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 Post subject: How to identify the AVC circuit?
PostPosted: Nov Sun 09, 2014 6:35 pm 
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How does someone (like me) who doesn't know how to do it identify the AVC Circuit on the schematic of a radio? Are there some basic steps to follow?

Thank you for your help.

Joseph


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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Sun 09, 2014 6:48 pm 
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AVC connects to the grids of one or more stages in the RF and/or IF sections of the receiver.

Look at the grid of the RF amplifier, and follow it back toward the detector.

You will find a capacitor of modest value (.05 or so), which connects to the detector section via a high-value resistor (megohms).
There might be a resistor connected between the grid and the capacitor.

That capacitor is the AVC integrating cap. That's where the AVC voltage is developed.

If the RF grid simply goes to ground through a resistor, or if there is no RF tube at all, use the same technique looking at the IF amplifier tube(s). At least one grid in the radio MUST return via the circuit as described if the radio has AVC.

- Leigh

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Mon 10, 2014 1:10 am 
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If you give us the schematic of the radio you are working on, we can point it out and you can see the things Leigh is talking about.

Jim Mueller

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Mon 10, 2014 5:33 pm 
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Thank you for the replies. I don't have a scanner but I do have the schematic for an RCA set model 9X561 which is on www.nostalgiaair.org. I would appreciate comments and help with this one.

Thank you again.

Joseph


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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Mon 10, 2014 5:44 pm 
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Here's a markup showing the AVC for that set.

referring to previous inputs, R5 and C8 set the time constant. 2 stages are controlled.
Attachment:
avc.jpg
avc.jpg [ 58.42 KiB | Viewed 3498 times ]

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Mon 10, 2014 6:33 pm 
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pixellany wrote:
Here's a markup showing the AVC for that set.
referring to previous inputs, R5 and C8 set the time constant. 2 stages are controlled.
Thanks, Pix.

To elaborate...

Resistor R5 and capacitor C8 form the AVC integrator (sets the time constant).

The voltage for the AVC circuit is sourced at the 12SQ7 detector, V3.

The AVC control voltage appears across C8. It's distributed to the tube grids from that point.

Some radios have isolating resistors between C8 and the individual grids.
Those do not affect the operation of the AVC circuit, since no current flows to/from the grids.

- Leigh

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Tue 11, 2014 1:06 am 
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Hi Joseph,
Since you are working on a 9X561, I would recommend Richard McWhorters book, The All American Five. He uses the 9X561 to explain the theory of the AA5, and takes you through each part of the circuit step by step.

Russ

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Tue 11, 2014 1:53 pm 
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Leigh, sorry for my ignorance but asking is the only way to learn.
What do you mean when you said "Set the time constant".
My understanding of the AVC is as follows: As you are picking up a stronger station you develop a higher voltage on the left side of the 3.3mohm resistor, this in turn changes the bias on the screen of 12sa7 and 12sk7 affecting the performance of the tubes, therefore adjusting the gain on them. As the control voltage is coming for the detected audio, C8 is there to eliminate and keep the voltage constant.

Thanks for your comments,

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Tue 11, 2014 2:15 pm 
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R8/C5 are sized to control how quickly the gain changes as you tune thru a station. It turns out that the values also serve to filter out the audio.

In some receivers, more sophisticated circuits are used such that the AVC comes on faster than it releases.

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Tue 11, 2014 10:45 pm 
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Quote:
What do you mean when you said "Set the time constant".


The "time constant" is how long it takes to charge the capacitor to 63% of its final voltage or to discharge it to 37% of its original voltage. It is given by the formula t=RC. See http://www.learnabout-electronics.org/ac_theory/dc_ccts43.php. The AVC circuit in a radio is more complicated than a simple RC circuit (it includes R4, R6, and V3 in your example) but the general idea applies.

If the time constant of the AVC circuit is too short, audio from the detector gets applied to the various stages and changes their gain at an audio rate. This reduces the modulation on the signal. If the time constant is too long, the gain changes can't follow fading of the signal.

Quote:
changes the bias on the screen of 12sa7 and 12sk7


No, it changes the bias on the control grids. The screen grids are connected to B+. In a pentode, the elements are, in order, cathode, control grid, screen grid, suppressor grid, plate.

Jim Mueller

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Tue 11, 2014 10:48 pm 
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The time constant also affects low frequency distortion. Many communications have poor low frequency audio specs because the time constant is too short. W1VD, Jay Rusgrove, made extensive measurements on numerous BA receivers and his investigation revealed that was a problem with many vintage AM ham receivers.

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Wed 12, 2014 12:04 am 
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If someone were to nitpick, they might point out that the AVC voltage (and the audio) is developed by rectifying the IF signal at the secondary of the 2nd IF transformer. The IF frequency is filtered out by the small cap between pins 2 and 5 on the IF transformer.
The combined audio and AVC passes thru R4 to the volume control, thru a cap that removes the "DC" AVC voltage; and the "DC" AVC voltage is filtered by R5 /C8 (to remove the audio and set the response time constant)

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Wed 12, 2014 5:38 pm 
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My thanks to those who replied to my question about identifying the AVC circuit on a radio.

I now have a clearer picture of the circuit and how it ends up controlling the fading in and out of the signal from a particular station.

Thank you for your help again!

Joseph


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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Wed 12, 2014 7:20 pm 
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Hi Joseph,

The "time constant" of the AVC circuit is so that it won't respond to modulation on the signal, but only to its average strength.

If the time constant was significantly shorter (smaller C and/or R), the gain would vary with audio peaks. Not good.

It would be more accurate to call the RC circuit an "integrator" since its purpose is to average the voltage over time.

- Leigh

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 Post subject: Re: How to identify the AVC circuit?
PostPosted: Nov Thu 13, 2014 5:41 pm 
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I found a helpful book to understanding AVC circuits is an old book called "An Hour A Day on Automatic Volume Control" by John F. Rider. They are usually available on EBay. In fact all the "An Hour A Day" books are helpful to understanding how radios work.


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