"Li'l 7" AM Transmitter

[old_coot88]

"Knock a buzzard off a sh**wagon." Gotta remember that gem.

[Winky Dink]

I added the innards of the tube to the schematic. I do get 125V from pin 2 to pin 7, and the singed hair on my knuckles indicates that the heater is working. Sometimes I can learn something from tube data sheets, so I downloaded RCA and Tung-Sol sheets, but I've gained little from that. I keep jumping from one question to the next, and I thought it best to post the revised schematic now instead of waiting for a revelation.


There seems to be a consensus developing that I've mixed up the coil terminals. It'll take me a while to follow up on the latest advice.

Gotta cook dinner now. Thanks.
- Winky.

[old_coot88]

Hmm. Assuming the circuit is wired as shown, the first thing that jumps out is that the B+ supply is shorted to ground through the primary of the coil (lugs 2 and 3). Assuming that that coil has continuity (not open), resistor R3 would be really smokin' (assuming it's 1/2 watt rating). What voltages do you have at the top of filter caps C3 and C4?

[Winky Dink]

I corrected the schematic of the RF coil--I had placed the numbers arbitrarily. Now I think they'll make sense. Following is the P-C70-RF instruction sheet, the latest revision of my schematic, and a picture of the coil showing how the terminal numbers were determined.







Voltage and resistance data for the coil:
Terminal 1 to chassis ground: 0V
Terminal 2 to chassis ground: 0V
Terminal 3 to chassis ground: -156V
Terminals 4 to chassis ground: -156V
Resistance terminals 1-2: 10 ohms
Resistance terminals 3-4: 78 ohms (Curiously, when I use the continuity tester between 3 and 4
I get an "open" reading or an occasional quick flash of "short.")

Voltage at C3: -156V
Voltage at C4: -165V

I see how my error misled everyone. If I've made any other errors in the schematic, it's clearly the fault of the United States Congress and the previous Presidential administration.

I considered trying the connection swapping, but I found it too confusing to follow the advice based on the erroneous schematic.

With sincere appreciation,
- Winky

[old_coot88]

First you need to address that intermittent between lugs 3 and 4. The break is most likely right where the coil wire joins the lug.

[old_coot88]

I wanted to double check that the tube basing (pinout) shown in the schematic is correct, and indeed it is, as verified here.. http://www.bunkerofdoom.com/tubes/sy...1949/117L7.GIF

After you've gotten that break in the coil fixed, there appears to be a couple of errors in the schematic. It shows pin 8 (cathode) of the pentode section going to ground. I don't believe this is correct. Try ungrounding pin 8.

Now connect lug#1 of the coil to pin 8 (after disconnecting lug#1 from the junction of C5/C6). Be sure lug# 2 connects to ground.

Connect the jct. of C5/C6 to lug# 4 of the coil (lug# 4 should already be connected to pin 3 {plate} of the tube, and to C1 which feeds the antenna).

If the circuit doesn't oscillate, try reversing polarity of either the primary or secondary of the coil (but not both).

[Winky Dink]

Thanks. I see six individual things to check out. I have to ask about the grounding of pin 8 first. That's something that actually didn't make sense to me, but I left it alone because it seemed to be consistent with source schematic:

So I'll break the circuit as above?
I'll be back later after I try to address the other issues.

- Winky

[old_coot88]

No, you want to leave the 47K resistor (R1) connected to ground, and break between pin 8 and ground.