Help troubleshooting my Theremax

Posted: 5/20/2016 12:45:19 PM

From: The bottom of the world

Joined: 5/17/2016

Ok man!

I'll let you know any news as soon as I can get my paws on that scope


Posted: 5/27/2016 1:05:40 AM

From: Madison, WI

Joined: 5/19/2016

Carlos: When I built a Theremax recently I replaced the zener with an actual three terminal regulator. I think it was a LM7809 or equivelent. I had to modify the circuit board a little. I hate zener diodes for regulation. If you aren't drawing enough current to regulate at the specified voltage then the voltage change versus current change is pretty severe. You are most likely getting a lower voltage because the theremax doesn't draw enough current to "regulate" at 8.2V. That data sheet for the zener diodes shows a lot of current is needed for the zener to regulate at the specified voltage. I used modern op-amps to avoid the problems you may encounter by using the 748 below the specified minumum voltage as dewster mentioned before. Watch out for using coax to the antenna. There is a lot of capacitance in that short length of coax. I assume you bought good caps for the oscillators since you supplied your own parts. PAIA didn't supply good ones in the kit I got. They were NPO but the worst ones I've seen. 


Posted: 5/27/2016 1:16:38 AM

From: The bottom of the world

Joined: 5/17/2016

Thats one of the first mods I am hoping to do. Put a simple 78L09.

Btw I found the problem!!. I used normal shielded cable, not coax and for some strange reason the oscillators weren't working with that. The moment I desoldered them and tuned the coils the circuit worked ok.
I was going mad.
there are known problems about the sensitivity of the antennas I'm hoping to mod too.

What are the problems of underpowering the 748?

Posted: 5/27/2016 4:45:56 AM

From: Northern NJ, USA

Joined: 2/17/2012

"What are the problems of underpowering the 748?"  -- CBEST

Undervolting any opamp can lead to non-linear behavior.  One skirts the lower end at one's peril.  And any kind of coax / shielded cable probably shouldn't be used at high impedance RF points in a Theremin.  For audio it's OK.

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