National SW-54 Thoughts

[Notimetolooz]

Originally Posted by Dubis7

Looks like my signal generator isn't working. I hooked it into a USB scope. It shows an identical waveform when the unit is turned both on and off.

Guess I'll need to debug this before I can get back to the radio.

I think it would be better for you if you stayed with one thing until the investigation is exhausted before jumping to something else.
The audio signal frequency should be somewhere between 300Hz and 500Hz, well within the range to use a USB scope. What scope is that anyway? You need to give us more details so that we can check the scope specs. A USB scope is a digital scope, what it displays is actually in digital memory. It may work like what would be a "storage scope". It sounds to me that with the generator off the scope isn't triggered so it displays what the last thing it saw before the triggering went away. Does the scope have an "AUTO" trigger mode? That would allow it to sample the signal without enough of a signal amplitude to trigger it.
Once you have the USB scope working you can find the frequency, amplitude and wave shape of the audio signal. A frequency counter would only cloud the issue at this stage since it also needs to be triggered in order to display anything.

[Dubis7]

Sorry, you're right, I'm jumping around. Too many things going on at once.

Let me get that info for you on the scope.

In the meantime, I'm investigating another route on the radio. I realize I've been reading the schematic and chassis wrong. C10 may be my issue.

C10 looks like a resistor, but it's a cap on the schematic. It's listed as 100 mmfd (which I understand is pf). I'm measuring it as .5nf, which doesn't appear to match up.

I've ordered some 100pf capacitors and will see if replacing that brings my signal back. It's moving from the tuning cap to pin1 of my 12BE6.

While I'm waiting on that, let me get more info on my signal generator. I'm using one of these little kit oscilloscopes: https://reference.digilentinc.com/re...iscovery/start

It's basically a 0 feature, simple scope. I technically have a slightly more advanced heathkit scope, but it's unrestored and untested so I'd need to tackle that before I could rely on its readings.

[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.