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Old 19th Mar 2017, 12:21 pm   #1
oldgeezer
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Default Cossor 737 radiogram - ganged volume control question

This set had been got at and the ganged volume pot replaced with a single that had burnt out. The replacement wiring looked a little strange too!

I've dug out the circuit diagram and strangely the ganged volume control has a dotted line on the slider for the gram input. There is no switch available to switch the input from 'radio' to 'gram' - only one that shorts out the first i.f. transformer on 'gram' to stop radio breakthrough.

Any ideas on how this should be connected? R17 is .5M + .5M, R16 is 50k, R15 is 2M.

Should I just wire the two sliders together??
Many thanks
Keith
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Old 19th Mar 2017, 12:42 pm   #2
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Default Re: Cossor 737 radiogram - ganged volume control question

Hi Keith

I have a 737 in my rountuit pile.

I don't know if its been got at.

However the way I recall this working is as follows

The Pot is a 1MOhm pot with a centre tap

When the pot is at midpoint it lowest volume for both gram and Radio.

Rotating clockwise from the midpoint increases the radio volume.

Rotating Anticlockwise from the centre point increases volume for the Gram.

I may be slightly confused on this its been quite a while since I had a play!

This arrangement is used on several Cossor Radiograms.

Cheers

Mike T
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Old 19th Mar 2017, 3:37 pm   #3
oldgeezer
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Default Re: Cossor 737 radiogram - ganged volume control question

Ah - That does make sense Mike. Different, but it would certainly work.

It doesn't make it easier to find a suitable pot though!

I may just get the radio audio working, and then perhaps put a capacitor from the pickup to the 'live' end of the pot - but I'll lose some radio volume through the pickup. Or I might just experiment with the best arrangement when its going!

Next stop ebay - just in case there's a centre tapped 1M pot out there!
Thanks
Keith
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Old 20th Mar 2017, 12:29 am   #4
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Default Re: Cossor 737 radiogram - ganged volume control question

A dual gang 500K linear potentiometer can be wired as a centre tapped 1M Ohm pot if you connect the opposite track ends together.
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Old 21st Mar 2017, 7:01 pm   #5
oldgeezer
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Default Re: Cossor 737 radiogram - ganged volume control question

Many thanks - Did think about that, but at half volume, both the 'gram' and 'radio' will be half there. Also at full volume on 'Radio' the audio will be shorted to earth through the 'gram' side being at zero. Unfortunately there is no switch to select 'gram' or 'radio' audio.
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Old 21st Mar 2017, 8:28 pm   #6
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Default Re: Cossor 737 radiogram - ganged volume control question

Hi Keith

Probably not much help but here is my 737 and some pictures of the original pot

Cheers

Mike T
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Old 21st Mar 2017, 10:41 pm   #7
oldgeezer
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Default Re: Cossor 737 radiogram - ganged volume control question

Many thanks Mike, That confirms what you said originally.

And it confirms the incorrect one was put into my set.

I have seen a couple of possible pots - but will use it just on radio until I can find a correct one.
regards
keith
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