[sparkydave]

Hi all, been away a little too long, and finally getting back to the Philco 89B that's langishing on my workbench.  So I had to rewind both the oscillator coil primary, and the second RF coil (between the 44 and 36 tube) primary.  The problem is it receives a few stations, then it becomes nearly silent once you tune below about 1kHz.  On previous advice, I changed the 36 cathode resistor from 15K to 10K, but it doesn't make much difference.  I finally looked at it with my scope, and saw something interesting.  When I turn it on, it warms up enough that I start seeing this on the 36 cathode resistor.  Looks good, seems to be oscillating!

[Image: http://i636.photobucket.com/albums/uu85/...wFile1.png]

But, wait a couple more seconds, and it's doing this:
[Image: http://i636.photobucket.com/albums/uu85/...wFile2.png]


So the primaries have the same resistance, I took careful notes on how many turns they had and which way they were wound, but it sure seems flaky.  Any ideas?  I'm starting to get pretty stuck!  It's as though this oscillator just doesn't want to oscillate nicely.

[Ron Ramirez]

Did you bake the coil prior to rewinding?

Moisture absorption is a serious problem with 19 and 89 oscillator coils. Sometimes, even baking does not help.

When I bake a coil, I literally put it in the oven (or electric toaster oven), standing upright on its steel terminals with aluminum foil underneath to allow a disposable surface for the wax to melt onto, for 30 minutes at 200 degrees Fahrenheit. This does not harm the coil form. Don't cook it at higher temperatures as that can ruin the form.

[markmokris]

Sparkydave,

I have been through this a number of times now.  I just made this post a couple of days ago, and maybe it will help you.

http://philcoradio.com/phorum/showthread.php?tid=13746

I have rewound tickler coils on 89/19 cathedrals probably at least a dozen times.  I experienced the exact same problem,with the oscillator dropping out below 1000 MHz, many, many times.

By the way, I used 8,200 ohms for my cathode resistor.  I didn't bake the coil, but I wouldn't hesitate to do that.

That post above contains what I think is the secret sauce.

Hope that helps.

Mark

[sparkydave]

Thanks guys, I didn't bake the coil. From markmokris' post, I think I've got it right in that I used #38 wire, and according to my notes I did count 27 turns on it. The latest development was I had briefly bypassed the tickler with a jumper to measure DC bias on the tube, and when I removed the jumper it won't oscillate at all now. Maybe I've got some other issues yet. Not ruling out a flaky 36 tube, I recall it was testing a little weak on my emission tester.

[Phlogiston]

I did not bake the last one I did either, still probably a good idea. I think I used 36 or even 34 wire just 'cause it was easier & I don't stock 38, 39 yes but it seem to be getting too small for me to see - funny how that works. Anyway the ones I have done worked. Now I need to redo the antenna coil on the 112X I did last year because it never worked.

[sparkydave]

Well, I took the coil out, baked it, checked resistances, and - no change. I took a look at voltages, and found I have about 200 volts on the plate, and about 126 volts on grid #2. Interesting, since the RCA tube data book shows 90 max for grid #2. Is there a voltage chart of what I should be seeing available? I re-tested my 36 tube, and I must have been mistaken, the emission test looks good.

[klondike98]

From Service Bulletin No. 146 on the audiophool site.

[Image: https://dl.dropboxusercontent.com/u/5462...Tables.jpg]

[Warren]

Well, I took the coil out, baked it, checked resistances, and - no change.  I took a look at voltages, and found I have about 200 volts on the plate, and about 126 volts on grid #2.  Interesting, since the RCA tube data book shows 90 max for grid #2.  Is there a voltage chart of what I should be seeing available?  I re-tested my 36 tube, and I must have been mistaken, the emission test looks good.

Don't you have to bake the coil before you rewind it? Not sure if it will help after it is rewound... maybe someone can clarify this. Also, are you using a dmm for the voltage measurements? As I understand it the voltages will be a bit higher than what they say if using a dmm instead of the old meters they used back then.

[OZ4]

Did you see the trouble shooting guide from Philco for the model 19,38,89 oscillator?

http://www.nostalgiaair.org/pagesbymodel...013876.pdf

[sparkydave]

Thank you for the links, especially the Nostalgia Air one. It does appear that the screen voltages on the first 3 tubes are significantly high, though not because I'm measuring with a DMM. Yes, old analog meters had lower input resistance that could cause lower readings when there was significant series resistance in the way, but not 36 volts lower. I checked the 51k resistor (#47) in the circuit and it measures nearly 87k ohms. Well, that would certainly drive the screen voltages up. If the oscillator is so temperamental that a little bit of moisture will make it stop, I don't doubt that a high screen grid voltage is diverting current away from the plate and changing the operation of the tube.

Thanks again, hoping I found the problem!

[sparkydave]

Huzzah! That was it! Not only was the 51K up around 87K, but the 39K had decreased to 24K, so no wonder the screen voltages were high. Now that I've replaced those resistors, it's down around 87 volts, and the oscillator is oscillating.

[klondike98]

[AI2V]

It's called phase cross-over distortion and can usually be resolved by tweaking the voltages on the 36. I've seen it in Silvertones but usually in the upper shortwave bands. Look closely at the screen voltage and don't be afraid to change some resistor values.