View Full Version : A perplexing puzzle: National NC-183D power transformer


Kamakiri
06-06-2017, 08:53 AM
Picked up a National NC-183D general coverage receiver at the Rochester hamfest, dirt cheap. Someone went through a lot of trouble with it....has an excellent repaint job, and appears to have been recapped maybe 20 years ago.

The problem is, the power transformer was missing. The seller told me that it had a meltdown many years ago. All the wiring is there, and I was able to clearly identify everything with a copy of the schematic.

Now here's the question.....

I can buy a Hammond transformer, if I want to drop a Benjamin....which I honestly don't. It uses a 5U4 rectifier, so I'd have to believe that I could use *something* else in there, as there's got to be a bazillion parts chassis out there that I could grab a transformer from.

The original is 5V @ 3A for the 5U4, 620V CT @ 180mA, 6.3V @ 6A. The B+ is normally 290 VDC but I understand that it'll operate even better at 240 VDC.

I do have a couple TV parts chassis, but those of course operate at a B+ of about 100 volts higher than this does.

Anyone have any thoughts as to how I can rescue this quality boat anchor on a budget?

dieseljeep
06-06-2017, 10:41 AM
Picked up a National NC-183D general coverage receiver at the Rochester hamfest, dirt cheap. Someone went through a lot of trouble with it....has an excellent repaint job, and appears to have been recapped maybe 20 years ago.

The problem is, the power transformer was missing. The seller told me that it had a meltdown many years ago. All the wiring is there, and I was able to clearly identify everything with a copy of the schematic.

Now here's the question.....

I can buy a Hammond transformer, if I want to drop a Benjamin....which I honestly don't. It uses a 5U4 rectifier, so I'd have to believe that I could use *something* else in there, as there's got to be a bazillion parts chassis out there that I could grab a transformer from.

The original is 5V @ 3A for the 5U4, 620V CT @ 180mA, 6.3V @ 6A. The B+ is normally 290 VDC but I understand that it'll operate even better at 240 VDC.

I do have a couple TV parts chassis, but those of course operate at a B+ of about 100 volts higher than this does.

Anyone have any thoughts as to how I can rescue this quality boat anchor on a budget?

Most of the later 50's BW TV sets didn't use that high of B+ 280-310 volts max. If the set has a filter cap rated at 400 volts, chances it'll be a good match. The potted type RCA transformer, used in the later 50's chassis would be ideal.
The original is a potted type transformer, so possibly, there is no room under the chassis to cut a hole in the chassis to use a half-shell type. :scratch2:

Electronic M
06-06-2017, 10:57 AM
IIRC the LM317/LM337 family of regulators can drop 40V (input to output) a chip and source an amp of current. Three such regulators in series could comfortably drop 100V no problem...Plus they would reduce B+ hum to well below spec, and give you rock stable B+. If I were doing this I'd only drop ~20v in the first reg so if before warmup the B+ goes high without load there is safe headroom.

Kamakiri
06-06-2017, 08:34 PM
It's enough of a stretch for me to do this as it is, let alone trying to figger out how to add that in place ;)

maxhifi
06-06-2017, 10:49 PM
Stay a bit longer at work one day and order up one of these?

http://www.edcorusa.com/xpwr008

I didn't verify dimensions, but specs look fine.

Plenty of ways to kludge something together, power up those junk transformers and see what you've got?

dieseljeep
06-07-2017, 10:51 AM
Stay a bit longer at work one day and order up one of these?

http://www.edcorusa.com/xpwr008

I didn't verify dimensions, but specs look fine.

Plenty of ways to kludge something together, power up those junk transformers and see what you've got?

The transformer shown doesn't have enough 6.3 volt current capacity. :scratch2:

maxhifi
06-07-2017, 11:38 AM
The transformer shown doesn't have enough 6.3 volt current capacity. :scratch2:

You're right, my bad!

Kamakiri
06-07-2017, 12:26 PM
Stay a bit longer at work one day and order up one of these?

http://www.edcorusa.com/xpwr008

I didn't verify dimensions, but specs look fine.

Plenty of ways to kludge something together, power up those junk transformers and see what you've got?

That's the odd part, I don't have any! :)

jr_tech
06-07-2017, 01:02 PM
The transformer shown doesn't have enough 6.3 volt current capacity. :scratch2:

Even the 5 amp spec is a little on the weak side, as the heater currents add up to about 5.25 amps. I suspect that it is the same xfmr used in the 173 and 183 and a couple more tubes were added in the 183D (double conversion) design. No worry there, as xfmr ratings of that era tend to be quite conservative. :scratch2:

jr

Electronic M
06-07-2017, 01:28 PM
It's enough of a stretch for me to do this as it is, let alone trying to figger out how to add that in place ;)

Only need 2 resistors (and maybe a cap for whichever side input or output don't have one) to make one of those regulators tick...Wikipedia has a great wiki on designing circuits for them (and some of the datasheets have even more design templates)...You can pick them up at any radio shack (if you can find one), or many of the online parts houses we get our caps from. A few minutes of napkin math is all that is needed to set one of those regulators up....
Or alternately the lazy man's approach to setting up one of those regs: use a pot in place of the two resistors, set it for max output voltage, hook it to the circuit with a ~100K ohm dummy load and cap on the output, meter the output and adjust the knob till you have what you want....So easy a grade-schooler could do it.:D

If you unlock the mysteries of these regulator chips and have usable scrap power transformers and passive components around you can make virtually any power supply you want, and as many as you want...

Findm-Keepm
06-07-2017, 04:35 PM
I'd use something this, and convert the 5U4 to silicon diodes:

www.ebay.com/itm/Basler-Tube-Amp-Power-Transformer-120-VAC-6-3V-6-5A-350-400V-28-VCT-BE32901001-/201585915421?hash=item2eef74f61d:g:7iQAAOSwcvdXOjS q

EDIT: A nice article on subbing power transformers, 5 pages:

http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0073.pdf
http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0074.pdf
http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0075.pdf
http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0145.pdf
http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0146.pdf

jr_tech
06-07-2017, 05:15 PM
I'd use something this, and convert the 5U4 to silicon diodes:

ww.ebay.com/itm/Basler-Tube-Amp-Power-Transformer-120-VAC-6-3V-6-5A-350-400V-28-VCT-BE32901001-/201585915421?hash=item2eef74f61d:g:7iQAAOSwcvdXOjS q

EDIT: A nice article on subbing power transformers, 5 pages:

http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0073.pdf
http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0074.pdf
http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0075.pdf
http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0145.pdf
http://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Poptronics-IDX/IDX/50s/59/Pop-1959-04-OCR-Page-0146.pdf

this one?
http://www.ebay.com/itm/Basler-Tube-Amp-Power-Transformer-120-VAC-6-3V-6-5A-350-400V-28-VCT-BE32901001-/201585915421?hash=item2eef74f61d:g:7iQAAOSwcvdXOjS q

I didn't get anywhere with the first link

jr

Kamakiri
06-08-2017, 01:46 PM
So give me a rough sketch of how the circuit would be set up.....just a chain of diodes?

Electronic M
06-08-2017, 03:42 PM
So give me a rough sketch of how the circuit would be set up.....just a chain of diodes?

Three (maybe 4 if the B+ drifts high before heaters warm and allow B+ draw) of these circuits strung in series ought to work.
https://upload.wikimedia.org/wikipedia/commons/thumb/2/20/LM317_typical_schematic.svg/330px-LM317_typical_schematic.svg.png

Math to determine resistor values if you want it here:https://en.wikipedia.org/wiki/LM317#Voltage_regulator

The important thing to remember is that these regulators can have any input/output voltage, but the difference between input and output must not exceed 40V. So if say your input voltages changes by more than 20V above that rating you quoted during warmup you need to know how much and plan additional regs set to feed voltages lower than that peak in 40V decrements per stage.

I've used these regs (both the positive and negative voltage variants) in all manner of circuit with good results. I've regulated 200V tube B+ supplies and supplies under 30V with them. I've put 3+ in parallel for high current applications, and I've put several in series where I had to drop some multiple of 40V....It would not be a stretch to say they are my favorite SS part to build/modify things with (I've been buying them by the dozen).

Kamakiri
06-11-2017, 07:27 PM
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?

dieseljeep
06-12-2017, 10:51 AM
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! :scratch2:

Kamakiri
06-15-2017, 10:52 AM
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.....

Electronic M
06-15-2017, 12:27 PM
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....

Kamakiri
06-15-2017, 12:49 PM
Tried the 100 uF, no change. The set's got push pull 6V6s, what am I grounding out?

Findm-Keepm
06-15-2017, 02:37 PM
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.

Kamakiri
06-15-2017, 02:59 PM
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+??

Kamakiri
06-15-2017, 03:27 PM
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?

dieseljeep
06-15-2017, 09:18 PM
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.

Kamakiri
06-19-2017, 07:51 AM
I GOT IT!!! :banana:

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 :D

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 :D

She sounds great! This project was a success!!!!

Titan1a
06-19-2017, 11:05 PM
Good show! Another great radio restored to service.

Kamakiri
06-20-2017, 06:44 AM
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....

Electronic M
06-20-2017, 09:37 AM
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 :pistols: , and cause I can be a bit of a hoarder. :D

jr_tech
06-20-2017, 06:28 PM
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

davet753
06-22-2017, 06:43 AM
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.

davet753
06-22-2017, 06:53 AM
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.

Kamakiri
06-22-2017, 08:25 AM
Oh, it's all recapped....I had everything in stock. Replaced a few out of tolerance resistors as well as the voltage dropping resistors. For caps, I used standard fare. I had to modify the thing enough to get it going.....

Um, these boat anchors seem to be multiplying. I realized this morning that I have to stop as I'm rapidly running out of room :sigh:

Kamakiri
06-22-2017, 08:26 AM
And more :tears:

Kamakiri
06-22-2017, 08:28 AM
BTW - every one of them works, all except the R-390A have been recapped. The NC-303 isn't here yet, I just bought it off the Facebook radios for sale page.

And just think, six weeks ago I didn't have any boat anchors!!

davet753
06-22-2017, 03:35 PM
Oh, it's all recapped....I had everything in stock. Replaced a few out of tolerance resistors as well as the voltage dropping resistors. For caps, I used standard fare. I had to modify the thing enough to get it going.....

Um, these boat anchors seem to be multiplying. I realized this morning that I have to stop as I'm rapidly running out of room :sigh:

Ok, now I'm officially jealous. I love that Hammarlund with the clock. I saw that model at the Knoxville Hamfest last Saturday, but it didn't have the clock option. I need another boat anchor like I need a hole in the head, but I'd make the space for one of those in a heartbeat. I've heard those are really good performers.

Kamakiri
06-23-2017, 06:55 AM
The Hammarlund is seriously impressive....out of all of the boat anchors I have, for listening to SSB QSOs or CW it's probably the best. Drift is minimal....when turned off the heaters on two of the tubes are constantly powered in order to keep the radio on frequency. It's a ham band only receiver. I picked up the Hammarlund and the National that I stuck the power transformer in for $75 for the pair. The Hammarlund only needed a control cleaning and new caps just for good measure. It had seen almost no use over its life.