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  #16  
Old 06-12-2017, 10:51 AM
dieseljeep dieseljeep is offline
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Okay, for grins, I have three parts chassis here......an Admiral 20X1, an 8TS30, and a GE 810. I'll assume the 810 and 8TS30 are out because each uses two LV regulator tubes, leaving the Admiral chassis.

Think I could make that work, or should I just pony up for the Hammond?
The Admiral transformer has the 5U4 socket on top, so it probably wouldn't fit.
I might have a transformer that would be a closer fit. I'm looking!
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  #17  
Old 06-15-2017, 10:52 AM
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Yep, but damn that Admiral transformer is the right one!!!

B+ levels off at 285, and everything's looking good! I'll just make a socket adapter for the top of the transformer and wire it in to the 5U4 socket underneath.

NOW!! Here's the problem. This thing hums so loud it howls!

The filter caps it called for were two 40 uF @ 475V....someone did a sloppy job of sticking in two pair of 22 uF @ 400, so I replaced them. But the damn thing just howls so loud that I can't tell if I'm getting any stations!

Possibly the filter choke? Spec says it's 17 henries, but I have no idea how to measure that....and no idea what beyond a filter cap would cause that hum. Damn well sounds just like a bad filter cap too.....
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  #18  
Old 06-15-2017, 12:27 PM
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Got a 100uF cap laying around? Try connecting it with clip leads in parallel with each lytic in turn. If that fails try grounding the signal to the output tube grid. Sometimes radios have component failures cause oscillation or tubes develop heater-cathode shorts which inject hum....
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  #19  
Old 06-15-2017, 12:49 PM
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Tried the 100 uF, no change. The set's got push pull 6V6s, what am I grounding out?
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  #20  
Old 06-15-2017, 02:37 PM
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You could be going into some sort of oscillation - do you have the transformer mounted, or hanging loose like in the photo? You may be inducing hum with those clip leads so close to other circuits.

Sometimes an under-filtered power supply will "motorboat" - did you replace the filters with proper 40uF units?

https://en.wikipedia.org/wiki/Motorboating_(electronics)

When it howls does the B+ vary? An under rated (current handling...) transformer could also cause motorboating.
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  #21  
Old 06-15-2017, 02:59 PM
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I replaced them with 40 uF 500V caps I had here. B+ is rock solid. The transformer is now properly mounted, and still motorboating. And the specs of the transformer exceed those of what the radio had.

Here's the schematic: http://bama.edebris.com/manuals/national/nc183d/

The radio screams loud even in "standby" mode, which I assumed shut off the B+??
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  #22  
Old 06-15-2017, 03:27 PM
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Wait, I think I'm on to something.....

The transformer out of the Admiral has two grounds....one large, stiff wire, and one smaller wire both soldered to a chassis lug. The main filter cap in the radio has a ground that is not connected to the chassis....it goes to pin 8 of the 6V6s. Me thinks we're getting somewhere!!

But now what?
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  #23  
Old 06-15-2017, 09:18 PM
dieseljeep dieseljeep is offline
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Originally Posted by Kamakiri View Post
Wait, I think I'm on to something.....

The transformer out of the Admiral has two grounds....one large, stiff wire, and one smaller wire both soldered to a chassis lug. The main filter cap in the radio has a ground that is not connected to the chassis....it goes to pin 8 of the 6V6s. Me thinks we're getting somewhere!!

But now what?
The smaller thinner wire should go to the point where it goes to the main filter cap and also to the pin 6 of the output tubes.
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  #24  
Old 06-19-2017, 07:51 AM
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I GOT IT!!!

After relocating the ground to the filter caps and then finding an open ground for the 6V6 bias, I got everything running smoothly except the B+ was stupid high....about 100V too much. Tried playing with dropping resistors, but the closest I could get it was by changing the 5K dropper to a 1.2K dropper and running it at around 95V. And everything was still getting red hot.

Cliff Benham suggested that I disconnect the positive wire of the 40 uf filter cap on the 5U4 side of the 17 henry choke and reconnect it in parallel with the other 40 uf on the radio side of the choke, making the power supply a "choke input type"...which got me right to a B+ of 260 and the radio was very happy right there

It also provides a much better regulated power supply....one that should technically outlast me

And now lastly, to locate an octal socket extender so that I can plug it into the top of the transformer and wire everything to the chassis 5U4 socket so I can close the lid. It's either that or make a 5U4 hole in the lid, and naaah

She sounds great! This project was a success!!!!
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  #25  
Old 06-19-2017, 11:05 PM
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Good show! Another great radio restored to service.
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  #26  
Old 06-20-2017, 06:44 AM
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Here's one last shot.....a 5V4 gave its life so I could make a socket adapter for the top of the transformer so that I could wire everything to the existing 5U4 socket in the chassis enabling me to close the lid

Weird thing about this receiver.....the gain is off the charts. Local AM stations will overload the RF even with the gain on minimum unless I slightly de-tune them. Even the religious stations on shortwave do the same thing.

I'm not sure if this thing is just an incredible performer, or if there's an issue. To its credit, I was listening to the Voice of Vietnam and caught a QSO between two hams in the UK on it last night, and I've never heard either before....
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  #27  
Old 06-20-2017, 09:37 AM
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Quote:
Originally Posted by Kamakiri View Post
Here's one last shot.....a 5V4 gave its life so I could make a socket adapter for the top of the transformer so that I could wire everything to the existing 5U4 socket in the chassis enabling me to close the lid .
Part of the reason I keep a selection of dud tubes on hand: so I can crunch a dud and reuse it's base if needed....Other reasons include having tubes to rip apart guilt free while explaining their inner workings to a novice, target practice , and cause I can be a bit of a hoarder.
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  #28  
Old 06-20-2017, 06:28 PM
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Quote:
Originally Posted by Kamakiri View Post
Weird thing about this receiver.....the gain is off the charts. Local AM stations will overload the RF even with the gain on minimum unless I slightly de-tune them. Even the religious stations on shortwave do the same thing.

I'm not sure if this thing is just an incredible performer, or if there's an issue.
You might want to check/replace a few wax caps connected to the rf gain control and the avc amp (6AH6) in the back corner of the chassis. Couple of wax .1 uf and some smaller wax caps in that area are prime suspects, IMHO.

jr
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  #29  
Old 06-22-2017, 06:43 AM
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That is a very nice looking National you've got there. I restored an NC98 last year (a baby brother to your radio). For a middle-of-the-road radio when new, the National's seem to perform quite well. Yours is a step up in performance from my NC98, but for casual shortwave listening, both are good sets.

Where your model really outshines the NC98 (and even more so the NC88) is in the ham bands. The NC183 was a very desirable model for ham use, and the NC98 being merely "acceptable". I've been told the lower priced NC88 was known as the "No Copy 88" in the amateur radio world.
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  #30  
Old 06-22-2017, 06:53 AM
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If you have to re-cap that particular National, the website "Hayseed Hamfest" sells a complete capacitor replacement kit for around $40 that includes the correct multi-section filter cap (along with all the other capacitors). The price is a little high, but that filter might be hard to find otherwise if you want a direct replacement.
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