National SW-54 Thoughts

[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.

[Dubis7]

Trying to make sure I'm understanding.

Am I correct in saying that I can measure the AF output with a DC voltmeter, and should see about 10VDC coming out of that if I do? Or are you just saying that, were I to plug this into an oscilloscope, I should see it moving from 20V above the 0 line to 20V below the 0 line?

[Electronic M]

Originally Posted by Dubis7

Trying to make sure I'm understanding.

Am I correct in saying that I can measure the AF output with a DC voltmeter, and should see about 10VDC coming out of that if I do? Or are you just saying that, were I to plug this into an oscilloscope, I should see it moving from 20V above the 0 line to 20V below the 0 line?

You can't measure audio with a DC meter. He was explaining to you the difference between AC Vpp, Vpk and Vrms (and how rms delivers equivalent power to the same DC voltage will when driving a resistive load).

[jr_tech]

Originally Posted by Electronic M

You can't measure audio with a DC meter. He was explaining to you the difference between AC Vpp, Vpk and Vrms (and how rms delivers equivalent power to the same DC voltage will when driving a resistive load).

If your DMM has an AC range, likely it has a wide enough frequency response to reasonably measure a 400 Hz output. Using an inexpensive DMM, I measure about 3 Volts rms from the audio out of my SG 8. See picture on my post # 71 for peak to peak (about 9 Volts), rms Volts and frequency of my un-modified SG.

jr