National SW-54 Thoughts

[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

[Notimetolooz]

Electronic M and jr_tech took the words right out of my mouth.
The measurements I did were with a 10Megohm scope probe and a 10Megohm input DMM, so they did not load the output very much at all. Part of the mod I did was to take the plate signal on the 6C4 through a 0.1 uf capacitor directly to the output connector. It does not connect to the 1 Megohm pot so it isn't variable. It is an example of the maximum signal you might see. The original circuit allowed it to be decreased with the pot.
Also I confirmed that you need to set the control for internal modulation.
When you select external modulation the oscillator circuit is rearranged to form an amplifier/driver that takes the input audio signal and drives the modulation. In your video I think you had it set to external.

[Dubis7]

Finally had a chance to get back to this. I've gone ahead and taken a video.

https://www.youtube.com/watch?v=gLhlQPAO7_4

I tested the AF output again, this time with a direct connection to the same speaker. Still no tone. So I opened it up and ran some continuity and value tests on the AF section. Everything in the AF section (including the wires to the AF out socket) shows continuity and appears to be within tolerance.

I traced the filament pins of the 6C4 tube to where pin3 on the 6C4 connects to pin 4 and 5 on the 12AU7. I discovered that pins 4 and 5 on the 12AU7 appear to be soldered together, which seems odd to me, but it also looks like it works on the schematic.

One question I do have: is Pin 3 of the 6C4 meant to connect there? The schematic I'm looking at is a little blurry, and Pin 3 there leads off to either an M or an N or an X or something. Pin 5 on the 12AU7 has another hard to decipher mark on it. I think those two link up but maybe someone with a better schematic can confirm. If my 6C4 is miswired of dead, that looks like it could kill my AF out.

I also need to check my pot values, but I'm not sure how to test when I have the three pins coming out of it. Any notes on how to best check the resistance reading on one of those?

[Dubis7]

Finally back with a major update.

Regarding the SG-8: It's finally working. It turns out that the generator fine the whole time, but my probe had an intermittent break in the cable. A new probe sorted it out, and I've been able to get steady and strong RF and AF output.

So now, back on the SW-54. I started at the input pin of the 50C5 (pin 5)and worked my way backwards. I got a steady, strong tone from Pin 5 of the 50C5, pin 7 of my 12AV6 (which is output but it means my network is good) and pin 5/6 of the 12AV6.

I ran into trouble at my 2nd IF transformer. Injecting the AF signal to pin 6 of the transformer block gave me a loud but heavily distorted signal, and injecting the AF signal into pin 1 of the block gave me a loud click but no signal.

So I'm thinking maybe my issue is inside my 2nd IF transformer, but I'm still seeking input before I do a swap. I have a spare if necessary.