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  #136  
Old 03-12-2014, 06:33 PM
HighHopes HighHopes is offline
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Quote:
Originally Posted by bandersen View Post
Well yes, that's what the photo is above - the actual response curve of a tuner.
I connected a sweep generator to the antenna terminals with proper termination and connected my scope to a test lug they provide on the tuner.
Does the tuner have a detector in it?? I am thinking it should look something like this with the largest output centered on the middle frequency to the IF, lower frequency on the left and increasing to the right.
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File Type: jpg sweep copy.jpg (40.7 KB, 6 views)

Last edited by HighHopes; 03-12-2014 at 08:58 PM.
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  #137  
Old 03-12-2014, 09:06 PM
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bandersen bandersen is offline
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No, you wouldn't align a tuner or IF like that. Scopes back in the 40s didn't have the bandwidth to directly observe the IF waveforms. The oscilloscope is connected to a grid leak bias network on the mixer.

You can download the service info here. It covers the circuit theory and alignment instructions. There are also books on ebay covering vintage TV servicing and alignment if you'd like to learn more. http://www.earlytelevision.org/pdf/a..._rider_tv4.pdf
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  #138  
Old 03-12-2014, 11:08 PM
HighHopes HighHopes is offline
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Hi Bob,

The manual for the Admiral 20A1 was very helpful. In the description of the RF section there is mention of a trap for "adjacent channel picture rejection." There are also several caps for alignment and setting a bandwidth. You are right. The tuner bandwidth is adjusted for one channel. The full skirt of the tuner RF response curve in Figure 32 (page 15) is over 10 MHz wide, but the tuner 3 db down points are about 6 MHz apart. The station channels are 6 MHz apart. This is all designed tighter than I thought.

I saw the tuner alignment test point(labeled W). This is the grid leak circuit you mentioned. Why isn't this RF, instead of the passband response voltage shown? This point must be detected to get the response curve, no?

Steve

W is near the middle of the picture.
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File Type: jpg RF.jpg (125.8 KB, 26 views)
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  #139  
Old 03-15-2014, 11:44 AM
HighHopes HighHopes is offline
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I guess nobody knows. I don't either.
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  #140  
Old 03-15-2014, 12:02 PM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by HighHopes View Post
I saw the tuner alignment test point(labeled W). This is the grid leak circuit you mentioned. Why isn't this RF, instead of the passband response voltage shown? This point must be detected to get the response curve, no?

W is near the middle of the picture.
It is detected, by the tube's grid/cathode performing the diode function.
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  #141  
Old 03-16-2014, 06:19 PM
HighHopes HighHopes is offline
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I never heard of a cathode-to-grid diode action. If there is a diode action, then wouldn't the results be the video (plus some weird sound on the FM audio). Were would the IF frequency come from if the RF is detected?

Is the sweep input so strong that the tube is driven into Class C operation?

Where can I read about this circuit?

-Steve
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  #142  
Old 03-16-2014, 09:55 PM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by HighHopes View Post
I never heard of a cathode-to-grid diode action.
Google 'grid leak detection'. It's the classical AM detection method used in the earliest tube radios. It utilizes the natural diode action that occurs between a tube's filament (cathode) and grid ('anode') to rectify the amplitude-modulated carrier.

In the case under discussion here, the sweep generator's carrier, rectified and displayed on the scope, denotes the bandpass of the TV tuner.
Quote:
If there is a diode action, then wouldn't the results be the video (plus some weird sound on the FM audio).
The two fixed-frequency markers or 'blips' on the trace are the sound and video carriers.
And yes, the TV would probably display some pretty weird stuff just from the genny's 60-hz sweep rate (though i never tried looking at it during an alignment).

Quote:
Where would the IF frequency come from if the RF is detected?
The IF is not derived from detected RF.
It is the 'beat' or difference frequency between the local oscillator (LO) and the
incoming carrier.
Or in the case here, there are two IF strips. The first one is tuned to the difference frequency between the LO and the video carrier.
The second strip is tuned to the difference frequency between the LO and the sound carrier.
The video and sound carriers are 4.5 mhz apart.
Hence the two IF strips are tuned 4.5 mhz apart.

This system was superceded by the intercarrier method, in which the sound carrier is included in the video IF, and picked off right after the video detector at 4.5 mhz, amplified and detected. This simplified things and greatly reduced parts count
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  #143  
Old 03-17-2014, 01:31 AM
HighHopes HighHopes is offline
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Hi 88,

I think I am getting this. With a capacitor in the grid circuit, the charge is held to a level depending upon the amplitude of the ac signal, charging only on the positive swings. So you get the envelop / detected waveform on the grid.

AND / BUT, the mixed radio frequencies still appear on the plate.

Horray! I got it.

Thanks for your patience.

Steve
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  #144  
Old 03-17-2014, 09:51 AM
old_coot88 old_coot88 is offline
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BOOYAH !!
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