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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Tue 21, 2021 9:16 pm 
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Location: Ukraine
I also want to thank all here for the time spent on a great explanation.
Unfortunately, less and less people are interested in this knowledge ...

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Wed 22, 2021 1:35 am 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1785
Location: Champaign IL 61822
As to scopes probing radios and especially TVs, suggest the following for looking at RF:

Get a 2 to 4 foot piece of coax with a on one end. At the other end attach a five inch or so
lead with an alligator clip on it to the shield and a 1 pF ceramic capacitor to the inner condutor.

1 pF won't load down things enough to stop them from working and won't even load down
TV IFs enough to make servicing useless. This of course needs calibration as the signal
reduction depends on the sum of the cable plus scope capitance. Its usually in the 50-100X region.
It of course falls off in response below the horizontal frequency.

For DC measurements make a similar gizmo with the 1 pF cap replaces with one
5.1 meg and one 3.9 meg resistor in series (to cut down capacitance). This is
a 10x probe at DC for the usual 1 meg scope input. Its frequency response is
down about 10x versus DC at the horizontal frequency. It has the advantage over
a regular scope probe that its capacitnce is so low that it
won't load down anything even a TV tuner oscillator.


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Wed 22, 2021 2:48 am 
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Joined: Sep Mon 16, 2013 2:42 am
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Location: Tucson, Arizona U.S.A.
If you suspect an image, it will be at twice the IF from the desired frequency. For example, if the IF is 455 KHz, the image will be 2X that or 910 KHz away. In AM broadcast sets it is normally above the desired frequency. For example, if the set is tuned to 600 KHz, the image would be at 1510 KHz. A signal at 1510 KHz might be heard at the same time as the 600 KHz signal. Of course harmonics of the local oscillator and the signal generator can create a real zoo of undesired responses but fortunately they are generally weak enough and far enough off the frequency you are working at to be recognized as incorrect.

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Wed 22, 2021 9:14 pm 
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Joined: Mar Mon 17, 2008 5:05 am
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Location: Ashhurst, New Zealand
One thing that Jim and Norm didn't mention in their excellent texts is that it's possible to get the 455kHz signal from a 1000kHz station by having the local oscillator set to the 'minus' frequency, in this example 545kHz. Now that's not easy to accomplish in the broadcast band as the oscillator coil adjustment doesn't have the range but if you have a multi-band radio which goes up to a high reception frequency then it's very easy to get the oscillator below the signal frequency and still get good reception - this is called the 'image' frequency, but the dial scale will not be accurate over it's full length.

One of the most handy tools I use and recommend is a reasonably cheap digital display radio, hopefully with the single sideband function. Then you can actually hear and see the frequency of the oscillator and set it to the correct frequency so you avoid the 'image frequency' problem.

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