[Chris55000]

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