help please!!

hi every one, i´m new here and i star to meake muy own theremi, well i see a stematic fro this web site and i need know the name an specifications of une of these components, any one how know please tell me, here the stematic.

pict stematics (http://www.thereminworld.com/pics/schematics/SiliconChip/sc-diagram-modified.gif)

the component are T1, T2 , T3 y T4 , please help, any info please, i realy apreciate any help.

hi from Chile!

Hi Dany,

These are 455kHz IF transformers..

You can buy these from Mouser Electronics - IF Transformers (http://gb.mouser.com/Search/Refine.aspx?FS=TRUE&N=4294759688+254428+4294620437+4294741289+4294620443+4294401844)

You need part numbers: 42IF202 [white] and 42IF203 [Black].. you need 3 white and one black.
The above transformers do not fit the PCB (they are smaller) so some fiddling is required - but they are much better than the ones specified..

The specified ones are from Jaycar Electronics.. Jaycar only supplies kits containing one of each [black,white,red,yellow] - so you need 3 of these kits to get 3 white transformers.. but the kits are cheap.. Jaycar kit part number = LF1050.

The Jaycar parts work fine, and fit the PCB.. The Mouser parts are more thermally stable and less microphonic - but with this paricular Theremin, that doesn't matter ;-)

If you are building this without a PCB (you have not bought a kit) I strongly advise you to check out EPE on Theremin Resources E-14 group (http://www.element-14.com/community/docs/DOC-16872) as this is a MUCH better version of the Theremin.

And welcome to Theremin World!

[i]If you have real problems getting these parts, email me (my email is in my picture <--).. I have some spare LF1050 and could send you the parts you need for about £5.[/i]

Sorry -
I only quickly glanced at the schematic, and mistook it for another SC Theremin..

This schematic makes a reasonably good Theremin - it has equalizing coil for improved linearity, and a tuning control on the reference oscillator.. it is fine!

But you must include all the modifications. The coil on the antenna (equalizing coil) is usually somewhere between 8mH and 10mH (it depends on the antenna capacitance).. getting this right is the trickiest bit of building the Theremin..

A really minor beef about that schematic ...

It shows "+6V" on the output of the 7805. The 7805 will regulate the 9-12Vdc input down to +5V.

Don

@djbp_designs: Didn't you see the diode between the 7805's ground pin and ground? It adds 0.7V to the output, so there are almost 6V!

Yes - the 7805 output is elevated to about 5.7 volts by having a diode in its Gnd (ref) path.. Also, it will take an input from 8V to 24V when used like this - but for higher input voltages the dissipation increases so a larger heatsink is needed.

There is a (minor) problem with using a diode to elevate the output voltage from a linear regulator like the 7805.. The voltage across the diode (Vf) is dependent on current through it and its temperature - these are variables which affect the accuracy of the output voltage.. Because oscillator frequency of this Theremin is not too critically dependent on supply voltage (the supply does marginally affect the capacitance of the 'tuning' FET so there is some effect) one can get away with this simple diode trick.

The best 'fix' is to replace the 78xx fixed regulators with variable regulators like an LM317.. This allows the voltage to be 'programmed' to whatever one wants with 2 resistors - calculating the values for these resistors is easy [Vo = 1.25*(1+R2/R1) for the LM317] .. The LM317 is easier to obtain than the obscure regulators specified in the EPE-2008 Theremin.

Have to agree with Fred on that one ... not a good idea to use a diode to raise the voltage of a regulator ... kind of defeats the purpose.

... and yes, a LM317 would have been a better choice.

The quickest and easiest way is like I did: Go to the next electronic component store, buy a 7806 regulator for a few cents and replace the diode by a bridge. No additional resistors needed.

... and, oops, I did miss that diode.

So is there really an important reason for 5.7V as opposed to 5V for this theremin's electronics?

If there is, then maybe there is a "sweet spot" for rail voltage that can be "dialed-in" by using a 10 turn trimpot to adjust the LM317.

There is some voltage headroom needed, especially by the mixer (MC1496) and the audio amplifier (LM386).