From:   Max Baars   
Reply-To:   m.baars@maxiespages.com  
To:   "Mike" , levnet@korrnet.org  
Subject:   Re: [Levnet] question about digital theremin from the univ.of Glasgow  
Date:   Sat, 08 Dec 2001 18:05:53 +0100  


On Sat, 8 Dec 2001 17:23:30 +0100, you wrote:

>Hello,
>
>can anybody please help me to fix my digital theremin from the univ. of 
Glasgow. 
>My problem is to adjust all the potentiometers, when I adjust 
the pitch the volume antenna
>is not responing well and vice versa.
>Further I can't seem to get a nice sine wave out of it, altough it sounds 
like a sine wave but then
>with a lot of noise into it. This is also exactly what I'll see when I 
connect the output with a scope.
>
>Thank you, and I hope that there's someone able and willing to help me......
>
>kind regards.



Hi.
I haven't build that theremin but here are some ideas that may help:
Dit you decouple the ic's that are used as oscillators? You may need
to solder a 100nf capacitor accros the power supply of each ic (thus
from VCC to gnd), as close to the ic as possible, to avoid unwanted
oscillations. It might be wise to simply decouple all ic's to be safe.

If there are any unused gates, make shure their inputs are either
grounded or connected to vcc.

Make shure the power supply is free of noise. Sometimes supplies with
the standard 78xx regulator ic's tend to oscillate.

If you have done the above and still cant tune the volume and pitch
sections separately, check their frequencies. Your volume oscillators
should be on a different frequency as your pitch oscillators eg.
volume on 200 kHz and pitch on 600 kHz.

Still no luck? Make shure that all oscillators are physically
separated as well as possible. Like avoid crossing and twisted wires.

Using logic circuits in there kind of applications is possible but
sometimes you'll find that things dont work well, and then it
sometimes helps to try a chip from a different brand.
 
Still no luck?
Try to disconnect the connections that join the different modules of
the instrument together, than test each module separately and then
connect each module one at te time to another to try to isolate where
the unwanted interaction comes from.

There is a FAQ for this theremin on the thereminworld website, there
are some mods as well, perhaps the problems you have are adressed
there.

Got it right? Let us know.

Max

/s/
Max PA3GEC
www.maxiespages.com