Tom
We know the heptode works from when you substituted the signal generator in place of the oscillator and a station came in. So I was trying to determine if the triode was conducting, and it is. It should be drawing a couple milliamps. Have you tried another tube? These are loctal tubes; have the tube pins been cleaned? If they are black or dark grey, clean them with 0000 steel wool and then alcohol. I believe the choke coil in the plate circuit was a work around to start the oscillator. It's hard to say at this point but keep it handy. I'm running out of ideas. When this happens I usually start poking around with a wooden or plastic stick to see if something is loose mostly around the tube socket.
To add to Rod's thoughts you could connect a milliampere meter between the hv lead and the bottom of #28. This would indicate the current flow to the plate of the triode section of the mixer tube.
On #28 is the hv winding the outside winding or the inside winding? When you had the 2.2MH choke connected did you try the pb and did they work?
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: 05-18-2020, 12:07 PM by Radioroslyn.)
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I tried 4 different value chokes in place of shortwave/broadcast coil #28. In every instance I checked both push button and manual dialing. I have set my SG to 910 Kc and I have tuned the radio to that same frequency because that is the broadcast frequency of a 1000 Watt local and it's mid point on the dial. My presumption being if I manage to get the oscillator working I will hear the generator at least strong enough that I could peak any trimmers. I have the SG tied to antenna lead marked white-green, no other antenna attached.
HV in your response confuses me a bit, “hv lead and the bottom of #28,” I have already tied a current meter set to MA in series with #28, attached to pin # 3, 7J7 and terminal 3 of #28. There was no current drop. I tried 2 different meters to make sure I was getting an accurate measurement and clip were attached correctly. Meter will produce voltage readings but 0 current.
The HV is the inside winding. They used a cardboard tube, around which is the HV winding. They then put that tube inside a second tube and created the SW winding around the outside. The whole thing is encased in wax. The HV break was the connection inside of that inner cardboard tube. Hair-like wire was brittle and easily broke. I finally heavily solder tinned a chunk of copper solder wick. I put that physically inside of the tube with the lead laying on top and using a pencil iron I heated that solder and pressed the winding into it. I have continuity but I have no idea where I picked up that winding. Was the short piece of wire I found the end of that winding? No idea.
Chokes I've substituted for shortwave/broadcast coil #28: 10uH, 47uH, 100uH, (slug tuned) 2.2 HM, (slug tuned) and the oscillator coil I got from Tubes and More. Last night I tried that Tubes and More coil in place of the existing broadcast oscillator coil. I have tested that coil for continuity and it matches the specifications on the print, but I had never tried to sub it out before.
HV in your response confuses me a bit, “hv lead and the bottom of #28,”
Ok let's do this break the junction of #33 and #35 and connect your meter there (in series with the hv).
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!
I'll do that. I haven't made any current checks in a while so it's time to do that again. Correction to my earlier response. The current check I made was between 33 and 28. I'll do that test again in a bit too.
The circuit is drawing 8 Ma, 175 VDC. I removed the shunt resistor across 28 that the factory installed but is not on the schematic. Readings remained the same. Because the circuit was already unsoldered between 33 and 32/28, I put the series meter there. Everything passed through the current meter then onto 4.7K resistor, 33. Tell me if that matters and I'll do the test again.
0 VAC.
Tom
(This post was last modified: 05-18-2020, 05:15 PM by Tomie.
Edit Reason: Forgot material
)
I'm good with your readings, the triode is conducting, although a little high. So back to oscillation. I have 2 more thoughts. The 26 coil is BC band but is a separate coil from 28 which is the primary. Is it possible that they are not close enough to get "excited"? Can they be moved closer together? Additionally, could the connections to 28 be reversed? Polarity is important to sustain oscillation. How did the tube pins look?
FWIW early on I did all the basic things, including I polished all the pins on all of the Loctal tubes. I've also checked all of the small components, to make sure I have them wired correctly, no cold solder joints, they have not drifted in value somehow or to insure I was not sold a bad part. All of the parts I used in this radio are new, modern equivalents.
Rod I know nothing about the proximity of the coils. They are mounted as they were from the factory. One is mounted on one side of the, "coil" shelf and the other on the other side. I'd have to Frankin something to get them closer.
But your comment got me thinking. I posted a link to this oscillator coil earlier in my comments but here is the schematic for it.
I have tried it in various configurations in place of either or coil 28 primary, and 26. But I have never ran them both through the coil simultaneously. When I've got time again I'm going to try that. You can't get the two coils any closer together than that.
FWIW the old man that has white hair and the long beard told me that I would never encounter a more difficult circuit to service or diagnose than the oscillator circuit in any of the "40s" Philco radios. I smiled to myself when he said that, I am now grinning out of the other side of my mouth.
Right, those 3 circuits are the ones I'm used to seeing. I'm pretty sure the problem is with the coils, especially since you replaced everything else. When you started, did 28 look like it had been replaced?
And I assume you have tested the tuning condenser plates for shorts.
Gentlemen, I have an oscillator. It is very dirty reception but the barn burner at 1310 Kc is coming in with a great deal of static but very discernible. The only combination that I can find to work is using the primary winding, terminals 4-5 of the P-C70 oscillator coil. Wafer switch positions B1 to coil #4, B4 to coil #5. Won't work the other way around. I have attached trim cap, #29 to switch position B4, and it will let me peek and the dial position is very close. Works on switch position B1 too, slightly different trim. It's not perfect but I am pleased in as much as this is the first actual AM radio reception I have had out of this radio since I took it out of the box.
The 910 Kc radio station that I was trying to tune into before is only a 1000 watt station but the transmitter is lest than half mile from me. 1310 Kc is a 100K watt daytime station but it is about 20 miles from here. Now I have to trace the static, 1310 should blow me out of the garage. It's strong but so dirty. So far PB does not work. I might work on that next only because it will tell me if the antenna coil, which is shared by both PB and MT, is the source of this noise. FWIW adjusting trim cap 4B makes no difference in reception. Makes me wonder about that coil too. More coil rot?
Tips appreciated.
Have a great holiday weekend. Stay safe, stay well.
Excellent, Tom. It was narrowed down to the coil but I kept looking for a wierd solution. I gave you everything I have come up against but the coil did it. I would say that if you have problems with other tank circuits the coil would be very suspect.
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