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 Post subject: National NC-183 running hot
PostPosted: Mar Wed 13, 2019 4:14 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 216
Location: Potomac, MD 20854
I have replaced all electrolytics and about 1/2 of paper caps so far. Radio plays great, but transformer is HOT and I even noticed a little smoke this morning from transformed (upside down on my bench). I THINK original transformer, part SA 3901.

I usually run radios at low line voltage, but this morning measured filament voltage (at transformer pins)
100V on Variac: 5.0 volts filament
110V on Variac: 5.4 volts filament
115V on Variac: 5.9 volts filament (this was when I noticed a little smoke).

I am surprised at 115V not even 6V AC filament voltage. Have not measured B+ yet; I know I have to finish recap before I get too concerned. But any thoughts? Is this a sign my transformer is dying or is this in the range of “normal” behavior? Can I continue to use radio at 110V?

Curiously, I have not found a single resistor over 20% high.

Paul


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Wed 13, 2019 5:29 pm 
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I would disconnect secondary windings from the circuit(s) and see how the transformer behaves w/no load.

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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Wed 13, 2019 5:41 pm 
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Paul,

Compare the ratio of the DC voltage at the rectifier cathode to the screen voltage at the 6V6 plates and see if the drop seems excessive. If so then something is probably drawing too much B+ current.

Have you replaced the two coupling caps from the phase inverter to the output tube control grids? Leakage here will cause a great increase in current draw for this stage and the output stage is one of the few parts of the receiver that can draw enough abnormal current to overheat the power supply transformer and choke without immediately killing itself although the audio output transformer is also very much at risk when this happens.

Rodger WQ9E


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Wed 13, 2019 6:28 pm 
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Joined: May Tue 16, 2017 3:03 pm
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Location: Potomac, MD 20854
rsingl wrote:
Paul,

Have you replaced the two coupling caps from the phase inverter to the output tube control grids?
Rodger WQ9E


Nope, not yet. Only replaced .1mfd and some .05 mfd paper caps. I won’t do anything till I am completely done. Thanks

Paul


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Wed 13, 2019 10:59 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1681
Location: Rochester, NY.
Replace all paper caps. Be aware of any Micamolds that are really paper/foil caps.
I had a leaky tubular ceramic cap (C71) that was passing B+ to the detector on mine. There was also a leaky 6V6 suppressor grid-to-plate 'neutralizing' cap (.002uF), if yours has them. The solder joints for my caps looked factory, although the caps weren't shown on the schematic.
A tube socket pin voltage test may show where the abnormal current draw is by noting any low voltages.
I had a 5U4 and 0D3 tube that would 'flash' brightly occasionally. Replacing both, ended the flashing (internal short?).
Is yours the original transformer? I suspect that mine is a substitute, but all voltages are close to the specs given.
I leave the lid up to reduce thermal frequency drift and to admire the fire-bottles of this beautiful receiver. :D


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Thu 14, 2019 2:18 am 
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Joined: May Tue 16, 2017 3:03 pm
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Location: Potomac, MD 20854
OK, all paper caps replaced. I did not see the .002 mfd neutralizing cap in there; I DO see C71, can I replace that with a 10 pF mica?

Filament voltage still seems low. I PULLED the 5U4GB tube:
at 115V on variac the "5V winding" on transformer measures 5.0V AC
at 115V on variac the "6.3V winding" on transformer measures 6.1V AC. (I measure 5.8V at tube sockets)
at 115V on variac the "HV winding" on transformer measures 580V AV

at 118V on variac I measure 6.3V AC on transformer (I measure 6.05V AV at tube sockets)
at 118V on variac I measure 344V AC on pin4 of rectifier tube (it should be 400V AC)

To get 6.3V AV at tube sockets I need line voltage of 125V, now I measure 361V AC on pin4 of rectifier.

Did not check for hot transformer, wanted to make these measurements quickly and get back to ya'll and turn off rig.

paul


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Thu 14, 2019 5:13 am 
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Location: Long Beach Ms. USA 39560
Is the meter calibration OK in low AC volts?
Pat

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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Thu 14, 2019 5:44 am 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 216
Location: Potomac, MD 20854
Meter is a new DMM, never had problems before. But also low B+ makes me think meter is correct in reading low filament voltage.

Full disclosure; when I got this a couple of weeks ago it was working (I couldn't resist plugging it in) but electrolytic can was a replacement; dual 40 mfd instead of dual 10 mfd as called for in schematic. Also electrolytic minus went to GROUND instead of center tap of transformer; bias was screwed up to the point that RF gain control didn't do anything. I replaced with two new 10 mfd caps and put minus to center tap.

The radio was used by previous owner this way for years; could that have "damaged" transformer? It seems like a 6 volt winding!? If I pull ALL tubes I would remove anything drawing current from system, right?

P


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Thu 14, 2019 12:17 pm 
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Paul,

Pop the rectifier out and see if the power transformer is getting hot without it in place. Only twice have I seen a failure that put a heavy load on the filament line (Viking 1 due to short in base of brand new indicator lamp, RCA ACR-175 with shorted lamp socket) and smoke from the wiring was the immediate result. A failure in the filament bus that will draw unusually heavy current generally makes the cause apparent very quickly.

If there is a problem with the transformer it is probably because of compromised insulation in the B+ secondary from the previous owner running it for a long time after "defeating" the bias system via the miswired filter caps which would put an abnormally heavy load on the transformer due to excessive output stage current draw. The audio output transformer would also have run hot under these conditions so it is suspect if the power transformer stays cool with the rectifier tube removed. Usually that results in one or more shorted turns on the secondary that causes the transformer to very quickly overheat and current draw will be very high even with no load. The filaments can be disconnected by temporarily opening the 3 to 6 link on the AC jumper plug but the 2 to 7 path must remain intact to provide primary power. With the 3 to 6 link open, the only 6.3V AC supplied is to the accessory socket.

I remember reading about this turns short checker several years ago but never tried it. Maybe it is something that would be useful to you, see the attached jpg image. Do NOT hold the button down but just press for an instant and release to watch for the neon lamp to fire, you are basically using your transformer winding to act like a spark coil as it creates a back emf from the collapsing magnetic field. Heavy current will flow so it is a quick press and release, use with caution.

Rodger WQ9E


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Thu 14, 2019 4:23 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 216
Location: Potomac, MD 20854
With rectifier tube pulled I ran the set about 3 minutes at 115V, the transformer DID get warm! (not to top of transformer, but shell under the chassis). I don't know how hot this would compare to when the rectifier is IN, but certainly there was a change from ambient temperature. Is SOME heating in 3 minutes or so with no rectifier normal?

I will look at the jumper plug tonight; i wonder if that could be a suspect and then I will remove jumper 3-6 and see if I now can get 6.3V on my accessory plug with no other filaments in line. I do not know if my transformer in there now is "original" or if it was a replacement. It LOOKS like the others I see on the web (part #SA 3901) but from what I read there was no real consistency and maybe a National replacement.

Thanks to all for the advice, as always. I plan to pull ALL tubes tonight and see if that lightens the burden on the transformer (i.e. no heat).

p


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Thu 14, 2019 11:30 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 216
Location: Potomac, MD 20854
Hmmm, I with the rectifier tube pulled and the 3-6 jumper on the shorting plug open I get no filament voltage on my tube pins (as expected) but I ALSO do not get 6.3V AC on the accessory socket! I have to trace this out, but when I measure resistance from post #3 of transformer to pin#7 of accessory socket I DO measure a short circuit. Hmmm

Also, with no filament voltage to tubes I now measure 6.52V AC at the transformer with 115V line voltage. I wonder why accessory socket not showing filament voltage.

Also, when I have shorting plug open the bottom of transformer DOES still get warm after 3 minutes or so. Doesn't seem to matter if shorting plug is connected 3-6 or not.

paul


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Thu 14, 2019 11:33 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1681
Location: Rochester, NY.
If it is like mine, the power transformer is mounted on a rectangular metal terminal box with solder lugs on the underside of the chassis to connect the wiring harness to. I lifted my transformer up to check the wiring to the solder lugs on the topside. It looked OK.
My transformer gets mildly warm. I can rest my hand on it for a minute without any discomfort, even if it has been on for hours.
You should never see or smell smoke. I would like to see higher voltage readings from yours. A modern high-impedance DMM should actually display voltage readings 5%-10% higher that the published expected tube socket voltages in the NC-183 manual when volt-ohmeters would load some circuits more.
There were some component value changes and mine was closer to the Sams photofact than the factory National manual. The National Service Notes (natsvcnotes) on Bama, has some service tips and test data info for the NC-183 (model 251 or earlier). Mine is serial # 281-0096, which I suspect is a later build.
My choke must have got warm at one time in the past as the surface of the can has surface rust. The paint is gone, if it was painted. It stays fairly cool now.
Having the two 6V6s and the 5U4 in the same corner as the transformer will radiate a bit of heat into that area. Those tubes do run hot normally.
A mica or ceramic should be fine for C-71, if yours is leaky. Mine was causing a rustling sound in the speaker that was intermittent. The National manual says C-71 is 10 pF.
Mine was a 47 pF, which is what the Sams manual calls for.
R3 may also be different as a 470Ω or 270Ω may be called for.
By reducing the transformer load, the voltages should increase, it should run cooler and possibly quieter.


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Fri 15, 2019 1:14 am 
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Location: Costa Mesa, California
Some photos of the radio for those following along.

Norm


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Fri 15, 2019 3:48 am 
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Location: Potomac, MD 20854
Norm, here are my pix. My transformer clearly different than yours. I guess mine was replacement buy LOOKS the same from the top. I wonder if I have a filament short somewhere, will pull all tubes and see if transformer gets hot. I love the look of this radio and I paid nearly nothing for it; Peter Dahl sells a brand new transformer for $118.

I like the color of yours. Did you spray yourself of powder coat? I might have to mail mine away to powder coat; super expensive here.

p


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Fri 15, 2019 4:33 am 
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Location: Potomac, MD 20854
Norm, did you paint your knobs or are they NC-183D knobs? If painted how did you do it? I hate the grey color.

p


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Fri 15, 2019 5:33 am 
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My radio was powder coated. The nice thing about these is that the panel has indented lettering and marking. I then used Lacquer sticks to fill in.

https://www.ebay.com/itm/Lacquer-Stik-P ... 2627!US!-1" -="window.open(this.href);return false;

I just cleaned up the knobs with a scrub brush and Windex and then applied a plastic restorer type spray --like Amor all.

Norm

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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Fri 15, 2019 12:11 pm 
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I am not sure of the exact start and stop dates but for a time during this production period National was installing a .1uf capacitor from one rectifier plate terminal to ground. National referred to these as a "plate filter" for the rectifier tube and it carried a 600 volt DC rating. It shows up on various versions of the schematics of several receivers of the era including the HRO-50T1 and HRO-60. My first NC-183D came with the original manual which did not show the capacitor BUT it had a note in the manual that the receiver was sent back to National for a replacement power transformer and the .1uf capacitor was shorted and removed.

This capacitor was an extremely high failure item and served no useful purpose other than selling replacement transformers. It was removed from most receivers and when it shorted it would take out the transformer before the fuse would open. It is the reason why you find a lot of otherwise original National receivers built during this time that have a non-original power transformer.

Whenever restoring a National receiver built in this time period, always check for that capacitor which will be connected directly between one of the rectifier plate terminals and ground at the rectifier socket. It really should just be removed but if you insist upon having it in place then use a proper capacitor rated for the AC voltage this capacitor will see in normal operation and not one that won't stand the AC environment including all of the line spikes. Nobody else before or after felt the need for such a capacitor at the rectifier terminal and National removed them when the transformer was replaced.

I suspect that is the reason why you have a different transformer Paul.

I wouldn't worry about the lack of 6.3 VAC at the accessory socket with the filament jumper connection pulled at the control/power socket. It really doesn't make sense that the accessory socket would be powered when the rest of the filament circuit is powered down and I suspect that instead of being wired like the schematic your receiver (and probably most others) were wired with the accessory socket filament feed common to the main filament bus so it would only be powered when the rest of the receiver has filament power.

Rodger WQ9E


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Fri 15, 2019 1:19 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 216
Location: Potomac, MD 20854
Norm Johnson wrote:
My radio was powder coated. The nice thing about these is that the panel has indented lettering and marking. I then used Lacquer sticks to fill in.

https://www.ebay.com/itm/Lacquer-Stik-P ... 2627!US!-1" -="window.open(this.href);return false;" -="window.open(this.href);return false;

I just cleaned up the knobs with a scrub brush and Windex and then applied a plastic restorer type spray --like Amor all.

Norm


i thought all NC-183 knobs were grey? Your 183 looks almost exactly like a 183D.


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Fri 15, 2019 2:03 pm 
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Paul,

I have also seen two NC-183D with factory fake wood knobs; same style/dimensions as the other HRT and HRS type knobs but with a wood grain center disc. I have never seen any ad that showed an option for black versus gray and I am not sure how/why the factory chose one or the other but they were available in both colors starting with the 1947 catalogs.

I don't know if the wood knobs were a special build and I have never run across them for sale in a National catalog.

I think either color looks fine on 173/183/183D receivers but the HRO-7 definitely looks best with its beautiful gray/chrome knobs. It was one of the first to use the new "modern" series National knobs.

If your transformer is bad, Hammond sells a replacement transformer through their acquisition of the P. Dahl line but it requires new mounting holes. The price is $118 and they sell through Digikey. You could likely also find a suitable replacement using those basic specs in the stock Hammond line for a better price, especially if you are willing to consider going to a SS bridge rectifier to provide wider options in secondary configuration. You might want to measure the no load primary current draw (either clamp AC meter or put a low value resistor in series and measure voltage drop across the resistor) to see what it is doing but given the smoke, temperature rise, and discoloration I suspect there is a shorted section in the HV secondary putting a heavy load on the transformer.

Rodger WQ9E


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 Post subject: Re: National NC-183 running hot
PostPosted: Mar Fri 15, 2019 7:28 pm 
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Joined: May Tue 16, 2017 3:03 pm
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Location: Potomac, MD 20854
Seems like I have a decision to make with this radio if the transformer is "dying". Putting $118 into a radio I paid almost nothing for isn't too bad, but it also needs a new paint job.

QUESTION: If the power transformer is dying anyway what harm could come from using it until it COMPLETELY dies? (other than burning my house down)

paul


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