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 Post subject: Resistor wattage
PostPosted: Mar Wed 23, 2011 8:13 pm 
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Trying to figure out the wattage needed to replace these filament resistors in an Apex 36. The values are way off and my voltage is way down. Image

Schematic is here http://www.nostalgiaair.org/PagesByMode ... 040746.pdf

These are wirewound but not very large. I'm thinking 2 watt should do but wanted others opinions.

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 Post subject:
PostPosted: Mar Wed 23, 2011 8:28 pm 
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I assume you are talking about the tapped resistors across the 2.5-volt and 1.5-volt windings.

1. How would a wirewound resistor be "way off" without being open?

2. Unless they are shorted, they should not affect the filament voltage substantially.

3. Since they are shunting the transformer windings (and the tubes), you'd have to disconnect them to get good readings.


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 Post subject:
PostPosted: Mar Wed 23, 2011 8:52 pm 
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I did remove them from the windings as well as disconnect the wires running to the tubes. NOW, due to my slow moving brain, I realized that someone had swapped the 6 ohm and 20 ohm resistors.

I had measured 7ohms across the 20 and 22 ohms across the 6. Old age plays hell on processing information. :(

Anyway, I still have a bad 600 ohm off the CT of the 20 ohm resistor, reads almost 900 ohms so I'll need to replace that one. The 2K on the 5 volt filament winding is OK.

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 Post subject:
PostPosted: Mar Wed 23, 2011 9:24 pm 
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I looked at the schematics. Here's my take on it:
As far as that 600 Ohm resistor is concerned, it is the cathode resistor for the third tube. Square the max anode current that the tube will safely handle and multiply the result times 600 Ohms. This gives you the max design wattage for the resistor. Now, double that for safety's sake, and that gives you the wattage of the resistor that you should replace it with. As it is the cathode resistor, and only sees low voltage (normally), there is no need for any significant concerns regarding high voltage withstanding. That's the way I see it, anyway.
Let the comments begin..

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 Post subject:
PostPosted: Mar Wed 23, 2011 9:44 pm 
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Hi Jim

I wouldn't change any of the resistors. Wire wound resistors shouldn't change in value. Notice values were hand written in. There may be errors.

Low value resistors across filaments are there just to make center taps. Exact value isn't critical as long as both sides have similar resistance. The 600 or 900 ohm resistor is there for bias on #26 tubes. Schematic shows 9 volts bias. Measure bias voltage. This will determine resistor value. Carbon resistors do go up in value.

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Last edited by Norm Leal on Mar Wed 23, 2011 9:46 pm, edited 1 time in total.

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 Post subject:
PostPosted: Mar Wed 23, 2011 9:46 pm 
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OK, I found the specs for RCS 226 tube and it is rated at a max. of 6.2mA. This gives a design max. wattage of 0.023064 Watts. Thus, one could easily get by with a 1/4 Watt resistor. However, as I am sure you know, many folks would use a 2 Watt here because it is a cathode resistor. 1A of filament current probably makes this a really pretty radio, no?

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 Post subject:
PostPosted: Mar Wed 23, 2011 9:54 pm 
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There are 4 number 26 tubes on this one filament line. If they each draw max current it would be 25 ma.

This radio has around 115 volts on each tube plate. Tube manual shows 6.2 ma with 180 volts on a number 26. Since voltage is lower each 26 will draw less current. If 600 ohms current for the 4 tubes is 15 ma.

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 Post subject:
PostPosted: Mar Wed 23, 2011 10:03 pm 
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From my website, Parts Values section, Carbon Resistors, comparison pictures:

http://www.philcorepairbench.com/resistcompare.htm

A handy scale rule in each pic to gauge yours against.

Chuck

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 Post subject:
PostPosted: Mar Wed 23, 2011 10:03 pm 
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I have fixed wirewounds of that sort by giving the connection a good squeeze with a pliers. Seems the connection between the resistance wire and the connecter can go bad over time


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 Post subject:
PostPosted: Mar Wed 23, 2011 10:12 pm 
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Norm Leal wrote:
There are 4 number 26 tubes on this one filament line. If they each draw max current it would be 25 ma.

This radio has around 115 volts on each tube plate. Tube manual shows 6.2 ma with 180 volts on a number 26. Since voltage is lower each 26 will draw less current. If 600 ohms current for the 4 tubes is 15 ma.


Wow, I missed the connection dots! What can I say, my eyes are going South on me too..
Good catch. The math seems right, and thank you for the correction. I can still learn, even if I can't see very well.

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 Post subject:
PostPosted: Mar Wed 23, 2011 10:35 pm 
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Jim,

Bingo!

Quote:
I have fixed wire-wounds of that sort by giving the connection a good squeeze with a pliers. Seems the connection between the resistance wire and the connector can go bad over time


Fix only what is broke!

26's, because of there high current draw, need to have solid, tight connections. IMHO, tighten and clean both the sockets and the tube pins, re-solder the filament pins even if they look good. Vacuum out the old solder and scrape the leads, yes, I know hard to do, but worth getting the few 1/10's of a volt needed to get the filaments of the '26 a little brighter. If there are any chassis connections related to the filament or it's center tap, especially if they are mechanical connections, loosen and re-tighten.
GL

Chas


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 Post subject:
PostPosted: Mar Fri 25, 2011 8:23 pm 
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After swapping the 6 and 20 ohm wirewounds and replacing the 600 ohm resistor I removed all the tubes and powered it up to check the voltages.

While watching the 5 volt filament for the 71A I saw the meter start jumping up and down between 4 volts and 4.4 volts. Pretty sure I have a bad winding thats arcing so now I'll need a replacement transformer for this set.

Thanks for the replies to my question.

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 Post subject:
PostPosted: Mar Fri 25, 2011 8:53 pm 
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Jim

Does the transformer get hot? It should, if shorted, with only 4 volts on a 5 volt winding even without tubes. Can you hear arcing or see smoke?

Having so many low voltage windings makes it harder to find a replacement transformer. You might be able to find a transformer that's close and add a 1.5 or 2.5 volt winding?

Here is a place with power transformers. Ones near the bottom have 2.5 volt windings.

http://www.oldradioparts.com/2a23efl.txt

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 Post subject:
PostPosted: Mar Sat 26, 2011 1:32 am 
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Everytime I get an early AC set in here its intermittent tube sockets and tube pins, open resistors, or wires held on by one strand. Ive yet to have a bad transformer......knock on wood.

Carl


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 Post subject:
PostPosted: Mar Sat 26, 2011 2:32 am 
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Norm Leal wrote:
Jim

Does the transformer get hot? It should, if shorted, with only 4 volts on a 5 volt winding even without tubes. Can you hear arcing or see smoke?

Having so many low voltage windings makes it harder to find a replacement transformer. You might be able to find a transformer that's close and add a 1.5 or 2.5 volt winding?

Here is a place with power transformers. Ones near the bottom have 2.5 volt windings.

http://www.oldradioparts.com/2a23efl.txt


Norm, suprisingly I don't feel any appreciable heating of the transformer nor is there any smoke. The transformer does pull around 250 ma without any tubes installed.

Carl, I too have been lucky in the past, but this is one case where I'm not. This will be one tuff puppy to replace and try to keep looking original.

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 Post subject:
PostPosted: Mar Sat 26, 2011 3:44 am 
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Jim

May be a bad connection? A bad transformer will either be shorted or open. If a winding is shorted the transformer will heat. If open you won't have voltage. Could be a poor connection at the leads. This is unusual and should be fixable.

Do you have an analog meter? non digital.

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 Post subject:
PostPosted: Mar Sat 26, 2011 4:42 am 
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Can you get an oscilloscope and look at it?

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 Post subject:
PostPosted: Mar Sat 26, 2011 5:32 am 
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I'm using an analog meter, Simpson 270, and it clearly shows the voltage fluctuating. None of the other filament voltages show this symptom. I will try resoldering the terminal tabs on the transformer but I sure don't want to try and open this tar filled baby up.

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 Post subject:
PostPosted: Mar Sat 26, 2011 5:36 am 
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With the tubes out, put your ear close to the transformer; if it's arcing, you'll actually hear it. It sounds like a ticking noise...and every time it tics, you'll see the voltage dip.


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 Post subject:
PostPosted: Mar Sun 27, 2011 12:10 am 
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Tried resoldering the transformer tabs but that didn't help. Then I remembered that the dial light was still installed so I thought HEY, this might be the problem.

Removed the lamp and powered it up. After about 10 minutes running time I saw no fluctuation, then DAMN, there goes the needle on my voltmeter as well as the needle on my AC current meter.

Unless I'm able to find a replacement transformer I fear that this chassis has just become a parts donor. I don't want to have to kludge up a replacement since there is no place to install it except on top of the chassis. Shame too, since I have a very nice cabinet to go with it.

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