|| PHILCO SERVICE HINTS & TIPS |
|PHILCO SERVICE HINTS & TIPS - NUMBER 43
Model 80 Performance Modifications
Excerpted from postings on The Philco Phorum, December 2005
by Ron Ramirez
MODEL 80 PERFORMANCE MODIFICATIONS
As originally designed, the Model 80 has a front end as shown at left. The set's performance can be improved by connecting one end of a wire to the cathode of the Type 36 detector-oscillator tube. Make the wire long enough to reach, and wrap around, the lead going to the center terminal (wiper arm) of the volume control.
Wrap 3 turns of this wire tightly around the volume control wiper arm lead, close to the antenna coil. At the same time, change the hookup of the detector-oscillator cathode resistor and capacitor, as shown below.
This wire, illustrated above as (A), is known as a "gimmick" and adds a small amount of capacity between the input of the antenna coil and the cathode of the detector-oscillator tube, resulting in a small amount of feedback which increases amplification.
This "gimmick" was used in Models 81, 84 and 37-84. I believe it is the main reason why these sets perform better than Model 80 receivers (other than the improved tubes used as detector-oscillator and second detector).
When I refurbish Model 80 sets, I also change the value of resistor (9) from 9,000 ohms to 13,000 ohms, as used in Model 84.
What prompted me to come up with this was in comparing the Model 80 schematic to the Model 84 schematic, I knew there had to be some reason why the 84 performed better than the 80. And I felt there was more to it than just the use of Type 77 tubes in place of the Type 36 tubes used in the 80.
Then, I noticed the "gimmick" used on the 84 but not on the 80. Adding the gimmick to the 80 I just finished working on allows it to bring in the local stations without an antenna! And this is in the basement, in my work area which is a sort of null area for radio signals!
I wonder why Philco didn't make this a production, or recommended, change; considering Model 80 was still in production when the Model 81 was on the market. I once worked on a Canadian Model 80; it HAD the gimmick, and used Type 77 tubes.
Remember, if you want to really extract the best performance from these sets, replace ALL carbon resistors as well as all of the paper and electrolytic capacitors. The old dogbone resistors, especially in these sets, tend to be way off-tolerance; and Model 80 receivers respond especially well to new resistors in place of off-tolerance units.