National SW-54 Thoughts

[Dubis7]

Quick check confirmed it, pins 2 and 3 were open on that tube. I swapped a new resistor in and swapped in another 35Z5. I'm back to testing, though I'm still waiting on the parts for my station before moving forward much more.

Are there any solutions for injecting signals without a signal generator? I have a signal generator, a Heathkit SG-8, but I haven't restored it yet. I'm trying to decide whether I should step back and get that work so I can start injecting signals, or if there's something else I could use.

I have one of those AM radio transmitter kits, tested and working. Do you think I could clip in a lead from the antenna output via a coupling capacitor and start injecting along the grids of each tube?

[Dubis7]

I need a crash course in signal tracing or something.

I rebuilt (or think I did) my Heathkit SG-8 signal generator. New electrolytics, replaced the paper caps, and I swapped in a 1N4004 for the selenium.

I tested that by placing it along the antenna of an AM radio and was able to pick up some warbling along the B band when I moved the dial around. It moved with the dial, and went away when I switched off the generator. So I think I have that working. This thing has RF out and AF in and out.

From what I understand, I need to connect the ground clip to the chassis through a capacitor, then connect the other end about 450 KHZ on the B band to the input of the grids on the tubes. So I started at Pin 5 of the 50C5, no dice. Just that humming.

Help? Any suggestions? I'm really at a loss on this at this point. I've tested components multiple times and can't seem to find any more that are out of spec. I'm reaching a point where I'm tempted to just replace everything and hope that gets me something.

[jr_tech]

Use the audio output for testing the audo stages, 455 khz for testing the IF circuits and 550 khz and up to test whatever frequency the radio is tuned to.

jr

[jr_tech]

AF is audio frequency ... use for audio stages.
RF is radio frequency ... use for 455 khz IF and frequency that the radio is tuned to.

yes

jr

[Dubis7]

Okay.

So additional question, how do I know if my signal generator is outputting anything on its AF output? I haven't found anyone that has a good test for deciding if that part works yet.

Say I connected the leads directly to a known good speaker. Should I hear something coming out on that?

[Electronic M]

Originally Posted by Dubis7

Okay.

So additional question, how do I know if my signal generator is outputting anything on its AF output? I haven't found anyone that has a good test for deciding if that part works yet.

Say I connected the leads directly to a known good speaker. Should I hear something coming out on that?

If the audio output is strong enough it should drive the speaker to where you can hear with your ear next to it...if not do you have a set of computer speakers or an audio amp with an audio input? You can connect a weak audio output from the generator to those and hear the audio tone.

Generator opperation note: if your generator is similar to my Heathkit IG-102 the compound audio input output jack will only output audio if the internal/external modulation switch is set to internal. And the modulation/AF out pot will set the audio out level.
You can probably use this generator as an AM transmitter if you want...if you set modulation to external and connect an music source to the audio input jack it. Should transmit it...Some of these tube generators (like my Heathkit) need a powerful audio input (output of an amp) to modulate noticably/sufficiently.

[jr_tech]

Too weak to drive a 8 ohm speaker... I can faintly hear mine when connected to earbuds or my Sony 7506 headphones. Switch set to internal, output pot set at max. Scope (high impedence) sez it is about 10 volt peak to peak at 535.7 hertz.

jr

[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.

[jr_tech]

What frequency?
What amplute?
How is the scope connected to the signal generator?

jr

[jr_tech]

The audio output of my SG 8 is a nice sine wave around 500 Hz, should make a pleasing tone on your amplifier... I could hear it with headphones connected directly to the SG 8 without needing to use an amplifier.



jr

[Dubis7]

I decided it was easier to show what's going on with the AF and RF output using video. That way you all can see what I'm talking about.

AF test: https://youtu.be/kNM5KqScNIY

RF test: https://www.youtube.com/watch?v=0GfKehD-BKw

I also checked the AF output with a pair of headphones. Didn't hear anything.

[jr_tech]

Can you connect the amplified speaker directly to the AF output of rhe SG 8 without going through the transmitter? Directly to the output connector, without using the old SG 8 cables? use clip leads or just hold the connector of the amplified speaker against the output connector of the SG 8.
Set SG 8 to “internal” to turn on the audio generator.
RF appears to be working well enough to drown out 60 Hz interference buzz from power supplies, lamp dimmers, chargers LED house lights and other switch mode power supply pests.
Do you have a manual for the SG 8?
http://tubularelectronics.com/Heath_.../SG-8/SG-8.pdf
jr

[Notimetolooz]

It looks to me a problem with the connection. It would be better to diagnose if the wiring was as direct as possible. Making various adapter cables is an important project. I would not use the detour to the transmitter even if it isn't plugged in, it could act as an antenna for hum. just clip the wires end to end. None of the RF controls should effect the AF output. I've replaced the AF connectors with RCA (phono) connectors on my SG-8. I'm very busy today or I would check some things myself.
The probe adapter for that USB scope may have a 10X attenuator that will increase the max voltage you can look at.