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 Post subject: Rogers 10-12 Automatic Tone Control Circuit
PostPosted: Mar Mon 05, 2018 2:56 pm 
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Joined: Jan Wed 28, 2015 5:27 pm
Posts: 12
Location: Calgary Canada
I am working on a Rogers 10-12 chassis for a friend of mine and when the Sensitivity control pot is rotated fully clockwise to where the switch becomes engaged there is a loud pop in the speaker. I have done some basic DC voltage measurements on the 88M tube used in this section as well as looking at the circuit with my scope. I do see that the 88M is cut off when the Sensitivity pot is active ( turned CCW from fully clockwise stop )...ie the cathode is at zero volts DC.

Pin 4 of the 88M is normally sitting at 13VDC and goes to 10VDC when the pot is rotated fully clockwise and the switch is engaged....

So does anyone know how this particular tube functions in this circuit?...he has another 10-12 chassis that does not do this ( I do not have access to that one at the moment so I cannot compare them unfortunately).


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 Post subject: Re: Rogers 10-12 Automatic Tone Control Circuit
PostPosted: Mar Mon 05, 2018 8:14 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4205
Location: Near Brandon, Iowa
The first audio tube and the "automatic tone control" tube appear to function roughly in parallel, each acting on one side of the audio inter-stage transformer. It really isn't surprising that you hear a "pop" in the speaker when the switch on the sensitivity control is actuated, since it acts on the auto tone control tube which in turn feeds the audio output stage. In fact, it would be more surprising if you didn't hear it.

It's not obvious to me what the "automatic tone control" tube circuit is supposed to accomplish; but it probably is a form of loudness compensation, where bass tones are actively boosted for low volume control pot settings. Since tone control beyond that is mostly a matter of personal taste, I can't imagine it doing much of anything else.


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 Post subject: Re: Rogers 10-12 Automatic Tone Control Circuit
PostPosted: Mar Tue 06, 2018 5:00 pm 
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Joined: Jan Wed 28, 2015 5:27 pm
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Location: Calgary Canada
Here is the section of the schematic if anyone is interested...in this chassis I have the 88M cathode is at zero volts DC with the switch shown in the position indicated ( left ). Another chassis I have worked on does not make the pop sound...the one I am working on now makes a popping sound when the switch is moved to the right position.


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