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Old 13th May 2009, 08:30 PM   #1
Klimon is offline Klimon  Belgium
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Default Mysterious OPT winding tap

I just dismantled a Grundig Mandello console from the late sixties with ELL80 SE power stage. The opts are very small and have a winding tap I can't identify: I'm measuring 700ohms DCR on the primary winding which has another tap (on the same winding) that adds about 10 ohms DCR. Any ideas what this could be? A special kind of feedback winding???

Thanks! Simon
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Old 13th May 2009, 09:02 PM   #2
JohanB is offline JohanB  Sweden
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This is probably a hum balancing tap. The HT+ voltage in these radios has a lot of hum due to a singel capacitor power supply. HT+ is connected to the tap of the OPT and a ellyt-capacitor at the end of the winding is connected to ground. This balance the hum in the transformer like in a push-pull output stage. Saved a few bucks in components for the manufacturer.
This trick was done in most european singelended radios, TV's and small amps in the 50's & 60's

JohanB
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Old 14th May 2009, 12:05 AM   #3
llwhtt is offline llwhtt  United States
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What is pin9 (screen grid) connected to? Could it be an Ultralinear tap?

Craig
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Old 14th May 2009, 07:44 AM   #4
Klimon is offline Klimon  Belgium
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Quote:
This is probably a hum balancing tap
Never heard of it; cost-cutting is the name of the game when you look at the innards so this would make sense

Quote:
What is pin9 (screen grid) connected to? Could it be an Ultralinear tap?
I broke the circuits boards to remove the opts so it's too late to check. Also thought of UL but wouldn't 700 / 10 ohms seem like a strange proportion?

Cheers,

Simon
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Old 14th May 2009, 10:33 PM   #5
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My money is on the hum balance tap. I parted out an RCA console that had these. IIRC the B+ was feed to the tap and the output tube plate was connected to the longer end (higher DCR) while the B+ supply for the tuner/premp was taken from the other end.
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Old 14th May 2009, 11:35 PM   #6
llwhtt is offline llwhtt  United States
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Looks like the UL tap got ruled out. So with a capacitor on the tap and a capacitor on the short end you wind up with a pi filter(CLC) for the everything before the output stage. Never heard of that but sounds logical.

Craig
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Old 15th May 2009, 09:49 AM   #7
JohanB is offline JohanB  Sweden
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There is also a power-resistor in series with the short end before the capacitor is connected. The value is usally around 2 KOhms. B+ to the rest of the circuitry is taken from that capacitor. This gives a hum current in the opt to cancel the hum in the speaker.
Maybe also lower hum to the other stages. Someone can make a simulation of this?
Here is a typical schematic diagram of a cheap 50's radio.

I don't know if the resolution is enough, otherwise copy this link.

http://medlem.spray.se/nostalgiteknik/5020s.jpgClick the image to open in full size.

JohanB
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Old 15th May 2009, 09:57 AM   #8
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I have a few of those transformers here. Yup, hum balance/choke winding.
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Old 15th May 2009, 10:06 AM   #9
JohanB is offline JohanB  Sweden
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OOps, my schematic was w i d e !! Sorry, not used to this media and to lazy to read instructions

But without that trick the hum would be tremendous, with a half wave rectifier and 25 uF on B+......

JohanB
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