I have a Philco 89 late chassis which I just restored, with a problem that it receives well up to about 1350 kHz, then the dies out above that. I aligned it on a reference signal generator, and indeed it can be aligned at 1500 kHz, at zero dB, which is a very strong signal indeed. However it is too insensitive at that frequency to receive a broadcasted signal. With a quality antenna attached, it has no problem receiving signals up to about 1200 on te dial. In order to get this one playing, it was necesary to rewind the oscillator tube plate coil section, and the primary of the antenna coil. I probably have a little less wire in the antenna coil primary than the original did. All the tubes are good. The padder peaks up nicely at 560 or so. In my experience, loss of sensitivity at the high end of the BC and is unusual. Wondering if anyone has some thoughts about this. Thanks, Mike Schwartzman
Hi Mike
Sounds like an oscillator coil problem to me.
You mentioned rewinding the oscillator plate winding...is is usually the oscillator
cathode winding that goes bad. (This is the outermost winding.) The turns count, size of wire and direction of winding are all critical. Even though the late version 89 sets use a Type 77 instead of a Type 36, it is still that finicky autodyne circuit that is not inherently stable.
Besides rechecking your windings, I strongly suggest you bake your oscillator coil in an oven at 200 degrees F for about 30 minutes to drive out any residual moisture. Afterward, spray the coil with clear lacquer or coat it with clear nail polish. This will help prevent future moisture absorption.
Of course, for a permanent fix, you could perform my
Super 89 Mod by changing the 77 to a 6A7, and re-rewinding the oscillator coil to the specs given in the post linked here. It is far more stable than any autodyne circuit. But, of course, it also destroys the originality of the radio. Just a thought.
Thanks Ron. In fact you are correct; my plate winding was on the preceding interstage coil to the plate of the 39/44 RF. The cathode winding on the oSC was the one I rewound on the OSC coil. I assume it stops oscillating above 1300 due to the way I rewound it, so will rewind it tonight and give a followup report. In the past, whenever I have rewound these, they performed well and were not finicky for me.
While you've got it out, do the drying routine with it. On mine I was able to see the leakage thru the form between windings and to the mounting lug with a DMM in the 2-4 Mohm range. Once I got it all dried out the radio performed quite well and most of the 'oddities' I had been seeing had disappeared.
Since you can align around 1500 khz,but have a drop around 1200 khz , make sure that the plates of the variable tuning capacitor arent shorting somewhere in movement, dirt & debris on the plates? Sometimes a "magnifier glass" and close visual inspection of the variable tuning cap plates reveal "shutdown" in certain areas if they infact do touch together? Im sure you have already inspected this? Blue Shower electronics bath-cleaner does wonders sometimes cleaning these variable capacitors if needed?Just tryin to help.
Well, here's an update.There is no short in th etuning cap; it plays with the plates fully meshed, stops oscillating as they disengage, but by ohming it there is no short anywhere. I pulled the OSC coil again, tried winding the cathode section various ways, but the problem persists. I then found that in fact with the bandswitch in the Police position, it would resume oscillating at the high end of the band, play stations up there, and if switching to the BC band again, it will receive some SW stations up there!! Also, sometimes on BC with plates fully meshed it stops oscillating, but can be restarted by a signal at that frequency from the signal generator. So the OSC tries to function but is unstable in function and frequency. I replaced the 700 pfd cap with a nice silver mica, and the resistor in the cathode circuit as well. I did not try the hair dryer trick yet. I think tonight I will pull it out again, and rewind the main section on the outer coil form and hope that is a cure.
And the answer is, rewinding the main section on the outer OSC coil cured the radio. This was my first experience with a section that had continuity, yet was not able to function. Only about my 20th Philco 89 repair!! So more experience is a good thing.
All's well that ends well...
or is that all's well that's Boswell? (Boswell Sisters)
Oh, nevermind.
Congrats, Mike, on another successful repair!