National SW-54 Thoughts

[jr_tech]

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

jr

[Electronic M]

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.

Remember that on a tube generator the lytics store enough power to run .5-10seconds and the tubes typically don't loose emission immediately after loosing heater voltage either.

[Dubis7]

I did a little more research. Looks like if I pick up a frequency counter I can get a better idea on whether I have any output.

Any suggestions on models to pick up?

I have to admit, I feel like I'm burying myself in a hole here. It sounds like I need a known good signal generator before I can fix my radio, but I'm having a bear of a time figuring out whether my generator is any good. Is there really no way to take, say, an AM radio transmitter (which I have one of that I know works because I use it daily) and just directly run the antenna output to the grids of the tubes?

[Electronic M]

Originally Posted by Dubis7

I did a little more research. Looks like if I pick up a frequency counter I can get a better idea on whether I have any output.

Any suggestions on models to pick up?

I have to admit, I feel like I'm burying myself in a hole here. It sounds like I need a known good signal generator before I can fix my radio, but I'm having a bear of a time figuring out whether my generator is any good. Is there really no way to take, say, an AM radio transmitter (which I have one of that I know works because I use it daily) and just directly run the antenna output to the grids of the tubes?

That won't work unless the you only inject between the antenna and the converter/mixer stage, your transmitter is capable of being tuned to 455KHz, or the radio is a TRF set.
One other thing you can do is if you have a working analog tuned AM radio you can run it at the same time as the set you are working on and dial around the AM band tuning on both...at some point the oscillators will be at the same freq and beat against each other creating a whistle...if you have two working AM sets put them next to each other and you can observe this behavior.

I use a vintage Tripplet 7000 counter....not because it good so much as it works and was the first cheap used counter I could find.

At RF/IF tube service generators often are non-sinusoidal and produce a double humped wine wave some generators can't accurately read. My best solution to dial my tube Heathkit generator in on frequencies under 30MHz is to dial in the digital tuner of my Sony ICF-SW7600GR world band radio to the desired freq then play with the generator till the Sony gets peak reception then hook the Heathkit up to the vintage radio I plan to align.

[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.

[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.

[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.