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 Post subject: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 12:31 am 
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Location: SE Iowa 52626
I have a few of these P-T156 transformers as shown at https://www.tubesandmore.com/products/t ... tage-10-ma which I use to replace open transformers in antique battery radios. The resistance between the green wires is about 1.5K ohms and between the red and blue wires about 480 ohms. I just took one apart and measured the resistance of only the coil with no core in it. The resistance between the green wires was the normal 1.5K ohms but between the red and blue it was measuring about 11M ohms and falling the longer than I kept the meter connected. It was down to about 3M ohms when I disconnected it. Why was it this high and why was it falling?

I then put the core from a Crosley in the coil (see attached picture) and got the same readings as above. What is going on? Did I ruin it someway when I originally took it apart or what? Also, should it work in the Crosley metal as the picture shows or does it have to be the same size as the original? The core is .5" on both the original and the Crosley.

Don


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transformer in crosley.jpg
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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 4:48 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4789
Location: Lexington, KY USA
The DC resistance is not affected by the magnetic core. Somehow a wire got pulled loose or broken inside the windings.

OTOH, the AC resistance at 1kHz, say, will be quite different with a core, or with a different core, or a change to the magnetic gap in the core.

Ted


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 5:38 am 
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Location: SE Iowa 52626
Usually Lurking wrote:
The DC resistance is not affected by the magnetic core. Somehow a wire got pulled loose or broken inside the windings.

OTOH, the AC resistance at 1kHz, say, will be quite different with a core, or with a different core, or a change to the magnetic gap in the core.

Ted


I think that you are correct about a wire getting pulled loose or broken during the disassembly. The red spot if just a bit of my blood that I never got wiped off so it was not a gentle operation.

If the coil was still good would the transformer work the same as pictured as it did originally or does the core have to be the same length, etc.?

Don


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 1:29 pm 
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Try a new battery in your ohmmeter.... A dying battery is the only explanation I can come up with that would make the reading drop on a purely inductive coil, the longer you keep the meter attached.

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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 2:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1191
Location: Bristol TN 37620
Don,

The bobbin should read the same on or off the core.
(500 or so ohms primary and 1500 or so ohms secondary)
As the guys say, you either have a bad connection on
your primary winding, or maybe a weak battery on your
meter. Looks good on the original lamination though.
It will work well if you can get continuity. Length of
coil won't matter in this case.


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 4:38 pm 
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Joined: Jun Tue 23, 2015 4:37 am
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Location: SE Iowa 52626
brnhornet52 wrote:
Don,

The bobbin should read the same on or off the core.
(500 or so ohms primary and 1500 or so ohms secondary)
As the guys say, you either have a bad connection on
your primary winding, or maybe a weak battery on your
meter. Looks good on the original lamination though.
It will work well if you can get continuity. Length of
coil won't matter in this case.


Thanks, I will check the battery and also try another meter.

As I understand it the length of the core and the space around it will not affect the performance of the transformer, correct?

Don


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 6:18 pm 
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Joined: Jun Tue 23, 2015 4:37 am
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Location: SE Iowa 52626
Barry H Bennett wrote:
Try a new battery in your ohmmeter.... A dying battery is the only explanation I can come up with that would make the reading drop on a purely inductive coil, the longer you keep the meter attached.


Barry, I know that this sounds strange but I just did the same check with three more meters including my old dependable plug-in bench meter and got the same results. The reading starts with a reading of several meg ohms and continually drops as long as it is connected...go figure. Oh yeah, I also checked the batteries on the battery operated meters and they were all good.

If you are doubting me I guess that I could make a Youtube video of the test and post it but that seems unnecessary to me.

Don


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 6:27 pm 
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It was common practice to "zap" an audio transformer to establish a carbon path in the open windings. The transformer would work in a fashion but the path not being a real resistor was variable and often generate noises. This would tend to create a a variable DC resistance reading. But so would holding the test leads of the Ohmmeter :roll: An analog meter one with a D'Arsonval meter movement and a battery power source should give consistent readings of an inductors resistance be it on or off the core. Gently squishing the windings give an erratic reading then there is corrosion, shorting or a poor connections. a variable reading would also indicate some sort of electrolytic process occurring. I would not think that those windings are usable...

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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 9:09 pm 
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Joined: Jun Tue 23, 2015 4:37 am
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Location: SE Iowa 52626
[quote="Chas"But so would holding the test leads of the Ohmmeter[/quote]

Okay, I was holding the tips of my meter leads rather than taking time to put the clips on them. BTW, when done correctly the coil was open.

In short, it was my bad. Although I knew better, thanks a bunch for reminding me.

Don (case closed)


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 9:17 pm 
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IGot2P wrote:
Chas wrote:
But so would holding the test leads of the Ohmmeter
Okay, I was holding the tips of my meter leads rather than taking time to put the clips on them. BTW, when done correctly the coil was open. In short, it was my bad. Although I knew better, thanks a bunch for reminding me.
Don (case closed)


:lol: :lol: :lol: I'm sorry Don, I just had to laugh :lol: :lol: :lol:

But regrettable the coil is open

chas

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Last edited by Chas on Sep Fri 10, 2021 6:31 pm, edited 1 time in total.

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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 9:43 pm 
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Chas wrote:
IGot2P wrote:
[quote="Chas"But so would holding the test leads of the Ohmmeter
Okay, I was holding the tips of my meter leads rather than taking time to put the clips on them. BTW, when done correctly the coil was open. In short, it was my bad. Although I knew better, thanks a bunch for reminding me.
Don (case closed)



Try carefully cutting the top tape wrapping layer (or two as necessary) from the winding to expose the solder connection where the coil attaches to the lead wire. Perhaps the wire got pulled during disassembly breaking the connection. At this point, you've got nothing to lose!


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Thu 09, 2021 10:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1191
Location: Bristol TN 37620
The length of the core doesn't matter in this application.
Just get a new core from AES - PT431.


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Fri 10, 2021 4:32 am 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
There are a lot of different core materials. This can affect how the transformer works in a radio. As can the various dimensions of the laminations. How the core is assembled also makes a measurable difference.

However, if the core is reasonably similar, you may not be able to easily tell the difference by listening.

Ted


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Fri 10, 2021 4:41 am 
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Location: Lexington, KY USA
Just get a new core from AES - PT431.

The PT431 is listed as a coil assembly only, with no core. If you want the core, as well, you need the PT156.

https://www.tubesandmore.com/products/transformer-audio-interstage-coil-assembly-p-t156

But it looks as if you may have a suitable core, so perhaps the coil assembly is the thing to get.

I think I might just spend the extra two bucks and get the complete transformer, though.

https://www.tubesandmore.com/products/transformer-audio-interstage-10-ma

Ted


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Fri 10, 2021 12:42 pm 
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It looks like you could get two of those bobbin assemblies onto that core.

I would be tempted to put two on and wire the two primaries in series and the two secondaries in series of course paying attention to phasing.

-Steve

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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Fri 10, 2021 3:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1191
Location: Bristol TN 37620
The PT-431 coil works great on the original Crosley lamination. Don could use the PT-156 (whole transformer)
but I think he wants to hide the new coil in the old
transformer case. He could use the whole new transformer and hide it with appropriate spacers to reattach it.

As his replacement coil tests open, he could just pick up
a new coil and replace the open one.

Sorry for the error in terminology. I posted "core" for "coil"
While there is room on the laminations to use 2 PT431
coils, I don't know how this would work, especially in
regard to phasing. It might open a new can of worms.

I can testify that the PT-431 works great on original
Crosley laminations in the Model 51, 52, and Trirdyn.


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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Fri 10, 2021 5:12 pm 
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Location: Boston, Massachusetts
I believe PTOP used to sell an mdf spacer to mount the transformer in, not original but is a timesaver.

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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Fri 10, 2021 5:17 pm 
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brnhornet52 wrote:
The PT-431 coil works great on the original Crosley lamination. I can testify that the PT-431 works great on original Crosley laminations in the Model 51, 52, and Trirdyn.

I second this endorsement. The only limitation with the PT-431 coil is the 1/2"x1/2" opening. For larger, I use the ARBE III versions, if the radio is worth it.

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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Fri 10, 2021 5:57 pm 
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Posts: 976
Location: SE Iowa 52626
brnhornet52 wrote:
The PT-431 coil works great on the original Crosley lamination. Don could use the PT-156 (whole transformer)
but I think he wants to hide the new coil in the old
transformer case.


You are correct, I wanted to use the original transformer case. I appreciate all the advice and suggestions from everyone but here is what I actually did. I used my jig saw and a piece of wood the same thickness as the original laminate and cut it to fit the perimeter of the transformer case. I then cut out the center of the wooden piece and left only enough for holes for the case screws. I then took a complete PT-156 and clipped off the metal connection tabs. Now it would fit in the original transformer case with the wooden piece between the two halves of the case. I had to remove the terminal screws from the transformer case but with some black spaghetti tubing over the wires up to the solder connections it is good enough for who it is for. :D

Don


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wood insert.jpg
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 Post subject: Re: Interstage transformer 101 needed
PostPosted: Sep Fri 10, 2021 11:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1191
Location: Bristol TN 37620
Good job Danl


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