cost version of the Stancor A-53C design.

Rodger WQ9E


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 Post subject: Re: National SW-5
PostPosted: Jan Thu 04, 2018 1:57 am 
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Rodger,

Remove the potting carefully, possibly a heating process.

Shrunk potting breaks fine wires at the transformer, they may be repairable...

Chas

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 Post subject: Re: National SW-5
PostPosted: Jan Thu 04, 2018 2:02 am 
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Thanks Chas,

If I remove the potting I plan to do a full rewind given the failure mode of these old transformers. I HATE dealing with potting compound and the last time I got seriously into it was with my first Hammarlund SP-600 which had the filter chokes potted with the contaminated compound that became conductive over time. I was successful in removing it but I never fully trusted those chokes after the repair and a year later bought two good ones from a later production parts SP-600.

For the short term it will have a replacement stuffed in the shell.

Rodger WQ9E


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