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Old 26th Aug 2019, 6:27 pm   #1
ChristianFletcher
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Default Interstage Audio coupling transformer

Not sure if this is a radio repair or components section question so apologies to the moderator if I have posted in the wrong place. Thanks.

Iím working on a Cossor 238 which is a battery set from 1937. It uses a interstage transformer to couple the detected audio to the output valve. I would call it a auto transformer as one side is connected to the -4.5 volt GB the other end to the grid of the output valve and the centre tap has the detected audio from the previous stage.

My problem is that itís open circuit. Actually all the windings appear to be open circuit. What alternative do I have to replace this. Iím not sure where to start with the winding ratios and wonders if I could modify a output transformer from a old transistor set or something.

What are my chances of rewinding it. I have never wound a transformer but would be willing to try if thereís a chance of success.

Any ideas. I have put quite a bit of work into this set and donít wonít to give up on it.


Thanks for any suggestions regarding Chris


https://youtu.be/7vVlvPiefpg
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Old 26th Aug 2019, 6:33 pm   #2
G6Tanuki
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Default Re: Interstage Audio coupling transformer

Everything is possible.... if you have enough time and money.

The winding is undoubtedly of *very* fine wire and lots of turns, meaning you'd really need a coil-winder if you don't want the risk of snapping the wire by accident just after you've put on 2000 turns...

There's a forum-member who offers transformer-rewinds who will no doubt be along shortly to offer enlightenment.

Alternative option: keep the old transformer in-place for purely cosmetics, then fit another small transformer under the chassis. A very small mains-transformer (3VA or so) - one of the kind with two 110V primary-windings you could wire in series - might be an option, if only to get the radio working while you work out what to do with the failed transformer.
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Old 26th Aug 2019, 6:53 pm   #3
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Default Re: Interstage Audio coupling transformer

Yes I was hoping to bodge something up just to get the set running. I would consider getting it professionally rewound as I would not know were to start with the actual turns ratio.

I guess I don’t really understand exactly the functionality of the transformer I can see is delivering the bias but is it stepping up or down or impedance matching?

Thanks for the reply regards Chris
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Old 26th Aug 2019, 6:57 pm   #4
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Default Re: Interstage Audio coupling transformer

Hi Chris,

It's working as an auto-transformer giving a 2:1 audio-signal step-up ratio, but it also provides a DC path for the bias voltage to get from the GB-battery to the valve-grid.

There were various different ways of doing transformer/choke-coupling over the years, must admit I've not come across it being done the way you describe but I can understand the logic of it.
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Old 26th Aug 2019, 7:14 pm   #5
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Default Re: Interstage Audio coupling transformer

Thanks. Yes definitely just one winding with a tapping towards one end. Hoping someone can advise turns ratio etc. The cossor service hints doesn’t help me. I assume if I rang up Cossor in 1937 they would have sent me a replacement.

Thanks again Regards Chris
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Old 26th Aug 2019, 8:49 pm   #6
Ed_Dinning
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Default Re: Interstage Audio coupling transformer

Hi Chris, can you post a picture of the transformer.
Cossor often used their own design of transformer can with "Cossor " imprinted on it. These were not a good quality part and were often replaced with a similar 3:1 device which was readily available then. These were 2 winding transformers and common in the 235 series of sets.
A cheaper option was sometimes to replace them with an RC coupling unit which would appear to be o/c on a continuity test. This did reduce the gain of the set.

From memory there were often 3 bobbins inside the can; 2 sec, 1 pri, so not too difficult to rewind, but a few thousand turns on each of 0.05mm wire. It can take a while !

Ed
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Old 26th Aug 2019, 9:32 pm   #7
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Default Re: Interstage Audio coupling transformer

Despite the model number the 238 is a 1938 model so released in 1937 the transformer is not in the Cossor Box it's an open frame auto transformer of a similar style to your average output transformer of the same period.

The 234 and 235 were Battery and Mains Kit sets of 1932 model year they did used the boxed three bobbin arrangement.

You can as a temporary bodge with a loss of gain replace the transformer with a resistor of 470K or so and connect the coupling capacitor to the joint of the new resistor and the 100K grid stopper.

Yo may have to fiddle around with the values to prevent the output stage going unstable.

Cheers

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Old 26th Aug 2019, 9:57 pm   #8
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Default Re: Interstage Audio coupling transformer

Thanks Ed and Mike

Great info as always from you.

I am just Away from home tonight but will post some pictures tomorrow. The circuit diagram shows it a an auto transformer arrangement. I do a video log of my repair as a video diary so lots of video footage from 28 minutes in. It look like a normal transformer with E core style transformer.

Mike so if I go for a grid stopper I would remove the -grid bias. I’m no expert on valve configurations just learning. Many Thanks.

https://youtu.be/OOlxetppURk

I could dismantle the transformer but a professional winder may want to use it as a pattern. My plan would be to prove the set will work before paying for a rewind.

Thanks Regards Chris
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Old 26th Aug 2019, 11:14 pm   #9
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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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Old 26th Aug 2019, 11:37 pm   #10
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Default Re: Interstage Audio coupling transformer

Ey up,
Whatstandwell is not far from South Yorkshire and I have tried to learn the language!
My coil winder is lurking behind my tomato plants but could be accessed before long. I could count the number of turns taken off, the length and also the weight.
The traverse is not working, this is a round tuit job. I will need to cast a couple of half nuts, 0.5 inch diameter, 40 tpi left hand buttress thread.
I have scramble wound 30,000 turns on a solenoid for a slave clock. I may have some suitable wire.
At present, the motor is supplied from a battery charger and a Variac. It works well but is cumbersome.

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Old 27th Aug 2019, 6:54 am   #11
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Default Re: Interstage Audio coupling transformer

Quote:
Originally Posted by Argus25 View Post
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.
I have just been onto the AES website and it seems they no longer supply the wound bobbins, just the complete transformers.

Peter
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Old 27th Aug 2019, 7:31 am   #12
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Default Re: Interstage Audio coupling transformer

Thanks all and offers of help and information. Let me get the transformer out the set and take some pictures. I will try to have a look at the wire diameter. I think I really need to get some power on the set and see if it will tune. Funny that the audio output transformer was open circuit now the interstage I am fearing for the coils in the tuning. The chassis wasn’t rusty and it doesn’t look like it’s been wet. I will pull the output valve and sniff with the signal tracer.


Thanks Regards Chris.
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Old 27th Aug 2019, 2:09 pm   #13
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Default Re: Interstage Audio coupling transformer

Quote:
Originally Posted by Electronpusher0 View Post

I have just been onto the AES website and it seems they no longer supply the wound bobbins, just the complete transformers.

Peter
It looks to me like they still sell them, just on back order at the moment:

https://www.tubesandmore.com/search/node/PT-431

Oddly the whole assembled transformer is only a few $ more anyway:

https://www.tubesandmore.com/search/node/PT-156
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Old 27th Aug 2019, 8:06 pm   #14
ChristianFletcher
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Default Re: Interstage Audio coupling transformer

This is the transformer I have removed and started to dismantle. As you say the windings are very fine and the bobbin is totally full. I have tested again and all windings are open circuit but the outer layer look in good condition. I bought a very cheap coil winder off Amazon today. I am not sure if the winds will be to thin and fragile to rewind but I think I would like to try. Now I have the bobbin out it looks a bit small to fit the winder but i have a machine shop so should be able to rig something up even if it the lathe in a low gear. The winds were a bit stuck but I have wetted down with acetone and this has released the varnish / glue so I thing it will unwind if I can rig up a counter.

Is there a way to work out how many turns to fill the bobbin I was think I should go for a three to one ratio to get me in the ball park. Any advise on doing this or how to unwind the bobbin would be helpful.

Many Thanks Regards Chris


Link for Coil winder (very Cheap) https://www.amazon.co.uk/Tools-Windi...83560137&psc=1
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Old 27th Aug 2019, 8:29 pm   #15
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Default Re: Interstage Audio coupling transformer

Quote:
Originally Posted by Argus25 View Post
Quote:
Originally Posted by Electronpusher0 View Post

I have just been onto the AES website and it seems they no longer supply the wound bobbins, just the complete transformers.

Peter
It looks to me like they still sell them, just on back order at the moment:

https://www.tubesandmore.com/search/node/PT-431

Oddly the whole assembled transformer is only a few $ more anyway:

https://www.tubesandmore.com/search/node/PT-156
Thanks, I intend ordering a couple for a 1920s radio I am restoring, I rewound one intervalve transformer, secondary oc, 17,000 turns, but the other one is oc on the primary which of course is the bottom winding.

Last time I asked the p&p to the UK was about $15.

Peter
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Old 27th Aug 2019, 8:39 pm   #16
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Default Re: Interstage Audio coupling transformer

Quote:
Originally Posted by ChristianFletcher View Post
I bought a very cheap coil winder off Amazon today. I am not sure if the winds will be to thin and fragile to rewind but I think I would like to try. Now I have the bobbin out it looks a bit small to fit the winder but i have a machine shop so should be able to rig something up even if it the lathe in a low gear. Is there a way to work out how many turns to fill the bobbin I was think I should go for a three to one ratio to get me in the ball park. Any advise on doing this or how to unwind the bobbin would be helpful.

Many Thanks Regards Chris


Link for Coil winder (very Cheap) https://www.amazon.co.uk/Tools-Windi...83560137&psc=1
I used exactly the same winder for my intervalve transformer re-wind (see post above). I wound backwards and unwound the wire to count the turns, as I said, 17,000!. I then rewound using new wire, Quite frankly the winder is too robust / not fine enough for the job and I broke the wire several times. I confess I joined it and insulated the join with yellow transformr tape and carried on. I have tested the transformer on a sig gen and it has the desired 3:1 ratio.
I am not sure I can face it again so will be buying the bobbins or transformers from AES.

Peter
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Old 27th Aug 2019, 8:47 pm   #17
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Default Re: Interstage Audio coupling transformer

Hello Peter thanks for the heads up. I am just about ignorant enough to not yet know how difficult it may be. I do have some other options for winders. The wire are thinner than a human hair and more fragile. How long did it take to wind?

I also wanted to ask why are the winding so thin and why so many?

Thanks Regards Chris
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Old 27th Aug 2019, 9:22 pm   #18
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Default Re: Interstage Audio coupling transformer

Hi Chris, I cannot help with why the number of turns is so high, my theory is too rusty.
The rewind took one whole evening (several hours) to unwind and a futher evening to rewind.
My arm really ached afterwards despite the gearing in the winder.
Add to this the fact that the laminations were rusty so each one was shot blasted (very gently) and revarnished before re-assembly.

Peter
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Old 27th Aug 2019, 9:31 pm   #19
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Default Re: Interstage Audio coupling transformer

That’s useful to know the time frame etc. Yes I think my lamination are a bit rusty I will have to rub them down. I bent them a bit as the outer metal frame was clamped around the laminations. I am looking forward to giving it ago.
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Old 27th Aug 2019, 9:39 pm   #20
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Default Re: Interstage Audio coupling transformer

It's not just about turns ratio it's about inductance.

Intervalve transformers need to have a reactance (inductance and frequency based) that matches the driving stage at the lowest frequency to be passed. It's a bit more complicated due to other factors such as resistance and capacitance but in the main that's it.

You need a high primary inductance hence lots of turns

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