03-31-2021, 04:26 PM
Having had no answers to my questions about the problems I was having with my Philco 38-10, which I traced to the detector stage, I broke out my textbooks. I have a hard copy of Elements Of Radio Servicing by Marcus and Levy. I HIGHLY recommend that any of you serious about this hobby obtain a copy ! Here is a link to an online PDF of it https://worldradiohistory.com/BOOKSHELF-...y-1947.pdf . I studied the section on "Detector Stage - AVC", beginning on page 151 of this edition. There I found a clear explanation answering all my questions. First, let's look at this diagram from page 154...
[Image: https://64.media.tumblr.com/f5ad061f9eeb...2e18a4.jpg]
Here we see how the basic detector is acting exactly like a rectifier in a power supply. C26 and C27 act to smooth out the rectified voltage developed across R27 and R26. Note how C26, R26, and C27 form a pi filter to the IF frequencies present in the detected signal. Below, on the same page is this diagram...
[Image: https://64.media.tumblr.com/78c247544030...a10363.jpg]
Here we see R27 has become the volume control feeding the first AF stage, and R28 and C28 feed the AVC voltage back to the previous stages with C28 filtering out the AF component. Now, look at the schematic for a delayed AVC circuit on page 166...
[Image: https://64.media.tumblr.com/79e13ec94e24...beb971.jpg]
Here C110 acts to delay the response of the AVC circuit. Does this circuit look familiar? It should; it appears in the Philco schematic for the 38-10 almost EXACTLY...
[Image: https://64.media.tumblr.com/c42db0749b4a...0af14f.jpg]
So, I got all my questions answered. I hope this post helps anybody else who faces a similar dilemma. I now know that each of the components has its purpose, and what that purpose is. I have not gone into detail here, but you can read it for yourself from the link I provided. Again, I HIGHLY recommend any of you who wish to pursue this hobby buy a copy of Elements of Radio Servicing by Marcus and Levy.
[Image: https://64.media.tumblr.com/f5ad061f9eeb...2e18a4.jpg]
Here we see how the basic detector is acting exactly like a rectifier in a power supply. C26 and C27 act to smooth out the rectified voltage developed across R27 and R26. Note how C26, R26, and C27 form a pi filter to the IF frequencies present in the detected signal. Below, on the same page is this diagram...
[Image: https://64.media.tumblr.com/78c247544030...a10363.jpg]
Here we see R27 has become the volume control feeding the first AF stage, and R28 and C28 feed the AVC voltage back to the previous stages with C28 filtering out the AF component. Now, look at the schematic for a delayed AVC circuit on page 166...
[Image: https://64.media.tumblr.com/79e13ec94e24...beb971.jpg]
Here C110 acts to delay the response of the AVC circuit. Does this circuit look familiar? It should; it appears in the Philco schematic for the 38-10 almost EXACTLY...
[Image: https://64.media.tumblr.com/c42db0749b4a...0af14f.jpg]
So, I got all my questions answered. I hope this post helps anybody else who faces a similar dilemma. I now know that each of the components has its purpose, and what that purpose is. I have not gone into detail here, but you can read it for yourself from the link I provided. Again, I HIGHLY recommend any of you who wish to pursue this hobby buy a copy of Elements of Radio Servicing by Marcus and Levy.
