Leader LSG-16

[Torkrench]

I've been trying to troubleshoot a Leader LSG-16 signal generator today and I'm not getting anywhere.  When the unit is just generating an RF carrier, everything seems fine.  When I turn on internal or external modulation, things get strange.  The signal on my SDR (poor mans spectrum analyzer) shows about a 300khz wide signal with a ton of harmonics.  With a 1Khz tone, on the audio waterfall you can see a tone at 1khz, 2khz, and 3khz.  In fact, the 2khz tone is louder than the 1khz.  I get the same reaction when I insert a 1khz tone from an external source.  If I bring the external input down drastically, the signal is cleaner, but I can only get the clean signal at about 5% modulation.  The internal modulation level is about 30%.  I've replaced the electrolytic caps inside the unit and so far I've removed and tested just about every component and haven't found a part out of spec yet.  Anyone have any ideas what might be going on?

I've attached two screenshots of the SDR with examples of what is showing up.

[Torkrench]

Here's a schematic if it helps.

[oldway]

This device was a low cost frequency generator for use by radio and TV repairers.

The generated wave was strongly distorted (it was even intentional to be able to use the harmonics and it amazes me that you did not find harmonics in unmodulated wave) and the low frequency was also very distorted.

What do you expect from such a device?

It seems to me that it works perfectly in relation to its conception.

Have you check it with an AM receiver ?

A distorted carrier modulated by a low frequency signal also strongly distorted can only generate very many harmonics.

Amplitude modulation is also very imperfect and also generates harmonics.

NB: I did not see that it is yours first posts.....welcom to the forum.

[orbanp]

Hi Torkrench,

Do check with a scope the output of the audio oscillator, when running in internal mode.
On the collector of Q104 you should see sinusoidal signal, about 800Hz, about 8Vpp, and not that much distortion, about 3% - 4%. (This is from simulation.)
The internal modulation changes "modulates" the supply voltage of the RF-oscillator circuit.
It is done by the emitter follower, Q103, which works as a pass transistor. At the emitter you should see similarly "undistorted" sinusoidal signal, about 3.3Vpp, "sitting" on the supply voltage.
The sinusoidal amplitude on the supply voltage depends on the "raw" DC supply voltage, measurable on the collector of Q103, that I estimated 16V.
The bottom line is that you should see that "undistorted" sinusoidal signal on the supply voltage.
If you see that, then the rest is done by the RF-circiut in the generator, meaning that possibly it is as good as it gets.

"Plugging" the simulations together (the audio and RF-stages) this is what you get.
And here is the FFT of the output. The RF fundamental frequency is 2MHz.

Regards, Peter

[ferdieCX]

Hi Torkrench,
I also had a LSG-16 for several years. When it was unmodulated, the signal looked quite clean in the scope.
The amplitude varied with the frequency, so the modulation percentage with internal modulation varied quite a lot and was sometimes overmodulated.
It served me good to align broadcast receivers, but that was all. It is poor shielded and leaks a lot RF.
I have replaced it 3 years ago with a Heathkit IG-42, the waveform is not clean but it is shielded.

[oldway]

Your simulation was not even valid when the device was new, (in 1973 - 1975, the time of the commercialization of this device) because you forgot to take into account that the components were not precision components. It was a device for hobbyist and radio / TV repairers.

So, 40 years later, with the variations in values due to the aging of the components, your simulation has nothing to do with the device as it should be currently ....

https://www.kevinchant.com/uploads/7/1/0/8/7108231/lsg-16_sig_gen.pdf

[alitronix]

hello all ,
yes I know it past years from the last post here but...but...I have a question:

could tell me someone the output voltage of the transformer, what deliver?  in manual you don't find it , I checked twice.
(I believe 12-15 V but not sure)

I have one of this unit with fried transformer this thing is common in Europe, since you can change the input voltage from 110 to 230 V and there are too many dummies people who don't care about it
and even if it set on 110V they put the unit on 220V line (Europe line) ... of course they kill the transformer.

I know is a old and cheap unit but is fun to work with this things.
thanks for an eventual response..

Alin T.