"Li'l 7" AM Transmitter

[Winky Dink]

The last two posts are way over my head, but I'll point out that G1 and G2 are -.8V and -.9V.

In addition to curing cancer and establishing world peace, this is what I've done this evening.

Tube:
- Pin layout is consistent with Tung-Sol, RCA, bunkerofdoom, radiomuseum diagrams.
- No continuity between any pins except the heater, 2 & 7.

Coil:
- I don't think there's a break in the coil. Lugs 1-2 read a consistent 10 ohms, lugs 3-4 read a consistent 78 ohms. Checked the continuity tester through a couple of resistors, and it reads "open" at 75 ohms and above. Visually all connections appear solid.

Changes in circuit:
- Ungrounded pin 8.
- Disconnected junction C5/C6 from lug 1.
- Connected pin 8 to lug 1.
- Connected junction C5/C6 to lug 4.
- Tested the unit.
- Reversed connections on lugs 1 and 2.
- Tested the unit.

Here are the last two versions that tested negative. Voltages are updated. Don't understand why there's 0V at pin 8. I'm going to let it sit for now and try to get the tube tested tomorrow morning.



Is this actually a hazing? Are you guys going to come back with, "Gotcha! Nothing was wrong except you forgot to plug it in!"

- Winky

[old_coot88]

Hm. Possibly a bad tube. If the RF section of the tube is conducting, there should only be a very small fraction of a volt on pin 8, readable only on the lowest V scale on the meter. That's because the low resistance of the coil, only 10 ohms, will drop very little voltage at the current the tube pulls.

When you go to have the tube tested, be sure the tester will do both sections of the tube independantly (not a "drug store" type tester).

[wa2ise]

Originally Posted by Winky Dink

Is this actually a hazing? Are you guys going to come back with, "Gotcha! Nothing was wrong except you forgot to plug it in!"

- Winky

Oh, it can sometimes seem that way... And can get quite fustrating. Thing is, one small error can be the cause, and people can be their own worst proofreaders, as they become blind to their own errors. I would avoid extensive modifications to Phil's circuit, if it never worked for you. That usually leads to additional fustration. Maybe one of us who lives nearby can pay you a visit. Not me, though, I'm in NJ.

[old_coot88]

Originally Posted by wa2ise

Oh, it can sometimes seem that way... And can get quite fustrating. Thing is, one small error can be the cause, and people can be their own worst proofreaders, as they become blind to their own errors. I would avoid extensive modifications to Phil's circuit, if it never worked for you. That usually leads to additional fustration. Maybe one of us who lives nearby can pay you a visit. Not me, though, I'm in NJ.

Dang, you are right amigo. And i am wrong for coming up with an alternative circuit that obviously doesn't work. I need to apologize profusely for the wild goose chase. Upon visiting Phil's schematic, the circuit depicted is obviously the correct one, once the tube internals are added to the tube symbol. http://antiqueradio.org/transmitter.htm

Oh man. Sorry as hell.

The cathode (pin 8) should be grounded as it was originally.

The C5/C6 junction does go to lug# 1, not lug# 4.

All else remains as it was.

One thing that's still puzzling though, the resonant tank caps (C6a, b, and c) are now in parallel with the low impedance coil winding, whereas in seems like they should parallel the high impedance winding (as was the case in my "alternative" circuit).

[wa2ise]

Originally Posted by old_coot88

Dang, you are right amigo. And i am wrong for coming up with an alternative circuit that obviously doesn't work.
Oh man. Sorry as hell.

No problem, it's a learning process. I've been there.

One thing that's still puzzling though, the resonant tank caps (C6a, b, and c) are now in parallel with the low impedance coil winding, whereas in seems like they should parallel the high impedance winding (as was the case in my "alternative" circuit).

The high impedance winding should be connected to the resonant tank caps. And the low impedance winding should be in the tube plate circuit. Best to rewire to make it this way, and leave enough wire so you can swap the connections on one of the windings, if it doesn't oscillate.

[old_coot88]

Yeah, just gotta be sure C5 (the feedback cap) remains tied to its junction with C6 when swapping things.

[Winky Dink]

At Murri's Electronics, the man says, "I put my tube tester in storage. Only needed to use it a couple of times a year. You should try A-Gem." A-Gem Supply is the place with the bad tube tester.

On reading Old Coot88's comments about very low voltage at pin 8--well, it does consistently read either 8 mV or -8 mV.

Questions:
From the data--voltages, resistance--is there any indication that either the tube or the coil is bad?
Please identify for me high impedance/low impedance windings and which is primary/secondary.

Fortunately I followed Phil's advice to not solder the coil connections until the thing actually works. In fact, I've gone one step beyond:



I'm inclined to buy another tube and coil, revert to Phil's original design, and rewire it as if starting from scratch. That will wait until I open the Atwater Kent 37 and find out what I need for it.

Until next time, thanks much, and as my old professor said, "When you hear hoofbeats, don't look for zebras."

- Winky

[old_coot88]

Originally Posted by Winky Dink

...very low voltage at pin 8--well, it does consistently read either 8 mV or -8 mV.

Questions:
From the data--voltages, resistance--is there any indication that either the tube or the coil is bad?

The voltage drop across the 10K filter resistor (R3)indicates the tube is conducting. R3 is dropping 8V, which works out to .0008 amp, which squares just about perfectly with the 8mv you were seeing across the 10 ohm coil. Offhand i would say the tube is good, but will wait and see if Robert concurs.
If the coil shows good solid continuity on the ohmeter with no intermittency it is probably good.

Please identify for me high impedance/low impedance windings and which is primary/secondary.

The primary would be the 10 ohm coil and carries B+ to the tube's plate (pin 3), and the secondary would be the 78 ohm coil which forms the tank circuit with C6.
According to Phil's schematic, the primary is shown as lugs 3 and 4, and the secondary is shown as lugs 1 and 2. This appears to be backwards from the way it's been getting hooked up. Maybe Phil could have an eyeball at it and see if a little switchamaroo in the labeling mighta snuk in.

In truth, i was trying to devise a circuit that'd conform to the coil lugs' labeling shown in the schematic, while keeping the high impedance winding in the tank circuit.