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Old 23rd Jul 2009, 12:02 pm   #1
'LIVEWIRE?'
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Question O/C Hum cancelling winding in an output transformer

Apologies if this has been covered before, but I'm wondering what is the best way around this problem. If I simply short out the o/c part of the winding(DC resistance only 16 ohms acc. to the service sheet) presumably there is a risk of increased hum, though the radio in question-a BUSHVHF90A- would presumably operate normally otherwise. I guess a 16 or 18 ohm resistor could be fitted(there's no room to fit a smoothing choke, even if I had a suitable one!), or does anyone have any other suggestions?
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Old 23rd Jul 2009, 12:52 pm   #2
kalee20
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Default Re: O/C Hum cancelling winding in an output transformer

If the winding really is O/C then yes, you can short it out. But if it's intermittent, then during the times that it's 'good', then you'd be shorting out some of the output.

Best suggestion is to disconnect it. Putting a resistor in its place will hardly make any difference - you'll find there is a large value external resistance in series with it. This is calculated on the basis of steering some of the hum through itself, some through the ra of the output valve, such that the two lots of hum going in opposite directions in the transformer cancel out (taking into account the turns ratios).

Yes, you will have more hum without it, but whether it is bothersome remains to be seen!
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Old 23rd Jul 2009, 8:05 pm   #3
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Default Re: O/C Hum cancelling winding in an output transformer

I've decided to do as you sggested, kalee, then check the hum level. The winding is totally O/C anyway
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Old 28th Jul 2009, 3:27 pm   #4
peter_sol
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Default Re: O/C Hum cancelling winding in an output transformer

Hi
I have fitted a substitute transformer without the tap and found little difference in hum level
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Old 3rd Aug 2009, 10:33 am   #5
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Default Re: O/C Hum cancelling winding in an output transformer

Having fitted a substitute transformer to the radio in question, I would agree with you, Peter, there's little. if any, audible hum present. I guess, if the same problem exsisted in a HiFi Amplifier, and with speakers having a bass response to below 50hz then the increased hum level would be audible, but not in the average radio
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