04-30-2020, 03:15 PM
I have no oscillator. I may have had a failing oscillator when I first finished recapping the radio, but now I have none. The basic symptom is I can attach my signal generator and peak out the IF transformers as long as I am set at 455 KC. When I set to 1500 KC everything is dead. I attached the oscilloscope to it yesterday and it has flat line output. No oscillation.
I have read on various threads that some of oscillator coils and others have a thing called green insulation rot. I have walked through every aspect of the oscillator circuit and can find no problems. I have heard of a, “ringing,” test that can be performed on those coils to determine if the windings have shorted. Can someone point me to a thread so I can learn how to do that test? I read one thread where the guy would not try to restore the 40-201 chassis that he said had impossible Philco coil rot, he replaced the chassis with an earlier model instead.
Is the problem that bad? That oscillator worked once when that radio was new, I think it should work again?
Things I have done to the oscillator circuit already:
Replaced, 13,15,18,19,37,35,33, there is an additional 4.7K ohm resistor across the primary side of short wave coil #28 not shown on print, mention in other threads on this forum, replaced it too, Jumped across 11&12. I replaced 14, the mica cap was reading about 125 uuF. I didn't have another mica cap and used a disc cap in that location until I could find better stock. It didn't work before I replaced that cap and it doesn't work afterwards either. I have the same issue with 32 reading about 300 uuF. I have no cap in stock to replace it.
There was also a choke in the connection between the grid of 7J7 and 14, it was cracked and so I cut it out and replaced it. I'll try to post a picture of the old one. I interpreted the color bands as wide red band, black band, red band, silver, 2000 uH.
(Can't figure out how to share the picture of the old choke here.) The new one is red, red, red, gold, 2200 uH.
The cathodes of the 7J7 and the 7B7 are connected by a solid wire and share a 180 ohm, and .2 uF cap to ground. They were both originally populate on the base of the 7J7. I left the resistor at that location but moved the .2 uf cap to the base of the 7B7 where there was more room to work.
I may have made other changes that I do not recall as I write this. I am hoping to find help further checking the coils in the oscillator string. I am being a bit detailed in hope that someone will see something I did that would kill the oscillator.
In another thread here on this forum there was mention of the resistor across the coil #28. That is the short wave coil but the primary of that coil is the current path to the PT plate of the 7J7, pin #3. When I could not get any oscillation I removed it and almost immediately 33 fried, fast enough that I could not get the variac turned off. The resistor I had in there was a modern, ½ watt resistor. I had read that sometimes such resistors are installed to prevent noise in a coil. The fact that it burned up the main resistor in that current path, made me wonder if coil #28 is really shorted even though it shows about an ohm of resistance. Is that resistor which was added to prevent noise now carrying the current in that circuit? I'm confused. Sometimes I'm only smart enough to know that I'm stupid.
All figure numbers are based on Rider's Philco page 12-24.
Thanks for any help...
Stay safe guys, we can do this.
Tomie
I have read on various threads that some of oscillator coils and others have a thing called green insulation rot. I have walked through every aspect of the oscillator circuit and can find no problems. I have heard of a, “ringing,” test that can be performed on those coils to determine if the windings have shorted. Can someone point me to a thread so I can learn how to do that test? I read one thread where the guy would not try to restore the 40-201 chassis that he said had impossible Philco coil rot, he replaced the chassis with an earlier model instead.
Is the problem that bad? That oscillator worked once when that radio was new, I think it should work again?
Things I have done to the oscillator circuit already:
Replaced, 13,15,18,19,37,35,33, there is an additional 4.7K ohm resistor across the primary side of short wave coil #28 not shown on print, mention in other threads on this forum, replaced it too, Jumped across 11&12. I replaced 14, the mica cap was reading about 125 uuF. I didn't have another mica cap and used a disc cap in that location until I could find better stock. It didn't work before I replaced that cap and it doesn't work afterwards either. I have the same issue with 32 reading about 300 uuF. I have no cap in stock to replace it.
There was also a choke in the connection between the grid of 7J7 and 14, it was cracked and so I cut it out and replaced it. I'll try to post a picture of the old one. I interpreted the color bands as wide red band, black band, red band, silver, 2000 uH.
(Can't figure out how to share the picture of the old choke here.) The new one is red, red, red, gold, 2200 uH.
The cathodes of the 7J7 and the 7B7 are connected by a solid wire and share a 180 ohm, and .2 uF cap to ground. They were both originally populate on the base of the 7J7. I left the resistor at that location but moved the .2 uf cap to the base of the 7B7 where there was more room to work.
I may have made other changes that I do not recall as I write this. I am hoping to find help further checking the coils in the oscillator string. I am being a bit detailed in hope that someone will see something I did that would kill the oscillator.
In another thread here on this forum there was mention of the resistor across the coil #28. That is the short wave coil but the primary of that coil is the current path to the PT plate of the 7J7, pin #3. When I could not get any oscillation I removed it and almost immediately 33 fried, fast enough that I could not get the variac turned off. The resistor I had in there was a modern, ½ watt resistor. I had read that sometimes such resistors are installed to prevent noise in a coil. The fact that it burned up the main resistor in that current path, made me wonder if coil #28 is really shorted even though it shows about an ohm of resistance. Is that resistor which was added to prevent noise now carrying the current in that circuit? I'm confused. Sometimes I'm only smart enough to know that I'm stupid.
All figure numbers are based on Rider's Philco page 12-24.
Thanks for any help...
Stay safe guys, we can do this.
Tomie
