10-29-2022, 03:26 PM
As you may or may not recall, I first began to work on this radio back in January 2015. Here is the thread.
When I wrapped up that thread on February 11 of the same year, I was wondering out loud whether an RF coil from a 39-45 would be a better replacement in this 770 than whatever was in it (it certainly was not the original coil). So, since then, I acquired a 39-45 parts chassis and transplanted its AM-police band RF coil in place of coil (8) in the 770.
Once I did that, AM reception showed a definite improvement.
However, when attempting an alignment, I found that I had another problem. Trimmer condenser (20A), the AM band antenna trimmer which is mounted across the antenna tuning condenser section, had zero effect on alignment.
Forgot to mention: I measured the AM antenna coil, thinking that might be the problem. You can see the measurements in red, in the partial schematic below. I expected the secondary to be open, as that is how the radio is acting - it is not. And this appears to be the original coil; it does not appear to have ever been out of the radio.
Background:
The mica sheet that was originally inside trimmer (20A) was missing, and so I did what I have done for many years when faced with that problem - I replaced the missing mica with a piece of clear plastic cut from one of those "blister pack" packages.
Mica capacitor (12), 250 pF, was bad, and was replaced.
Resistor (13), 1 meg., was also replaced.
You can see the full schematic here:
https://www.philcoradio.com/images/39-770.jpg
and the entire service info here:
https://philcoradio.com/library/download...20Book.pdf
Below is a partial schematic showing the set's front end. Now, while you guys mull that over, I have another question on this same radio - but a very different circuit - which I will ask next.
When I wrapped up that thread on February 11 of the same year, I was wondering out loud whether an RF coil from a 39-45 would be a better replacement in this 770 than whatever was in it (it certainly was not the original coil). So, since then, I acquired a 39-45 parts chassis and transplanted its AM-police band RF coil in place of coil (8) in the 770.
Once I did that, AM reception showed a definite improvement.
However, when attempting an alignment, I found that I had another problem. Trimmer condenser (20A), the AM band antenna trimmer which is mounted across the antenna tuning condenser section, had zero effect on alignment.
Forgot to mention: I measured the AM antenna coil, thinking that might be the problem. You can see the measurements in red, in the partial schematic below. I expected the secondary to be open, as that is how the radio is acting - it is not. And this appears to be the original coil; it does not appear to have ever been out of the radio.
Background:
The mica sheet that was originally inside trimmer (20A) was missing, and so I did what I have done for many years when faced with that problem - I replaced the missing mica with a piece of clear plastic cut from one of those "blister pack" packages.
Mica capacitor (12), 250 pF, was bad, and was replaced.
Resistor (13), 1 meg., was also replaced.
You can see the full schematic here:
https://www.philcoradio.com/images/39-770.jpg
and the entire service info here:
https://philcoradio.com/library/download...20Book.pdf
Below is a partial schematic showing the set's front end. Now, while you guys mull that over, I have another question on this same radio - but a very different circuit - which I will ask next.
--
Ron Ramirez
Ferdinand IN
