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Old 26th Aug 2019, 11:14 pm   #9
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Interstage Audio coupling transformer

Quote:
Originally Posted by ChristianFletcher View Post
My problem is that it’s open circuit. Actually all the windings appear to be open circuit. What alternative do I have to replace this.
I have done a fairly detailed study of inter-stage transformers for vintage radios. Look at pages 11 through 17 of this article:

http://worldphaco.com/uploads/THE_GREBE_MU-1.pdf


There you will see a suggestion. The better way to repair these is to un-stack them and fit a pre-made bobbin from AES (unless you have the rewinding gear). These can readily be wired in auto-transformer or isolating mode.That is if you want to keep the original stack and brackets. The wound bobbins come from AES (Antique Electronic supply USA). There are essentially two sizes available, in one case you would have to buy the transformer and remove the bobbin.

Or if the transformer you have is not that special looking, just fit a new one from AES. Have a look at their stock lists.

You will see from that section of the article that audio inter-stage transformers in vintage radios are very interesting. Their job is voltage magnification (and often DC isolation an band pass filtering) not the transfer of power. The usual transformer equations and models are not that helpful here, so I had to derive the equation for the frequency response myself (with a lot of help from Terman). The plate resistance of the driving valve has a very significant effect on the frequency response. It is the only place on the net you will find the frequency response equation (unless it has been copied by now).

Unfortunately, being wound from very fine wire and often with organic insulation (paper), green spot corrosion takes many of them out.

Some people have attempted to replace them just with an RC network, but the loss of gain in that case is significant.
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