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.
>Is the problem that bad?
Well if you have enough time money and energy you can fix anything.
>I am being a bit detailed in hope that someone will see something I did that would kill the oscillator.
Here the thing parts placement and layout is important. Moving parts around so they may fit better or look neater can be problematic. The designers design the circuit and on paper it should work but in actuality some parts may not get along with others. This can be the difference between working and not working. There are slight differences in mica vs ceramic even though they are the same value.
>I removed it and almost immediately 33 fried
The only thing I see that would overheat #33 is a short to the chassis somewhere between the triode plate of the mixer down thru the coil.
If everything is working save the osc connect a small cap (100mmfd or so) to pin 4 of the 7J7. Connect the other end to your generator. Turn the modulation off. Set the chassis up for manual tuning and bcb. Set dial for the frequency of the local flamethrow. Adjust the generator for the same frequency + 455kc. With this configuration the generator is acting as the local oscillator. The generator must be 455kc higher than the signal that you are trying the receive.
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Your work around worked perfectly. I was able to tune in the local, strong AM station using my signal generator as an oscillator. It confirms what I had already assumed, my oscillator is dead. Now how to get it going? I have checked and rechecked all of the small passive components in that circuit. I am settling on the two coils which are employed in the broadcast band. Do you know where I might find specifications for those coils? Do you have an ideas about what they should be?
Coil #28 is identified as, “Short wave oscillator coil, part # 32-3242.” But the primary side of that coil, pins #3 and #4, are fundamental to all of the oscillator circuits. My failure is both manual tune as well as push button and as far as I can see that coil is the only component that is necessary for both circuits. I have ordered a coil in hopes I can replicate what's there, (https://www.tubesandmore.com/products/co...oscillator.) But I'm flying blind, I have no idea what value that original coil had. I don't care about the shortwave band, but I need that primary in order to get things working.
Correct me if I'm making bad assumptions here.
Thanks
Hi Tom,
Next I would check the resistance of #28 both pri and sec. Both should very low. Connect your ohm meter to the stator of the tuning cap the osc section and measure the resistance. Switch must but set for manual tuning. The PB are a separate tuned circuit so if the bcb osc coil was bad the PB should still work. You may need to adjust the ant and osc trimmers if the set came from out of your area. Maybe a switch issue.
On #28 the winding that has the hv on it is the feedback. All the other windings are the tuned circuits for each band.
>Now how to get it going?
Now isn't that the $64 question! Just keep chipping away at it till you find the ultimate cause. It can be really repetitive....
>I might find specifications for those coils?
I don't think they are listed anywhere.
>Correct me if I'm making bad assumptions here.
I hope so cauz the osc coil is a pita. Have to do some more troubleshooting.
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
After review the diagram some more I thinking the sec of 28 isn't really a feedback winding but it does act as a plate load for the triode section of the hexode mixer. So you are correct that if the winding was open or had some type of defect the PB would not work. With that in mind it could be replace with a rf choke. It would allow a load for the plate and present the rf osc signal to #32 which block the dc voltage and passes signal. The pri of #28 is connected to the grid circuit thru a bunch of switches and the other end of #28 is connected to the plate signal thru 32 and a bunch of switch.
Philco used a 2 or 2.5MH chokes in some of their '41 models.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
We are both circling the same questions, I've checked them all once already but I'll do it again to see if there is something I have missed. You say that the HV side of coil 28, pins 3 and 4, are the feedback circuit. Thanks, but is that coil or winding simply a choke? Or has it been tuned to a specific frequency necessary to make that circuit oscillate? Why doesn't that extra resistor across that coil also provide feedback for the plate? If I unhook that coil but leave the resistor in place I lose my B+. I am going to check that again too. That winding measures about 1 ohm. The broadcast oscillator coil, #26, measures about 6 ohms.
I assume the main tuning cap on the oscillator side is working, because during the test you had me perform where I used my SG as an oscillator, I was able to rock on and off the station by moving it. I'll make a specific test to capture the specifications. I have an accurate, dedicated capacitor checker. I've used it to measure the capacitance of that main capacitor already. I'll do it again and share the results.
Also, I have used clip leads to jump around the various contacts on the selector switch to be certain that they are all working. I have cleaned that switch.
I'm waiting on parts. I have ordered all new mica caps for the 100 Pf as well as the other two 250 Pf caps, #6, #14, #32. All of them are out more than 20%. As I already mentioned I have also ordered 2 adjustable oscillator coils.
Thanks, you replied at the same time i did, my answer here some place. I own a 2.2 MH choke, I'll try it to see if it helps. The primary side of coil 28 was broken and I repaired it, like 3 times, the lead kept breaking off. It has continuity now, but I have no idea about the windings, even where I'm connected to them. The resistance is high compared to the print specifications.
If I am looking at the correct circuit this has a 1232 as a pre amp. Its failure would reduce the aerial input. if there is an error in the AGC circuit or a problem on AGC diodes that could be shutting the first tube down, or causing it problems. That also applies in away to 7B7, as it also has AGC applied to it.
Thats rather a messy circuit around the AGC . It would have been better as two.
Marc
Again in that same post radiotechnician says, “Look for an open coil in the antenna or RF stages, or an open contact to the tuning condenser is those stages...”
Without saying it the answers implies that if the RF is shut down, perhaps at 1232, it would effect the oscillator. Is that true? If there is no RF, say a bad antenna coil, could that shut the oscillator down?
A bit of my radio history:
455 Kc runs loudly through the system, IFs can be peaked.
Anything above about 700 Kc goes nowhere.
Your Signal generator work around proved that the oscillator is dead.
Immediately after recap I had a semi-functional oscillator for a very short time. The reception was horrible with a good deal of noise, and then all of that simply went away, no oscillator at all now. I have done extensive voltage checks around the 7J7 tube. Both pins 6, (grid 1, hp) and pin 4, (grid 3, hp) show 0 volts. I measured that with a good quality digital meter to ground, not cathode. The cathode floats 180 ohms above ground. The specifications from the tube book for the 6J8-G tube which is supposed to be the electrical equivalent of 7J7, indicates that G1 should show a negative voltage up to -3. Does not mention G3, which I am calling the oscillator grid.
Could that lack of negative voltage shut the oscillator down?
I have felt that the antenna coil is functional only because when I am injecting signal from the signal generator to peak the IFs, I am connecting at the antenna input, pin #1, coil 5, “loop loading coil,” through the coil to G1, pin 6 of the 1232/7G7 tube. I would assume that if it comes through at that point the coil must be working?
If the oscillator voltages are correct it should run regardless of the RF signal coming in the antenna. The oscillator signal is combined with the RF input in the mixer tube. An AVC problem shouldn't affect the oscillator. When all the oscillator parts are good then maybe the oscillator coil is wired wrong. It should be connected to provide positive feedback to sustain oscillation.
Hi Tom,
As Rod pointed out the avc and the osc are two different circuits and don't really have much to do with each other. Like adding antifreeze when the gas tank is empty and thinking why won't it start.
>Could that lack of negative voltage shut the oscillator down?
Yes. The negative voltage is developed when the oscillator is oscillating. Just adding a negative voltage there isn't going the make it work.
Did you disconnect the 2ndary connections from #28 and add the choke?? At minimum I would think that it should get the pb working.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 05-16-2020, 12:08 PM by Radioroslyn.)
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Thanks,
FWIW the oscillator is wired exactly as I found it and according to the print. The only difference in that oscillator circuit is I found a choke connected at Pin #4, 7J7, G3. The schematic shows a solid wire. The choke was factory I'm sure. The color bands indicate 2 MH. I replaced it with a 2.2 MH. I have also tried a solid wire in that position like the schematic indicates. There is no mention of that choke in any of the literature I have found so far.
I've just taken a quick glance at that circuit and with the exception of the 330 ohm resistor on the cathode of the 1232/767, (illustration #7,) which is old but checks good, I have replaced all of the passive components in that circuit up through the 7J7. That includes all resistors, all the capacitors, including the three mica caps, illustration numbers 6, 14, 32. These have all now been replaced with high end mica caps I got from Mouser.
I did replace the choke and 10K resistor from the plate of 1232 with a solid piece of wire.
The replacements are all new parts. I intend to check my work again to make sure I didn't install any wrong values and I will check values to make sure I didn't get a bad part. I will once again make sure everything is wired as it is supposed to be.
Coil #28 has been a problem from the get go. It had a broken primary winding which I repaired 4 times before it stayed. I have no idea how far up that coil the break(s) go. It now has continuity and I have good plate voltage through that circuit.
I have tried 3 different components in that position. The original coil which I repaired, a 2.2 MH choke, (In stock I also have a 10 uH, 47 uH and a slug tuned 100 uH I could try I that position if you think it would make a difference,) and a coil from a Philco parts chassis. They all produced excellent plate voltage but no oscillation. I now own this slug tuned oscillator coil, https://www.tubesandmore.com/products/co...oscillator
which I'm experimenting with in that position. Could too much inductance at that position in the circuit block the AC or sufficient AC from returning to the grid?
Can you disconnect the wire to the 7B7 cathode? I'm trying to determine if current is flowing through the 7J7. If the other cathode is disconnected we can measure the cathode voltage of the 7J7 to see if the oscillator triode is conducting.
On second thought, don't do that. It takes the bias off the 7B7 and it may be detrimental. Let's see what the voltage measures on both sides of resistor 33.
(This post was last modified: 05-16-2020, 08:23 PM by RodB.)
B+ trace from rectifier to resistor #33.
These measurements taken from chassis ground, not B-. Voltage readings are what it is after the described component.
B+ out of rectifier, new electrolytic capacitors, 361.5 VDC.
1K dropping resistor to bring B+ closer to schematic, 292 VDC (I added this after recapping).
Speaker field, 230 VDC.
1K ohm resistor, illustration #35, 215 VDC.
4.7K resistor, illustration #33, 173.5 VDC (Measurement taken from pin #3, 7J7. There is very little voltage drop across the high voltage side of coil, #28).
The voltage across resistor #33 has remained consistent at about 175 VDC (about 40 or so volts drop across the resistor). That measurement was taken from pin #3, 7J7.
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