Keppinger Theremin Part Deux

OK

Reuploaded my assembly photos.

Keppinger Assembly (http://s980.photobucket.com/albums/ae286/panelectronica/Gabriella%20Photos/)

Some of my early waveforms had lots of ringing, probably due to mixing of the 2nd harmonic of the tone oscillator with the pitch oscillator.

The chrome chassis pix are not mine, likely Mr. Hasten's. Don't remember at the moment.

Philip Neidlinger

Brill!

It looks to me like my oscillators are actually working fine. I think that the volume has problems, as the oscillation range is only about 100mv - but I think the pitch ones were working OK.

Thanks for posting these.

Steve

No post (mail) yet :(

Check you biasing for the volume!

Philip

BTW Dude, Happy New Year!

Hey thanks - Happy New Year to you too...

So I added in the variable caps and...

... now I can't get the oscillators to function. :(

I did notice that they start to work again when I measure the plate voltage with my meter (either tube). When I do this I get back to regular sine waves on the scope. Not sure why.

MK suggested to move the coils further away. I will give this a shot.

One thing I did notice - I am getting about 110V on the plate for both pitch oscillators, but now only about 75V on the plate for the volume oscillator - should they all to be the same?

At the moment, I'm not even sure if the vacuum tubes are still working properly... I guess they must be as I can sometimes coax an oscillation from them.

Try grabbing the big volume/oscillator coils. If it starts working for an interval, you have the same problem I did. IF so, double the cap values I mentioned to .02 uF from 0.01 uF and see if it helps.

Fun, isn't it?

Philip

A) You may do a differential diagnose by interchanging the oscillator tubes.
B) The plate voltage of the volume osc. Is too low. Check the value of the grid resistor (compare the grid voltage with the fixed pitch osc. They should be almost identical, slightly below 0V. Make sure to use a Multimeter with an input resistance of >=10Mohms) and try with another tube.

Tried grabbing the volume coil - no permanent effect.

Swapped tube - no effect.

* Grid voltage on volume osc -> -10mV
* Grid voltage on fixed pitch -> -16V
* Grid voltage on variable pitch -> -44V

The 6J5 tubes came from an "old stock" place as Tube Depot could no longer get them - I don't know how reliable they really are.

[I have a love/hate relationship with these projects - I hate the problems, but I always learn a lot]

So...

I completely disconnected the volume oscillator circuit (come back to that later) and worked on the 2 pitch circuits.

I think that the variable pitch oscillator is working OK - I get a good clean output that varies nicely as I bring my hand closer to the antenna.

However the fixed pitch oscillator is only putting out about 1/2 the amplitude of the variable.

When I measure the grid voltage I see half the amplitude on the fixed pitch oscillator as I do on the variable.

So it seems like the fixed pitch oscillator is running at half the power as the variable.

I don't think it's the coils or the tubes, as I've swapped them around. All the other components are correct.

The 150V DC feed is configured as a "bus" with the variable pitch oscillator attached first, so in theory could be pulling too much from the supply and starving the fixed oscillator.

Anything else I can check?

Anyone know where I can get a good 750pf cap from? I can only find ceramics...

The first step of a systematical diagnose would be checking the pure DC operating points of V1, V3 and V5 without oscillation:

a) Pull V2 and V4 out.
b) Remove the grid capacitors C4, C7, C12, disconnect L2 from C1 and shorten the L2 side of C1 to ground.
c) Make sure with the oscilloscope that there is no more oscillation.
d) Check the plate voltages of V1, V3 and V5: They should be identical now. Post values here.
e) Check the grid voltages of V1, V3 and V5: They should be identical now.Post values here.

Now let them oscillate under free-run conditions without external influence of the antennae:

f) Disconnect L6 from the tank circuit, shorten L3 so that the non-plate side of R2 is directly connected to +150V. Put back C4, C7 and C12
g) Now all three oscillators should work under identical conditions at about the same amplitude on the cathodes (post values here), both pitch oscillators at about the same frequency (around 180 - 200kHz if I remember well) and the volume oscillator around 400kHz. Post values here. Tell us if this are clean sine waves.

Sorry for these many "post values here", but since I'm only based on the schematics and have no reference circuit here, I need as many details as possible in order to find the problem in your circuit.

OK, it's a theremin and that's why its circuit may show female (unexplainable) reactions, but, hell, this are simple Hartley oscillators...