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Old 18th Jun 2021, 12:28 am   #1
Chris55000
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Default Levell TG200 Series Oscillators

Hi!

I bought a parcel of four non–working signal generators recently containing two Levell TG200 RC Oscillators.

There's two circuit diagrams known to be in existence, one labelled "Issue 3" that uses 1S44 diodes and very complex wafer switches with dashed "ghost connections" all round the contacts, then there was an "Issue 10" diagram that deleted the "ghostly" switch contacts and quoted 1N4148 diodes, but the "Issue 10" diagram is worse than the "issue 3" one!

Two PCBs are known to be in existence, the brown SRBP type that was the PC200 shown on the original Issue 3 book, then there was a epoxy–glass PC202 main board that did have (some!) silk–screening on the component–side and a slightly neater layout, but still no component reference numbers!

Also, the physical arrangement of the resistors and contact rotors on the frequency range switch did change as well between the "Issue 3" and "Issue 10" diagrams!

I think it is a two–phase RC oscillator arrangement with the first switched RC bank providing 90 plase–shift using J1/J2, then a second 90 phase–shift using the second switched RC bank, uses J3/J4, making 180 in total, the amplified signal being inverted by the long–tail pair amplifier J6/J7 which gives a further 180 phase–shift, then after feeding through the emitter–follower stages J8, J9 and J10, the signal output from tapping down J3's emitter circuit means it's at the same phase to feed back to the frequency–range S2, section 2R as positive feedback to sustain oscillation!

For oscillation to be sustained, this means the gain from J1/J2 and J3/J4) has to exceed the negative feedback fraction given by the 3k9 and 1k02 resistors in series from J10 emitter back to the frequency range switches!

I am completely redrawing these circuits to show the principles in a bit better form but it'll be a few days before the diagrams are fully completed!

Chris Williams
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Last edited by Chris55000; 18th Jun 2021 at 12:34 am.
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Old 18th Jun 2021, 7:05 am   #2
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Default Re: Levell TG200 Series Oscillators

Quote:
Originally Posted by Chris55000 View Post
For oscillation to be sustained, this means the gain from J1/J2 and J3/J4) has to exceed the negative feedback fraction given by the 3k9 and 1k02 resistors in series from J10 emitter back to the frequency range switches!
In addition to this, there needs to be some mechanism to precisely hold the loop gain at unity, servo-controlling the amplitude in order to get a stable, fixed amplitude. Fixed gain will not do. A whisker too much and the amplitude increases until the output smashes int the limits imposed by the power supply and bias arrangements. A whisker too little and the oscillation decays and fades into the noise.

RC oscillators sometimes use thermistors to stabilise their amplitudes, sometimes detector diodes controlling an FET as a variable resistor. Some crude ones might let a valve or transistor drive itself into cut-off over a variable fraction of the cycle, but this introduces a lot of distortion. It's a common trick at RF where the tank circuit's stored energy acts as a flywheel/filter to give an adequate waveform.

So when 'reading' an oscillator circuit, the frequency control, amplification, and overall phase shift get covered first, but don't forget to look for the amplitude stabilisation mechanism. It isn't well covered in the literature, but it is still essential.

The uni labs had plenty of Levell oscillators and meters, but I don't remember ever looking at their manuals in the manuals rack. I was too interested in the Wavetek stuff.

David
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Old 18th Jun 2021, 8:18 am   #3
Chris55000
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Default Re: Levell TG200 Series Oscillators

Hi!

Yes there is a local amplitude control n.f.b. loop, an STC RA54 bead type thermistor is connected back from J12/J13 emitter circuit to the emitter circuit of J10 – I simply left it out of my description of the early stages as it is only local to the later ones, and writing it in the first paragrahs would have made the description more confusing!

Chapter 71 of my T & M book will be covering the TG200 in detail with all the salient points mentioned above!

Chris Williams
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Last edited by Chris55000; 18th Jun 2021 at 8:43 am.
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Old 20th Jun 2021, 9:57 pm   #4
Chris55000
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Default Re: Levell TG200 Series Oscillators

Hi!

The circuit form used by Levell in their TG200 RC Oscillator does exist in thermionic–valve form, allbeit with switched capacitors rather than switched resistors, and this may assist Members grasp the working principle of these units!

If you download the January 1952 issue of P.W. from "World Radio History", and study the article "A Stable Audio Oscillator" on P.9 by N. Wadsworth, you'll see that it is neither a "Wien Bridge", nor a conventional three–stage "Phase Shift" oscillator, but if you mentally replace the valves by transistors, and the switched capacitors by switched resistors, you'll find it's virtually the same as the Levell TG200!

Chris Williams
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