National SW-54 Thoughts

[Notimetolooz]

OK. You are still kind of getting ahead of yourself. If you tested C10 while it is still connected then the reading will be wrong. The reading would be effected by other components. You should ask about things here first, then if necessary order parts. At this point you don't know enough to make some decisions before forging ahead.
Wow, so that Digilent Analog Discovery can do a lot of things, it isn't just a oscilloscope. In a way that makes it harder to figure it out. You can change things around a lot in the software. Since I don't know the software I don't think I can help with setting it up right.
One thing that I did notice is that the maximum input voltage is +-25V.
That voltage is fine for most modern solid state circuits but tube circuits can have hundreds of volts DC on them. Unless you use a DC blocking capacitor on the inputs you could easily blow the scope input circuits. I would use a 0.1uf, 400 to 600V film cap.
I notice that there are trigger inputs on that device. They may or may not be what you should use with the scope inputs, the trigger inputs might be digital inputs used when the unit is used as a logic analyzer.
Probably few people on this forum know how to use that unit, you might get better understanding on it somewhere else.
The frequency and input resistance are fine. It may not be very sensitive to small signal levels however, but tube circuits do have a high signal level in general.

[Dubis7]

I've got a board that plugs into it to give it BNC scope probes, though the input voltage may still be an issue. I'm using it to read the probes, though, not anything internal to the signal generator, so I'm confident that I'm not seeing any dangerously high voltage there.

That being said, though, do I have any options to test my SG-8 without a known good probe or frequency generator? Assuming that device is out of the question (which it sounds like it just isn't a good option for this) where do I start? It's starting to feel like this whole thing just goes around in circles unless I get a known good piece of equipment that I can use to test everything else.

[Electronic M]

Originally Posted by Dubis7

I've got a board that plugs into it to give it BNC scope probes, though the input voltage may still be an issue. I'm using it to read the probes, though, not anything internal to the signal generator, so I'm confident that I'm not seeing any dangerously high voltage there.

That being said, though, do I have any options to test my SG-8 without a known good probe or frequency generator? Assuming that device is out of the question (which it sounds like it just isn't a good option for this) where do I start? It's starting to feel like this whole thing just goes around in circles unless I get a known good piece of equipment that I can use to test everything else.

Do you have amplified computer speakers or some audio amplifier with an audio input?...that's all you need to test the audio generator half of the generator. Any signal in the 100-5000Hz range will be fine for signal injection testing of audio.

I kinda make it a point not to buy scopes or any piece of test gear currently made broken...If there is a good used one I can afford then a broken one is not worth wasting my life fixing.

[Dubis7]

Let me check. I know I have a little 9V battery powered amplifier. I'll have to see if I have a cable I can use to connect to it.

[Notimetolooz]

Originally Posted by Electronic M

I kinda make it a point not to buy scopes or any piece of test gear currently made broken...If there is a good used one I can afford then a broken one is not worth wasting my life fixing.

Of course some of us consider getting old test equipment working just as interesting and useful as fixing up a radio or TV. But you do have to start with some working equipment you can believe in.

[Dubis7]

Originally Posted by Notimetolooz

Of course some of us consider getting old test equipment working just as interesting and useful as fixing up a radio or TV. But you do have to start with some working equipment you can believe in.

Therein lies my issue with this whole thing. But I do agree, I got this specifically because I wanted to learn how to get it working. I just didn't realize how complicated confirming it as working would be.

I decided last night to pick up another patch cable that I can use instead of jumping through the transmitter connection. It's the same cable I'm using there, just not connected to anything. I'll plan to open the cabinet, bypass the connector and cable with some alligator clips into the unit, then repeat the AF test. Maybe I'll have more luck.

While I'm in there, I'm going to compare the diagrams on the schematic with what's actually wired there. I know I didn't change anything, but I have no knowledge on the history of this unit. It's possible it was assembled incorrectly.

[Notimetolooz]

You could also check the cables and connectors for continuity with a meter.

[Notimetolooz]

I dug out my SG-8. I had modified it somewhat. I remembered that I had changed the audio connectors to RCAs and changed the RF output to a BNC.
I also remembered tweaking the audio oscillator to 400 Hz, which is a standard test frequency, and adding a tube to improve the modulation. What I didn't remember was how I changed the modulation switch hookup. I think you need to have the modulation switch set to "INT." for the internal audio oscillator to work.
At any rate, you should be able to get up to 20Vpp ( that means 20V peak-to-peak) out of the audio output.
Now a little education, in case you don't understand the subject, 20V peak-to-peak is what you would see with a oscilloscope, 20V between the positive peak to the negative peak. Since a sinewave with no DC on it is symmetrical around zero volts, the peak voltage (Vp) is half that or 10V. What most meters would display is RMS (Root-Mean-Squared) AC volts, the equivalent DC value. That means it produces the same power (and current) in a resistor as that DC value. For a sinewave only, the RMS voltage value is equal to 70.7% of the peak voltage. So for a sinewave with a peak of 10V the RMS value is about 7V.
That is a big signal, way bigger than what the battery powered amplifier you tried is intended to deal with. What you probably are hearing is hum due to a bad connection, maybe the ground. The audio oscillator (6C4) may not be working at all.