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 Post subject: How does the AVC work?
PostPosted: Sep Mon 21, 2020 12:12 am 
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Location: Forestville, NY
Hi guys, I'm still a novice at this and was hoping someone could explain in detail how the AVC works on my radio. I really have no idea lol. I've attached a copy of the schematic. Thanks in advance!


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McMurdo Silver 15-17 schematic.jpg
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 Post subject: Re: How does the AVC work?
PostPosted: Sep Mon 21, 2020 1:14 am 
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It should work very well :D

There is an AVC IF amplifier dedicated to only the AVC which taps off the signal from the grid of the 2nd IF amplifier through a 250 mmf cap.
It is amplified then goes through a dedicated AVC IF transformer to the AVC detector diodes.

The cathode of the right hand diode is biased with a small amount of DC to create a voltage delay. The actual AVC voltage is taken from the lower left hand corner connection (bottom secondary) of the IF transformer and fed through a 1/4 meg/1/4 meg voltage divider to feed the second IF amplifier.
It also is fed through a 1/2 meg resistor to feed the 1st IF, the Mixer and the RF amplifier. Each one having a 100,000 ohm resistor.

The IF signal also is coupled to the second diode, through a 250 mmf cap to create AVC voltage to operate the eye tube which does not have a voltage delay.
It feeds a 1/4 meg/1/2 meg voltage divider before feeding it through a 1/4 meg resistor to the eye grid.

I hope that helps?

-Steve

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Last edited by azenithnut on Sep Mon 21, 2020 2:24 pm, edited 1 time in total.

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 Post subject: Re: How does the AVC work?
PostPosted: Sep Mon 21, 2020 3:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Rochester NY USA
On a more basic level:

Without AVC, stations volume would vary with transmitter power and distance, tune from a weak station to a strong one and and you'd have to turn the volume down fast! Tubes like the 6K7 are "variable mu" - mu is amplification factor. The more negative the grid is, the less amplification. So a negative voltage is developed from the received signal, the stronger it gets, the more negative. So a really strong signal creates more negative AVC voltage, which reduces the amplification. Result is that weak stations get amplified more, strong ones less.

Simpler receivers usually use the same detector for audio and AVC. Taking it from a earlier stage results in more pleasant tuning, as the AVC responds over a greater bandwidth with fewer IF transformers ahead of it. Inter-station noise quiets when approaching a station; the tuning peak is easy to find without it.

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 Post subject: Re: How does the AVC work?
PostPosted: Sep Mon 21, 2020 12:26 pm 
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azenithnut wrote:
I hope that helps?

-Steve


It helped me. Thank you.


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 Post subject: Re: How does the AVC work?
PostPosted: Sep Mon 21, 2020 10:32 pm 
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Joined: Apr Tue 07, 2009 5:33 pm
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Location: Forestville, NY
Thanks guys! That was perfect!


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 Post subject: Re: How does the AVC work?
PostPosted: Sep Tue 22, 2020 10:20 pm 
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One other item of note for AVC - many manufacturers will have the technician disable the AVC during the alignment process. This allows you to use the strong signal from the generator (which would trigger AVC) without the signal being damped. The instructions may have you ground the AVC out or limit it by grabbing a bias voltage in the set and clamping the AVC. If you don't have full alignment instructions, keep the signal generator signal as low as possible.

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 Post subject: Re: How does the AVC work?
PostPosted: Sep Wed 23, 2020 6:02 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1759
Location: Colorado Springs, CO
This might help.
http://radioremembered.org/detector.htm


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 Post subject: Re: How does the AVC work?
PostPosted: Sep Sun 27, 2020 1:14 am 
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Location: Westland, MI, USA
On a more advanced level:

Some receivers will allow adjustment of the DC voltage on the cathode of the AVC rectifier (in this case the rhs cathode) to change the delay of the AVC, ie faster or slower. This can be important for the reception of signals that can vary quickly in intensity such as on shortwave. This feature is sometimes found on communications receivers of better quality.


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 Post subject: Re: How does the AVC work?
PostPosted: Sep Sun 27, 2020 6:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1759
Location: Colorado Springs, CO
kent3256 wrote:
One other item of note for AVC - many manufacturers will have the technician disable the AVC during the alignment process. This allows you to use the strong signal from the generator (which would trigger AVC) without the signal being damped. The instructions may have you ground the AVC out or limit it by grabbing a bias voltage in the set and clamping the AVC. If you don't have full alignment instructions, keep the signal generator signal as low as possible.


Another method is to connect an ac voltmeter on it most sensitive scale across the speaker terminals, turn the volume all the way up and then apply just enough signal from the generator to get a reading on the ac meter at half scale. This will keep the avc from kicking in. As adjustments are made and the signal gets stronger, adjust the output of the signal generator to keep the ac meter at about half scale.


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