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Small RF Oscilaltors. - Printable Version

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Small RF Oscilaltors. - morzh - 02-19-2014

I have at least two working (I have Leader LSG-11 for some time and recently got SG-8 Heathkit in Kutztown for 5 bucks). I have some more but did not get to them yet.

Now my main qualm with them is the waveform while perfectly sinewave at the input of the cathode follower (the schematics are fairly similar to the point of being the same between the two) comes out distorted from the follower's output. Usually the bottom is somewhat clipped. The load is the same in both cases - 1K resistor in series wirth 1K pot.

The chart in SG-8 suggests -2 to -15V at the grid of the follower depending on the frequency but I measure pretty much 0V. The only thing that could offset DC bias is the load itself.

Anyway, I was trying to make the Heath work as it refused to work at most of D (6 to 24MHz) and all of E (26 to 110 MHz) bands.
The tubes in the oscillators are 12AU7 in Heath and 12BH7 in Leader. The tubes are very similar (though not totally identical). I had a spare 12BH7, so I yanked out 12AU7 and replaced it with 12BH7. After that the SG-8 works on all bands. And in some of them the output is fairly sinusoidal in shape (well, it now behaves similar to the Leader).

Of course it is not calibrated - the frequency is roughly about what numbers tell, but if in Leader with exception of the C band they are right on the money (checked with the counter I have) in Heath they are good few percent off (higher).
I'm OK with it, I even check Leader with counter first before using it for alignment, so as long as I do that the only thing that matters is the oscillation itself being present.

Well....now I have two oscillators, and I have another SG-8 and then I have a larger something, forgot what, that Sam (Mafiamen) gave me. Will fix them and see which one I like best.

PS. Oh, and I replaced tacky sticky grimy power cord.....now I do not feel like "yack!!!" when I have to plug it.

So much for today's evening (other than performing the second re-aligning of the 70).

G'nite!!!

RE: Small RF Oscilaltors. - TV MAN - 04-04-2014

This is interesting.. I have been "freshening up" some of my test equipment, which has been idle for years (one was in a barn!). I recapped 2 signal gens, a Knight 38k166 I think and Eico 324. I observed something similar in the Knight when I scoped the output, but didnt investigate any farther. I havent checked the Eico yet. I need to see what tubes it uses and try your trick.

RE: Small RF Oscilaltors. - codefox1 - 04-04-2014

An inexpensive modern digital frequency meter is a very welcome addition to anyone using these aging signal generators. Been my experience that the lower the amplitude asked of the instrument, the truer the sine wave to a point anyway.

RE: Small RF Oscilaltors. - BrendaAnnD - 04-04-2014

Some distortion was designed into those old RF generators, in order to get harmonic output. Many radio manufacturers call for you to set a signal generator at a relatively low output frequency, and use harmonics of the output when moving from band to band doing an alignment. Also, that entire highest band is entirely harmonic output. IIRC, they use 3rd harmonic, as odd numbered harmonics are higher in energy than even.

RE: Small RF Oscilaltors. - morzh - 04-04-2014

Brenda

This distortion was clearly not desirable as otherwise it would be present on all frequencies and it is only noticeable on the lower ones. If you want harmonics, you need to keep the waveform across the spectrun.

RE: Small RF Oscilaltors. - TV MAN - 04-05-2014

Ok, now ya did it - I just HAD to hook things up an investigate some more. The one thing I did not do was connect a load resistor, but that didnt seem to affect anything other then the output level a little. I checked the Knight a little more carefully. It's waveform is very jagged (clipped) and low on the lower half cycle. Top part is strong and smooth. This may answer the question as to why my freq counter counts exactally 1/2 the actual freqency on the brodcast band setting. I think this one needs more work and probably will be shelved until I get back into it.

The Eico has an almost smooth, even waveform. It has a small 'crook' on the lower 1/2 cycle coming back up to 0. Freq counter reads almost dead on across the band. Seems this is the better generator. Dont laugh, but I actually "calibrated" this one by using an am radio with a digital tuner on the BC band. It got me very close, confirmed with my new to me freqency counter. Poor people have poor ways Icon_biggrin but it worked!

RE: Small RF Oscilaltors. - morzh - 04-05-2014

Tim

The lower half is clipped in all small gens especially at low freqs.
It is clipped due to the way the output cathopde follower is constructed - the actual signal that is generated and is fed to the follower is nice a good sinewave.

Still works for alignment. But will generate harmonics. Whether it is plus or minus - depends on application.