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Detector stage problems... - Printable Version

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Detector stage problems... - mikethedruid - 03-31-2021

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-ARH/Servicing/Elements-Of-Radio-Servicing-Marcus-Levy-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/f5ad061f9eebbe8c638246284f67b262/d9472aa1b013fe30-f0/s640x960/2090f6bbfb282cdde078d2464a587a3b542e18a4.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/78c24754403059d9b77516331cb1beaa/d9472aa1b013fe30-dc/s640x960/05d1bb75fe77ea63100d6f90ae90ce5748a10363.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/79e13ec94e24519b18ddf0e2125aa160/d9472aa1b013fe30-72/s400x600/ea602aa44c3e4688052b8dc6b565b92619beb971.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/c42db0749b4ae6a32ac30255ba36fa3a/ba98da8511b31bd5-c5/s1280x1920/0e519bcc57890cac5b5c7df1c54658af1c0af14f.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.

RE: Detector stage problems... - klondike98 - 03-31-2021

Thanks for posting that explanation!

RE: Detector stage problems... - Paul Philco322 - 03-31-2021

So sometimes it is good to have a good book! You are the Professor.

Paul

RE: Detector stage problems... - Forrest - 03-31-2021

Hello,
Very good explanation, good post.
Forrest.

RE: Detector stage problems... - John Bartley - 04-01-2021

Any of the Marcus and Levy books are good. I have one (packed away right now), called I think .... Basic Radio? It's theory, not practical repair, but it is one of two "essential" books in my collection.

Another "set" is the US Navy set from the 1950's, called "Basic Electricity", "Basic Electronics" and "Basic Radio" (as I recall). These are better than Marcus and Levy. The author apparently was a company "Van Valkenburgh, Nooger, and Neville, Inc.", but they were Riders manuals for all intents and purposes.

RE: Detector stage problems... - morzh - 04-01-2021

Mike

>>>Here C110 acts to delay the response of the AVC circuit.

My impression is, C110 is there to DC-decouple the AVC rectifier from the Detector. And the R28-C28 is what delays the AVC responce.

RE: Detector stage problems... - mikethedruid - 04-01-2021

Yes, John, a good library is a valuable asset. I have a few dozen books on the subject of electricity and electronics. Besides Elements of Radio Servicing, I also have Marcus' Radio Servicing Theory And Practice, which is a tome and goes into great detail. I find Elements better for quick, practical answers. I also have a number of the old Audel's books. Audel's Radioman's Guide is a very useful book for basic, practical learning about servicing radios. I also have the ten volume Audel's New Electric Library, which is full of useful information on everything from house wiring to radio. I don't have the electronics course books we used back in the Navy, they were actually classified "Confidential", although I know that there was nothing in them could not be found in any of the better electronics course books.