Philco 89B won't oscillate!
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City: Medford, Oregon
This is something I'm not understanding: I have the radio back together. With everything placed as it should be, I hear a small whistle. When I remove the cap from the interstage coil to the input grid (top cap) of the 36, I hear stations!! Garbled, yes, without any RF amplification. Put the cap back on and there's the whistle. With said cap off, I close-couple my hand near either the top cap or the connection and my reception picks up. So the O-coil is working. But what is going on??
SM
(This post was last modified: 05-09-2016, 07:32 PM by Steve McDonald.)
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(05-09-2016, 07:29 PM)Steve McDonald Wrote: This is something I'm not understanding: I have the radio back together. With everything placed as it should be, I hear a small whistle. When I remove the cap from the interstage coil to the input grid (top cap) of the 36, I hear stations!! Garbled, yes, without any RF amplification. Put the cap back on and there's the whistle. With said cap off, I close-couple my hand near either the top cap or the connection and my reception picks up. So the O-coil is working. But what is going on??
Check the resistance of the antenna coil primary. Sounds like it's open.
Terry
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Terry
From all indications, you are correct. But the only way I will be POSITIVE of this is to separate the 10K antenna resistor from the primary and look for 27 Ohms. If I can't find it then, BINGO! As a matter of fact, I have the chassis of my Philco 19LZ out (same chassis as 89B) and it has the same malady. In the case that they ARE open primary, I need to hire someone to rewind both of them.
SM
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http://www.philcorepairbench.com/tips/svctip36.htm
I don't know if this applies to ant. coil but you might be able to do it yourself.
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As for the 10K resistor you can measure the coil resistance with it connected. With the resistance of a good coil being so low 20 something ohms 10K isn't going to effect the reading very much at all. If your coil is open you'll be measuring the 10K resistor not the low value of the coil.
If it is it's not as fussy as the osc coil direction of the winding doesn't matter and number of turn isn't real critical either. 20 turns or so should be fine.
Terry
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I've had all the coils done, NOW it has a slow oscillation the whole way through. Of course, I can pop the top cap off the 36 oscillator tube and shut it up. I've worked on this radio so much and got nowhere. Sounds like it did with an open-primary interstage coil. D***it! Now what is plan C?? si-i-i-i-i-igh...
SM
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Try removing the 44 rf amp tube and connect your antenna to the grid cap of the 36. If all is well should receive stations. Have you tried to align any of the mixer/if trimmers??
Terry
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Well now, I have stations coming in! This thing is just picky as to lead dress in places. I will align it later. Wires going up to the tuning cap were evidently fighting each other, thus causing a parasitic oscillation. Now, I have to zero the stations just like you would with a ham rig on CW. I will review the alignment procedure and later align. WHEW!! Bullet dodged.
SM
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Glad you got it percolating!!! You may find that keeping a stage detuned just a bit is a good thing to stop it from oscillating. And as you mentioned lead dressing can be a little touchy.
Terry
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Now if I can find alignment instructions. It would be Service Bulletin number 146-B- how to align 89 and 19. Or is it 'unobtainium'??
SM
(This post was last modified: 07-11-2016, 09:01 PM by Steve McDonald.)
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It's a secret!!! Adjust trimmers in fig 3 for 260kc (IF) Apply signal to cap of 36 tube.
Adjust trimmers in fig 4 as noted. Low frequency is 600kc apply signal to ant post LF and HF are the oscillator adjustments.
If you use a 1/4" nutdriver put tape around it to insulate it as some trimmers have HV on them. Peak all for max signal w/generator set for low level output w/modulation.
http://www.nostalgiaair.org/pagesbymodel...013876.pdf
GL
Terry
(This post was last modified: 07-11-2016, 09:37 PM by Radioroslyn.)
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I still have the Delryn plastic Philco tool AES sold 20 years ago
SM
(This post was last modified: 07-11-2016, 11:20 PM by Steve McDonald.)
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I still have this oscillation when I put the cap on the 36, sqealing, un-cap the 36, quiet BC band. I'm losing patience here! What NOW??? The only thing that modulates the squeal is running the tuning cap up and down. GRRRRRRRH!
SM
(This post was last modified: 07-11-2016, 11:16 PM by Steve McDonald.)
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