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 Post subject: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 3:54 am 
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Joined: Feb Wed 15, 2017 11:49 pm
Posts: 457
Location: Kingsville, OH 44048
Hi all,
Just started to tear into this 110K and got a surprise when I removed the chassis from the cabinet. A big black puddle of burnt wax under the chassis!! When I flipped the chassis over there was wax splattered all over the one end. So I checked resistance of the HV winding and it was nonexistent.
I looked around some my donor chassis and found a Truetone chassis with a power trans about the same physical shape as the 110's. I got some chassis and speakers from a lady that was repurposing cabinets and this Truetone was one of them.
The 110 schematic calls for 150 ohms of resistance total for the HV winding. The Truetone trans. measures 158 total. Filament-.1 Heater-.3.
I heated up the trans. and got 695vac on HV winding, 5.6 vac on filament and 6.89 on heater winding, all with no load on them.
My question is would this trans. be a good enough match to replace the 100K trans.?
http://www.nostalgiaair.org/pagesbymode ... 014715.pdf
Here's the schematic if needed.
Thanks for any thoughts and considerations,

Dick


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 5:43 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8292
Location: Near Brandon, Iowa
It's not so much no-load voltages or winding resistances that are of interest in comparing power transformers, but current-sourcing capability. Whatever replacement transformer you come up with is going to have to supply heater power to nine signal tubes plus dial lamps, and B+ to service those tubes (four of which are power-hungry audio output tubes). According to the Riders info, total B+ current is around 137 mA, which is relatively high. The output tubes account for 100 mA of that figure.

Overloading the transformer (due to excessive current demand) will result in poor radio performance due to depressed voltages, power transformer overheating, and eventual failure.

To accurately assess whether or not your Truetone tranny will work, you will need to look up the specs of the radio from which it originates. That should tell you (by tube count) if you are going to be in the ballpark, power-wise.

One clue might be the rectifier tube that the Truetone donor chassis uses: if it is a single 5Y3 or similar tube, its power transformer is not likely to be up to the task of running a 110K (the 110K uses a 5U4 rectifier, which has a substantially higher output current capability).


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 5:54 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4785
Location: Lexington, KY USA
Yes, the donor radio info will tell you most of what you need to know about the prospective replacement.

The quickest way to evaluate a replacement transformer is by weight. The new one will have to be at least as heavy as the original. If it is heavy enough, then check out other factors.

Ted


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 8:15 pm 
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Joined: Feb Wed 15, 2017 11:49 pm
Posts: 457
Location: Kingsville, OH 44048
Thank you both for replying. Ted I weighed both trannies and the old cooked one is 5lb 13.5 oz. The replacement trannie is 6 lb 1.3oz. so pretty similar.
Lorenz as far as the rectifier tube goes I don't know what it had. When I got the Truetone chassis I took all the tubes out and tested them and put them in my stash. I don't know what model of Truetone this chassis came out of. There is a Stock # stenciled on the back D-1950 but after checking some sites online I could not find that number anywhere. There is also a Factory Chassis No. 10AF22. Couldn't find anything with that no. either. With that little bit of info I don't know if I can find a schematic to find what the rectifier tube was.

Thanks for your help of which I need more.

Dick


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 8:43 pm 
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Joined: Jun Sat 15, 2019 7:43 pm
Posts: 1510
Your donor transformer is from a Western Auto D-1950 which is rated at 122 watts. Your RCA is rated at 135 watts. That will be really cutting it close. It will work but I wouldn't run it for an extended period of time. Use a bucking transformer to decrease the incoming voltage and keep an eye on the transformer temperature.

DM


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 8:45 pm 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 8537
Location: Vancouver Island, Canada
I think I'd try the new transformer but keep it on the bench for a week or two.
You can use a silicon rectifier to reduce the load on the new transformer, do temperature tests, then decide if the added load of the 5U4 will be ok. I would use a laser thermometer for measuring temps.

Don't forget to test for a dead short somewhere. Cheers and good luck. 8)

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 8:59 pm 
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Joined: Feb Wed 15, 2017 11:49 pm
Posts: 457
Location: Kingsville, OH 44048
Thank you guys for your recommendations. I'm quite a ways from trying this replacement tranny because of so much crap that came out of the cooked one. Lots of cleanup and shining up the chassis. When I get it ready to test, after all the caps and resistors are changed and checked and all the other stuff( you guys know the drill) I will report back.
DM where did you find that info about the wattage of a D-1950? I couldn't find anything.
Thanks again

Dick


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 10:02 pm 
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Joined: Jun Sat 15, 2019 7:43 pm
Posts: 1510
http://www.nostalgiaair.org/Resources/723/M0023723.htm

DM


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 10:17 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1606
One advantage of modern replacement power transformers, especially those made by Hammond is they have much superior laminations (core material) and run generally a lot cooler than vintage transformers. Typically the core magnetization current is a fraction of the vintage version and the wire insulation is better too. A lot of older transformers, especially run on high range line voltage push the core too far up its B-H curve, this also increases the magnetic radiation causing hum issues in some cases. So likely if the replacement you select is close in power rating , or even just a little lower than your original you will often get away with this, it is important though to check the transformer's temp rise on worse case line voltage and check you don't push its currents over its manufacturer rating.


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Wed 29, 2021 11:59 pm 
Member

Joined: Jan Tue 16, 2007 7:02 am
Posts: 4785
Location: Lexington, KY USA
A smaller power transformer could be kept cool by reducing some of the load.

A solid state rectifier has already been mentioned.

The output stage of the RCA uses four 6K6GT pentodes in push pull parallel. A single push pull pair of 6V6GT output tubes would provide about the same audio power out, with much less heater and B+ drain.

Ted


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Thu 30, 2021 12:26 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8292
Location: Near Brandon, Iowa
It appears that the Western Auto D-5000 used a 5U4 rectifier tube, so that's a good sign for compatibility.
Signal tube count is about the same. Worth a try.

Not sure that I would want to reconfigure the push-pull-parallel output stage to use a more efficient arrangement; that's part of this model's "charm". Even though 1940s Lincoln flathead V-12 engines were horrible, I suspect that most restorers of this car would go to great lengths to retain the original engine.

Eliminating the redundant "parallel" tubes would be one way to reduce the power transformer load (by merely pulling two tubes). The radio should still have adequate audio power and could be put back to "as built" status in seconds.


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 Post subject: Re: RCA 110K Power trans. SMOKED
PostPosted: Sep Thu 30, 2021 2:21 am 
Member

Joined: Feb Wed 15, 2017 11:49 pm
Posts: 457
Location: Kingsville, OH 44048
You know you guys are the best. You give me all these options which I will try every one just to see which one produces the best results. And if this old man can remember he will report back so the results can be passed on.

Thank You all so much

Dick


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