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 Wacky STC Transformer
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 Return to top of page · Post #: 1 · Written at 8:48:08 AM on 8 April 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

HI, I have an STC A5130 radio which has an illogical output (to me).

It has a standard centre tapped secondary, when I measure with an ohmmeter one side from centre tap to the secondary output it reads 282 Ohms, the other side is open circuit. Measuring right across the secondary is open circuit as well. It would indicate that the transformer is no good.

However, when I apply power and the radio turned on it has an output of 240v from the 'good' side and 180v from the 'bad' side. The rectified output is about 200v but should be 230vdc.

The radio works very well except the voltages are down across the set.

Can someone explain why I am getting this result?

If it's open circuit why is there an output of 190v with the rectifier valve removed and 180v with it in?

Is it a phantom voltage with no current behind it? The rectified voltage (200vdc) seems too high for that.

I considered it may have an open winding that is shorted to ground on the bad side but it should have continuity.

I have put an ad for a replacement transformer in the Wanted section of this forum.

Thanks, David Smile


 
 Return to top of page · Post #: 2 · Written at 11:28:59 AM on 8 April 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 571

Its the high impedance of the voltmeter.
This does not "load" the open circuit winding much and so enough current can flow (microamps) to make the meter register.
If you measured with a old school needle meter the reading would be much lower.
If you load the winding with a milliamp size load it will then read zero.
Fred.


 
 Return to top of page · Post #: 3 · Written at 2:31:45 PM on 8 April 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

Hi Fred,

I think I worked it out, I put a light bulb across the faulty secondary and the voltage dropped to zero. When placed across the good secondary it lit the bulb and held the voltage at 200 or so volts.

I now understand the faulty secondary is open circuit but will still show a voltage when floating and somehow it still shows a voltage when the rectifier valve is in circuit, which, I can only assume is by magic or it's just sitting at the anode with no desire or ability to accept electron flow.

It all makes sense when it unfolds, Thanks Fred Smile


 
 Return to top of page · Post #: 4 · Written at 5:23:40 AM on 10 April 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 571

That's called a "light bulb" moment!
You are now a certified electronic gingerbeer!

Testing always gets back to applying a value to the device under test that is equivalent to the "nameplate rating" of the device.
Doesn't matter whether its ohms or Kw's or pounds.
All you have to do is figure out what the device should do and aim for that.

Magic is how I explain how things work when I have a dumb look on my face!
Of course the magic disappears when you work out the science.

Fred.


 
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