National SW-54 Thoughts

[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.