Signal Injection Question - Printable Version
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RE: Signal Injection Question - Electrothaumaturgist - 12-25-2017
check for 'cold" solder joints at the bottom of the tuning capacitor... That will kill the signal for sure...
RE: Signal Injection Question - Welder - 12-25-2017
Morzh:
Yes, I attached and secured it by soldering.
Welder
RE: Signal Injection Question - morzh - 12-25-2017
So....if you remove the Antenna wire from the GND and inject modulated signal into it, nothing comes out? At all?
RE: Signal Injection Question - Welder - 12-26-2017
Yes, I haven't thought of that. I'll check those.
Welder
RE: Signal Injection Question - Welder - 12-26-2017
Correct.
RE: Signal Injection Question - morzh - 12-26-2017
Correct means nothing comes out when the 550-1600 kHz modulated signal injected in antenna wire but when 455 kHz is injected in the grid of 7A8 it does produce sound. Correct?
Now, there might be a case of 7A8 not functioning properly. The fact that 455kC goes through means your IF filter's ok and the amp!ifying part of 7A8 is ok.
The fact that nothing else goes through might mean that 1) your oscillator doesn't oscillate or 2) badly off. This calls for looking at T3 coil, the C1 section of the tuning cap, and everything else around the 7A8 except the IF xfmr T1 as obviously it is ok.
RE: Signal Injection Question - Welder - 12-26-2017
What's the best way to test the osc.? I have ordered a new 7A8 tube and I'll check around this section also.
Welder
RE: Signal Injection Question - morzh - 12-26-2017
One way of checking it is putting your radio next to another radio, better to another superhet, and listen to the noise/howling as they are tuned an IF delta from each other (in your case 455kHz): tune one radio say to 1500kHz and the other (the one you are fxing) to 1045kHz. You should hear some radio noise / howling in the former as you rotate the tuning of the latter.
Another is with a scope, simply look at the output waveform of the oscillator.
RE: Signal Injection Question - Radioroslyn - 12-27-2017
Or in a bit more simple terms. Tune the broken set to 100 on the dial. Use a transistorize set set near the 7A8 tube to listen for a strong signal when it is tuned near 1450kc. The osc puts out a signal at 455kc higher than the dial seting. So when you set the broken set to 100 at equals 1000kc. The osc is running at 1000kc +455= 1455kc. Simple right!
RE: Signal Injection Question - Welder - 12-27-2017
Okay, I'll try the tests for the oscillator. I do have a scope. The two radios testing might also prove interesting. Just a note of humility: This set WAS playing before my bony hands got hold of it to change the old caps etc. I thought I could make it sound "better". The cord was dangerously rotted and I had to make the entrance hole larger to accomodate a new grommet. The Philco cord had it's own grommet molded into the cord. I confess that my enlarging the hole may have affected some component nearby. I've inspected the set with a magnifying glass and can't find any obvious damage.
Welder
RE: Signal Injection Question - morzh - 12-27-2017
Well. I'd start looking in places I touched, checking aps values, wiring etc, now that we know that the tubes are likely ok since it played before.
On using the scope: DO NOT usethe scope on this radio unless you have an isolation transformer and can plug the radio through it. Or unless it is a battery-powered scope.
RE: Signal Injection Question - Welder - 12-27-2017
Morzh:
Thanks guys again for sticking with this thread. I have everything hooked up to a variac and an isolation xfmr.
I had other issues at home so I'm just getting back with you folks.
RE: Signal Injection Question - oldswab37 - 12-27-2017
Just for chuckles. Are all of your tube voltages correct?
RE: Signal Injection Question - Welder - 12-27-2017
Yes, tubes are correct. I carefully checked the sockets and labeled them with a sharpie.
Welder
RE: Signal Injection Question - morzh - 12-27-2017
No, he's asking about the voltages at tubes' pins not the tibes themselves.