Modified 38-12 to what purpose? - Printable Version

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+--- Thread: Modified 38-12 to what purpose? (/showthread.php?tid=23453)

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Modified 38-12 to what purpose? - Tom45acp - 04-10-2022

I'm working on a 38-12 that has two modifications, one of which I don't understand.

Visit      http://www.nostalgiaair.org/Resources/236/M0013236.htm    for the schematics.

The modification I'm seeing is coming off the center terminal of the volume control. It consists of a 50pF mica in parallel with a 400kohm resistor which then connects to the .01 mfd cap that is shown on the schematic. After changing the filter caps and removing the modification in power supply, I turned the radio on. It works about as well as you would expect for leaky caps and many way out of tolerance resistors. I tuned in a station and listened to it then bypassed the modification. As far as I could tell, there was no difference between the two.

Does anyone have an idea as to what this modification as supposed to do?

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RE: Modified 38-12 to what purpose? - Mondial - 04-10-2022

Looks like someone was unhappy with the audio high frequency response of the radio and tried to modify the circuit. As you describe it, the mod would act to boost the highs a bit by placing a 50 pF in series with the volume control output. The parallel 400k resistor bypasses the 50 pF cap so the cap has only a limited effect. Actually, the circuit cuts all the lower frequencies so the net effect is an apparent increase in the highs.

As you noticed, the mod has very little effect as it only would relatively increase the highs by only 1 dB or so. Hardly worth the effort.

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Tom45acp - Tom45acp - 04-11-2022

Thanks for the reply. I knew there had to be a reason even though the modification wasn't very effective.

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