PT-6 antenna connections - Printable Version
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PT-6 antenna connections - bridkarl - 08-24-2024
I seem to have lost my way on this PT-6.
I see the antenna aerial transformer schematic - I’m reading pins 1 through 4 counterclockwise with a cap connected to 1, pin 2 connected to the chassis, pin 3 connected to 7C6, and a wire going from the tuning condenser to the 7A8, and the antenna leads going to pins 1 and 2 on the coil. Also a wire underneath the tuning cap going to the oscillator transformer.
I think I have this totally confused - does anyone have a good picture showing where the antenna leads etc go?
After attaching an outside antenna and checking the connections against the schematic -and realizing two were reversed - I now get loud volume and a rushing noise - no stations.. As the only tube I haven’t replaced with a known good tube is the 7C6 detector - I’m thinking that might be the problem. New one on order.
RE: PT-6 antenna connections - RodB - 08-24-2024
Hi, reading your explanation I can see the confusion. If you take it one connection at a time it should result in a desired outcome. One end of the loop antenna is connected to the tuning condenser and pin 6 7A8. The other end of the loop antenna is connected to pin 3 of the antenna transformer. Pin 4 of the antenna transformer is connected to the AVC wherever it comes from. Pin 1 of the antenna transformer is connected to a long wire antenna, which you probably won't need right now. Or you can just connect a short one. Pin 2 of the antenna transformer should have the 0.0015mfd capacitor connected then the other end of the capacitor to ground.
If you get the same results then this is a symptom of the local oscillator not working. If you have a signal generator, feed a modulated rf signal at the IF frequency to pin 6 of the 7A8 through a 0.047mfd cap. If you get a tone at the speaker then the oscillator isn't working.
You stated you connected pin 3 of the antenna transformer to the 7C6. There is no such connection on the schematic.
bridkarl - bridkarl - 08-24-2024
I have two PT-6 chassis. On both there is a blue wire that runs from the above chassis transformer to pin 6 of 7C6. So the wire is there connected wrong at the transformer - which would be one problem. As I look at the original picture I took the blue wire from 7C6 it seems to go to a pin - pin 2??
I think my confusion here is not understanding the numbering sequence. I’m reading the pins - looking from the side of chassis towards the tubes as 1 being on the top left and going clockwise. With the cap in the photo attached at 1. I’m not interpreting the diagram on the schematic with the pin numbers correctly I think. Pins 1 and 2 could be 180 degrees opposite.
I’ll draw myself a pic based on your answer and see if I can visualize the schematic better. Thanks.
RE: PT-6 antenna connections - RodB - 08-24-2024
Notice in the schematic the notch at the bottom of the antenna coil is between pins 3 & 4.
Yes, pin 6 of the 7C6 is the AVC connection I referred to above. It should connect to pin 4 of the antenna transformer.
RE: PT-6 antenna connections - bridkarl - 08-24-2024
Thanks. I never realized that was a notch noted in the schematic as described. I’ll recheck my wiring.
RE: PT-6 antenna connections - MrFixr55 - 08-25-2024
Hi Bridcarl
+1 on the others. Pin 6 of the 7C6 is the AVC Diode so it is appropriate that that pin 4 of the coil connects here. Note that it is not just pin 4 of the antenna coil that goes to this point but the secondary of the 1st IF transformer that goes to the same point, as well as a 2.2m resistor from the detector diode and a 0.1uF cap to filter the "pulsating" DC of the AVC voltage. This AVC voltage adjusts the bias on the mixer grid of the 7A8 and the grid of the 7B7 IF tube, thereby controlling the volume and preventing "blasting" when tuning a local station.
Try this:
- Take this radio and a working similar radio outside, away from the house and power lines, and compare. With all of the CFLs, LEDs, digital electronics, transmission of electrical meter info over the power lines, etc., the environment is hostile to broadcast AM.
- This radio should have a loop antenna. If it does not, you must find and install one.
- Unless you are in a rural area, stations should be received with the built in loop antenna.
Break out the VOM or DVM and verify that:
- Between pins 3 and 4 should be 1 Ohm.
- Between Pins 1 and 2 should be 38 Ohms.
- Verify wiring and polarity of the antenna coil and any other coil that was rewired. Seems that in the pic, the wiring is incomplete and the cap should not be connected to pin 1 but to pin 2
- Verify that the loop antenna exists, is in circuit and verify continuity. Ohm between Pin 6 of the 7C6 (AVC Diode) and pin 6 of the 7A8 (Mixer Grid). This should be the same as the resistance of the loop and the secondary of the antenna coil.
- If you rewired the radio (likely because this vintage likely had deteriorated rubber wiring), recheck all connections of the 7A8. Unfortunately, the schematic does not show pin numbers. Attached below is the 7A8 page from the 1940 RCA Receiving Tube Manual version RC-14
Hope that this helps!
RE: PT-6 antenna connections - bridkarl - 08-25-2024
Thanks. The transformer tested per your numbers. Checked the wiring and it appears to be good. I’ll move on to checking the oscillator circuit. I have good tested 7a8 and 7c6 tubes on order so I’ll keep rechecking the wiring until they arrive. Thanks all. Sort of a distracted month dealing with work, funerals , new cat