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  #1  
Old 06-06-2017, 09:53 AM
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Kamakiri Kamakiri is offline
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A perplexing puzzle: National NC-183D power transformer

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?
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Old 06-06-2017, 11:41 AM
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Quote:
Originally Posted by Kamakiri View Post
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.
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Old 06-06-2017, 11:57 AM
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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.
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Old 06-06-2017, 09:34 PM
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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
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Old 06-06-2017, 11:49 PM
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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?
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Old 06-07-2017, 11:51 AM
dieseljeep dieseljeep is online now
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Originally Posted by maxhifi View Post
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.
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Old 06-07-2017, 12:38 PM
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The transformer shown doesn't have enough 6.3 volt current capacity.
You're right, my bad!
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Old 06-07-2017, 01:26 PM
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Quote:
Originally Posted by maxhifi View Post
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!
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Old 06-07-2017, 02:02 PM
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Originally Posted by dieseljeep View Post
The transformer shown doesn't have enough 6.3 volt current capacity.
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.

jr

Last edited by jr_tech; 06-07-2017 at 02:13 PM.
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  #10  
Old 06-07-2017, 02:28 PM
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Originally Posted by Kamakiri View Post
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.

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...
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Old 06-07-2017, 05:35 PM
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I'd use something this, and convert the 5U4 to silicon diodes:

http://www.ebay.com/itm/Basler-Tube-...QAAOSwcvdXOjSq

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

http://www.americanradiohistory.com/...-Page-0073.pdf
http://www.americanradiohistory.com/...-Page-0074.pdf
http://www.americanradiohistory.com/...-Page-0075.pdf
http://www.americanradiohistory.com/...-Page-0145.pdf
http://www.americanradiohistory.com/...-Page-0146.pdf
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Last edited by Findm-Keepm; 06-07-2017 at 08:03 PM.
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Old 06-07-2017, 06:15 PM
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Quote:
Originally Posted by Findm-Keepm View Post
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/...-Page-0073.pdf
http://www.americanradiohistory.com/...-Page-0074.pdf
http://www.americanradiohistory.com/...-Page-0075.pdf
http://www.americanradiohistory.com/...-Page-0145.pdf
http://www.americanradiohistory.com/...-Page-0146.pdf
this one?
http://www.ebay.com/itm/Basler-Tube-...QAAOSwcvdXOjSq

I didn't get anywhere with the first link

jr
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Old 06-08-2017, 02:46 PM
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So give me a rough sketch of how the circuit would be set up.....just a chain of diodes?
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Old 06-08-2017, 04:42 PM
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Originally Posted by Kamakiri View Post
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.


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).
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Old 06-11-2017, 08:27 PM
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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?
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