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 Post subject: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 2:16 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 798
Location: Jackson, TN
Hi all,

I built this theremin last year, and have had an issue which I cant seem to get resolved.

The pitch section works fine, the volume, not so good.

The Hartley oscillator which controls the audio gain puts out only about 2V P-P at plate of V4-B (pin 6).
The 2V signal only gives a very small rectified voltage at the grids of the AF amp (measured at CD2)

Should I expect a greater voltage swing from Hartley oscillator?

Thanks, Tim


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Ther_1a.jpg
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Ther_2.jpg
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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 2:37 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16836
Location: Albuquerque, NM 87123
Did you wind your own coils? I guy I know built a Theremin years ago and he had a problem with the way he wound the coils.


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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 3:05 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 798
Location: Jackson, TN
Hi Johnny,

I used coils from AES, they are P-C70-OSC. These are 'supposed' to be direct replacements for the Miller 70-OSC.

The oscillator tunes the way you'd expect, just not much on the amplitude.

Also, I did some more measurements and see the V4B plate volts at 53 and screen at 70 volts. Also, the D power supply voltage is about 250 rather than 300.

I wonder how much impact that has? I wouldn't have thought that to be a show stopper.

Thanks! Tim


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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 3:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36085
Location: Livermore, CA
Are you using a scope to check AC voltage on pin #6? Believe you should have much more than 2 volts AC here.

Just reading plate voltage lower than screen may indicate a larger signal on pin #6?

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Norm


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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 3:45 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 798
Location: Jackson, TN
Hi Norm,

I measured the 2V with scope. Also, the waveform is far from a clean sinusoid. The shape looks like the bottom 25% of a normal sine.

When i saw this I tried several different values of C19 and R18. I was thinking that the bias was somehow incorrect and causing clipping. These changes made only small changes in the 2V output.

I've also tried smaller value of the plate resistor, no help.

Thanks! Tim


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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 6:45 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2076
Location: Saskatoon
The volume oscillator's frequency is fixed by the 500 kHz crystal. The amplitude is controlled by the tank circuit in the V4b cathode: C20, L3 and volume control antenna. Changing the resonant frequency of the cathode tank will change the gain of V4b and the amplitude of the 500kHz signal. If the tuning of that tank is way off, then you won't get much amplitude.


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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 12:03 pm 
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Joined: Aug Wed 26, 2015 9:41 pm
Posts: 750
Location: Baton Rouge, Louisiana
I used that circuit when I built my theremin and had a similar problem with the volume control.
What seemed to help the most was the size and shape of the aerial. I made the aerials with copper tubing, duplicating
the "original" theremin volume aerial as close as possible. While not perfect, it certainly improved the performance significantly.

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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 8:41 pm 
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Joined: Aug Wed 26, 2015 9:41 pm
Posts: 750
Location: Baton Rouge, Louisiana
Here are my prototype aerials. You can have them if you want, as I no longer need them.
I had a "real" set made.
P.M. me either way.
It has to do with the resonance of the tuned circuit, The capacitance of the aerial.
I used mini shielded coax type to connect the antennas to the circuits, grounded on one end only.
Attachment:
antennas.jpg
antennas.jpg [ 39.95 KiB | Viewed 56 times ]

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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 10:50 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 798
Location: Jackson, TN
Mark,

Thanks for your kind offer, but I think great minds think alike. I have already made some aerials that look much like yours.

Also, when I was initially building this thing, I tried several different 'temporary' aerials before making the final version that I have now. Unfortunately, when I tried various aerials, the basic low amplitude problem always seemed to persist.

Bob,
Thanks for your response, in this circuit, the hartley oscillator runs without the crystal, using the tank as you described. The crystal is then used to pass the oscillator output to the V4A, the cathode follower, which peaks up when the oscillator is producing a frequency that the crystal likes. The hand in proximity to the aerial does swing the oscillator frequency as intended, and produces a dc bias that peaks when it should.

The only failure that I can detect is the puny amplitude of the oscillator.

I'm attaching a couple photos of this aggravating thing. The last thing I'll try is another 6AN8, then these photos will come in handy to put this up for sale to some willing victim! :twisted:

Thanks, Tim


Attachments:
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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Sun 17, 2017 11:04 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2076
Location: Saskatoon
Tkilboy wrote:
in this circuit, the hartley oscillator runs without the crystal, using the tank as you described. The crystal is then used to pass the oscillator output to the V4A, the cathode follower, which peaks up when the oscillator is producing a frequency that the crystal likes.


Oops, right. i should have looked more closely at the circuit.

It does appear that something is be loading down the oscillator. Have you tried it with the oscillator completely isolated (i.e.,, with the volume control antenna disconnected and the crystal pulled)?


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 Post subject: Re: Help with this Theremin project...
PostPosted: Dec Mon 18, 2017 2:31 am 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 2419
Location: Tucson, Arizona U.S.A.
Are you using a X10 probe on your scope? Is it correctly compensated? If not, your scope will give you the wrong answer.

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