[dmblackburn]

   
After my interpretation of the schematic and physical layout, I have attempted to number the lugs on the schematic. At this point, I currently don't believe I have an open coil.

Lugs 1-3 : feedback coil (small band on the bottom outside) 7.0 ohms
Lugs 2-4 : primary with a common connection(4)  to the secondary winding 4.5 ohms
Lugs 4-5 : secondary with a common connection (4) to the primary winding 4.9 ohms

If I'm reading this correctly, the feedback coil (considered the secondary outer winding) should read 15.7 ohms but actually reads 7.0 ohms

My thought is just to rewind the feedback coil and hope for higher ohms.

Thoughts anyone would be appreciated

[Chas]

Try using another meter, a VOM is recommended. The readings, now, are not really out of line. Re-solder the terminals of the observed offending coil.

Check the capacitors in the circuit. Schematic seems to indicate four variable condensers. At least two are the main tuning condenser and its (mounted) compression trimmer. Any leakage from damage, corrosion or dust can kill the oscillator.

NOT associated with any particular radio is that the insulators for the stator of tuning variable condensers have been known to leak. Poor electro-mechanical connections to the rotor can also be troublesome.

I see no copper corrosion but, the decaying celluloid can be a source of low resistance by shorting turns of the tickler, unseen on the inside of the winding.

The tickler is at cathode potential, any leakage to the other windings which reference ground, will carry some of the total plate current.

GL

Chas

[morzh]

If my memory serves me right (and this is not always the case), is my recollection correct that the R18, a 15K resistor, had sometimes go down to 10K to oscillate?

[morzh]

One thing I did not hear from you (sorry if I missed it):

1. Have all the tubular paper caps been replaced?
2. Have all the resistors been checke for being within some reasonable boundaries from the specified values?

[dmblackburn]

Electrolytics and safety capacitors I replaced. There are some paper caps still remaining soldered outside the filter capacitor metal box (P/N 21).
Evident that previous repair bypassed restuffing. I will attempt to rebuild that component but was trying to avoid a "shotgun" approach to replacing components. Thank you for your reply. I have a lot more work to do. Have not decided to rewind the oscillator coil until someone can corroborate my previous post for what I believe is the mapping.

[MrFixr55]

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.