01-20-2019, 04:47 AM
[Image: http://philcoradio.com/phorum/images/smi...on_eek.gif]Hi everybody. Yes I am still around. I been lurking around without much success. Meanwhile Ive been working on old radios.
And currently it's a 40-140 electronic restoration (well restomode anyway).
After replacing the customary paper caps. and tired resistors and the volume pot. I found the audio sounded like garbage like arking audio input to the amp. very weak. Oh and it seemed that the AGC was varying it up and down while monkeying with the connections.
After much investigating the only thing I could find was the radio didn't match the schematic drawing at the volume control item #35. It was missing the ground for the low side of the pot. Now mind you it was made that way. There was no evidence that there ever was one. The terminal never had any solder on it, no wire, nothing. And to top it off I have another radio that has the same chassis and it has the same missing ground. So I got daring and added a ground which made it work.
Now to the question. Has anybody seen this issue before and how could this detector circuit ever work like it was. Of course both radios had been monkeyed around with by some previous orangutans.
Lokie Here. [Image: http://philcoradio.com/phorum/images/smi...on_mad.gif] http://philcoradio.com/library/download/...20Book.pdf
( "Foxit reader" for PDFs.)
Meanwhile doing all this exercise I decided to try and figure out how the Detector/Demodulator works so I redrew it as semiconductors.Now it make some sense.
It appears that this radio has two stage demodulator. The 7C6 tube has three separate little goodies inside. Seen as semiconductors Two diodes and NPN transistor with a common lead at the cathodes/emitter hard to ground. On the drawing. Caps 33C and 33D (both of which are inside the IF can) are the two stages I speak of.
It looks like both diodes rectify the IF. One is used for the AGC level. And the other rectifys the same IF and charges Cap 33C which charges into C 33D which discharges into the volume pot. The audio is then tapped off by the volume pot and sent to the base of the NPN transistor with the collector level being amplifyed copy of the audio. All this by one little tube.
So without the ground the audio "DC" level of the caps must have been floating up not following the audio level.
When I was a kid I use to think that radios were some kind of voodoo. Especialy those little square cans with screws in them. Twistie twistie.
Oh and I have been playing around with my SDR radio. Boy is it cool. I can see the oscillator sail accross the the noise floor and I can find the exact freq.
I wish it had a tracking counter. Using a calculator I set the exact freq for the dial readout. How long it will stay that is another question. (all this with the $200 SDR and a second hand laptop). I set the IF with my rather unstable generator.
I will try and Post some pics and maybe some videos in a few days. Can I post videos here? I never tried.
Chris
And currently it's a 40-140 electronic restoration (well restomode anyway).
After replacing the customary paper caps. and tired resistors and the volume pot. I found the audio sounded like garbage like arking audio input to the amp. very weak. Oh and it seemed that the AGC was varying it up and down while monkeying with the connections.
After much investigating the only thing I could find was the radio didn't match the schematic drawing at the volume control item #35. It was missing the ground for the low side of the pot. Now mind you it was made that way. There was no evidence that there ever was one. The terminal never had any solder on it, no wire, nothing. And to top it off I have another radio that has the same chassis and it has the same missing ground. So I got daring and added a ground which made it work.
Now to the question. Has anybody seen this issue before and how could this detector circuit ever work like it was. Of course both radios had been monkeyed around with by some previous orangutans.
Lokie Here. [Image: http://philcoradio.com/phorum/images/smi...on_mad.gif] http://philcoradio.com/library/download/...20Book.pdf
( "Foxit reader" for PDFs.)
Meanwhile doing all this exercise I decided to try and figure out how the Detector/Demodulator works so I redrew it as semiconductors.Now it make some sense.
It appears that this radio has two stage demodulator. The 7C6 tube has three separate little goodies inside. Seen as semiconductors Two diodes and NPN transistor with a common lead at the cathodes/emitter hard to ground. On the drawing. Caps 33C and 33D (both of which are inside the IF can) are the two stages I speak of.
It looks like both diodes rectify the IF. One is used for the AGC level. And the other rectifys the same IF and charges Cap 33C which charges into C 33D which discharges into the volume pot. The audio is then tapped off by the volume pot and sent to the base of the NPN transistor with the collector level being amplifyed copy of the audio. All this by one little tube.
So without the ground the audio "DC" level of the caps must have been floating up not following the audio level.
When I was a kid I use to think that radios were some kind of voodoo. Especialy those little square cans with screws in them. Twistie twistie.
Oh and I have been playing around with my SDR radio. Boy is it cool. I can see the oscillator sail accross the the noise floor and I can find the exact freq.
I wish it had a tracking counter. Using a calculator I set the exact freq for the dial readout. How long it will stay that is another question. (all this with the $200 SDR and a second hand laptop). I set the IF with my rather unstable generator.
I will try and Post some pics and maybe some videos in a few days. Can I post videos here? I never tried.
Chris
!!!!!!!!!!!!!!!!!!!RESISTANCE IS FUTILE!!!!!!!!!!!!!!!!!!!!!!
/\/\/\/\/\/\/\/\/\
[Image: http://philcoradio.com/phorum/images/smi...on_eek.gif] Chris
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