Yesterday, 03:50 PM
Hi DM,
First off, "divide and conquer" Touch the grid cap of the "44" 1st audio amp (may be referred to as the "Det Amp") . If you dont, fix the audio circuit before proceeding to the other circuits. You should get a rather strong hum.
If this is the coil indicated in the schematic that you attached to your thread, this is not the RF coil, it is the Oscillator / Mixer coil, otherwise referred to as the 1st detector coil. It may be referred on the schematic as the "oscillator coil. It consists of 3 sections. The "primary" is the long winding on the outside coil form, and connects to the plate circuit of the "1st Detector" through capacitor 12 and to ground through capacitors 15 and 16. The "secondary" is on the inner form, and connects to the oscillator section of the tuning capacitor (3). This and the trim capacitors comprise the oscillator tuned circuit. Please note that I may have the primary and secondary "switched, as I do not currently have one of these radios handy.
The "Tickler" is the short winding wound on the outside of the coil wound on the length of the outside form the primary?). This tickler winding is what provides the feedback to sustain oscillation. In the 71 (and the 80, 81, and the dreaded 89 and 19), it is very common for this tickler winding to fail and open.
This circuit is referred to as an "Autodyne" oscillator / mixer, as the same tube does the same function. This is as opposed to separate oscillator and mixer tubes or tube sections, or the "pentagrid convertor", both of which are superior to the autodyne 1st detector. However, all 3 circuits do the same thing- They create a "local oscillator" signal that is tuned to be a specific frequency above the frequency of the station being tuned in. the mixing of the desired station and the local oscillator signal produces a heterodyne frequency that is amplified by the IF stage, then detected and finally amplified for the speaker.
Of the 3 windings of this oscillator coil, only the tickler carries current in the Model 71 radio.
The main failure mode of this coil is what the other folks have described. The celluloid insulator between the primary and tickler breaks down, creates nitric acid which strips the enamel off the tickler (and possibly the primary), shorting windings or if the corrosion is sufficient, causing an open circuit.
on RF and IF coils, the primary opens but on this oscillator coil, the tickler opens.
The solution to this issue is to carefully remove the tickler section, counting the turns of wire removed, and most importantly, the direction of the winding. Examine the section of the primary that was under the tickler. If this section of the winding is corroded, the corrosion must be cleaned and the winding reinsulated. This may not be possible, and the primary must also be also rewound.
There are articles on rewinding these coils (Don't try to find one from another radio, if the coil has not failed, it will fail soon).
I am trying to find the wire size and specs for the tickler for the model 71 and will modify the post when I find it. There are articles on Philco coils in the Philco Library. There are also mods to the values of the cap and resistor that couple this winding to ground. These mods provide a stronger current, therefore a stronger feedback.
Hope this all helps.
First off, "divide and conquer" Touch the grid cap of the "44" 1st audio amp (may be referred to as the "Det Amp") . If you dont, fix the audio circuit before proceeding to the other circuits. You should get a rather strong hum.
If this is the coil indicated in the schematic that you attached to your thread, this is not the RF coil, it is the Oscillator / Mixer coil, otherwise referred to as the 1st detector coil. It may be referred on the schematic as the "oscillator coil. It consists of 3 sections. The "primary" is the long winding on the outside coil form, and connects to the plate circuit of the "1st Detector" through capacitor 12 and to ground through capacitors 15 and 16. The "secondary" is on the inner form, and connects to the oscillator section of the tuning capacitor (3). This and the trim capacitors comprise the oscillator tuned circuit. Please note that I may have the primary and secondary "switched, as I do not currently have one of these radios handy.
The "Tickler" is the short winding wound on the outside of the coil wound on the length of the outside form the primary?). This tickler winding is what provides the feedback to sustain oscillation. In the 71 (and the 80, 81, and the dreaded 89 and 19), it is very common for this tickler winding to fail and open.
This circuit is referred to as an "Autodyne" oscillator / mixer, as the same tube does the same function. This is as opposed to separate oscillator and mixer tubes or tube sections, or the "pentagrid convertor", both of which are superior to the autodyne 1st detector. However, all 3 circuits do the same thing- They create a "local oscillator" signal that is tuned to be a specific frequency above the frequency of the station being tuned in. the mixing of the desired station and the local oscillator signal produces a heterodyne frequency that is amplified by the IF stage, then detected and finally amplified for the speaker.
Of the 3 windings of this oscillator coil, only the tickler carries current in the Model 71 radio.
The main failure mode of this coil is what the other folks have described. The celluloid insulator between the primary and tickler breaks down, creates nitric acid which strips the enamel off the tickler (and possibly the primary), shorting windings or if the corrosion is sufficient, causing an open circuit.
on RF and IF coils, the primary opens but on this oscillator coil, the tickler opens.
The solution to this issue is to carefully remove the tickler section, counting the turns of wire removed, and most importantly, the direction of the winding. Examine the section of the primary that was under the tickler. If this section of the winding is corroded, the corrosion must be cleaned and the winding reinsulated. This may not be possible, and the primary must also be also rewound.
There are articles on rewinding these coils (Don't try to find one from another radio, if the coil has not failed, it will fail soon).
I am trying to find the wire size and specs for the tickler for the model 71 and will modify the post when I find it. There are articles on Philco coils in the Philco Library. There are also mods to the values of the cap and resistor that couple this winding to ground. These mods provide a stronger current, therefore a stronger feedback.
Hope this all helps.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
