#16
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I might have a transformer that would be a closer fit. I'm looking! |
#17
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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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"Restoring a tube TV is like going to war. A color one is like a land war in Asia." |
#18
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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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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#19
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Tried the 100 uF, no change. The set's got push pull 6V6s, what am I grounding out?
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia." |
#20
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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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Brian USN RET (Avionics / Cal) CET- Consumer Repair and Avionics ('88) "Capacitor Cosmetologist since '79" When fuses go to work, they quit! |
Audiokarma |
#21
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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+??
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia." |
#22
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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?
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia." |
#23
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#24
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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!!!!
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia." |
#25
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Good show! Another great radio restored to service.
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Rick (Sparks) Ethridge |
Audiokarma |
#26
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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....
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia." |
#27
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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.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#28
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jr |
#29
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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. |
#30
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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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Audiokarma |
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