Jaycar/SC theremin Manual lost, any help?

Hello all.
A while ago, I made a modified Jaycar theremin as one of my first electronics projects. I played it constantly the first few months, but since then it has seen less and less use. Now, it has been waiting patiently collecting dust, but I want to get it back into business. Unfortunately, there are now problems with the tuning when I try to play. I forget now where I placed the manual, and and am lost as to how I can solve the tuning. Volume aerial is doing pitch, pitch aerial is doing nothing, and as this was an early project of mine, the craftsmanship is especially suspect. I really hope the thing isn't broken, and I remember my worry before I tuned it the first time that it was broken, so I am a bit hopeful.

If some kind soul on the Theremin World Forum could supply me with a scan, photograph, or transcription of the Jaycar manual, or at least the tuning section, I would be very greatful!

Hello Sky,

There are several SC / Jaycar Theremins - tell us exactly which version and that might help!

You could look at http://www.element-14.com/community/groups/theremin-general-resources?view=documents

I have placed a number of files here, and some relate to SC / EPE / Jaycar Theremins or use SC boards.. Best bet is to look at the Enkelaar documents, as this uses the basic SC board.. I have drawn the schematic for the Enkelaar variant - I think there may be the original SC schematic in one of these documents.

I am sure that somewhere I have schematics for all the SC Theremins - and I have complete instructions for some as well.

I would be happy to help you bring your Theremin back to life - either here (which may benefit others) or if it gets too technical / risks infringing copyright, you can contact me : fred [at] fundes [dot] co [dot] uk [no spaces]


[b] ADDED -> [/b]

TW has link to S/C modifications:

http://www.thereminworld.com/silicon_chip_theremin_modifications.html

and there is this recent thread http://www.thereminworld.com/forum.asp?cmd=p&T=4646&F=3 where Thierry is helping debug a Jaycar.
Fred.

My theremin is a Jaycar "Theremin Synthesizer" Mk I. The link from which I bought it can be found here: "http://www.jaycar.com.au/productView.asp?ID=KC5295"
It is CAT. NO. KC5295. It is the same theremin as modified by "Max" on this site, and I copied his methods (save for the L-shaped wires and multi-turn pot). It is re-cased and I have made new aluminum aerials.

All I really require is tuning instruction. My primary problem is that the volume antenna seems to want to control pitch, and my pitch antenna does nothing. While I own an oscilloscope, I do not want to learn how to check each component... and am not very skilled that way. I'm sure the circuit is working... yet the solution eludes me.

In the instruction I had, I remember the process being very direct. Begin with the tuning potentiometer, fiddle with one inductor until something happened, then fiddle with the rest until you had a wide range of space and tone. If anyone can simply describe this process to me, I would be very greatful.

You can buy the manual here: http://www.siliconchip.com.au/cms/A_103277/article.html

I doubt that the problem you describe is a simple tuning issue - I suspect you will find a dead FET in either the reference or pitch oscillator. (I suspect that the pitch oscillator is not functioning, and that volume oscillator frequency is being coupled back via the pitch antenna and is being heterodyned with the reference oscillator)

You need to use your 'scope! and follow Thierry's excellent debugging regime!

http://thereminworld.com/forum.asp?cmd=p&T=4430&F=3

You could determine if any of the oscillators ar not functioning by disconnecting the drain resistor on the FET.. Disconnect the 1k between Q2D and +6V.. If there is no change in behaviour of the Theremin, you know that you have a problem with the pitch oscillator!

[b]NOTE: ->>[/b] Disconnecting the drain resistor from any oscillator will safely disable that oscillator. Another useful trick is to disconnect the 6k8 resistor connected between the volume CV op-amp and the bias pin on the multiplier - If one connects the bias pin to +6V VIA A RESISTOR (the 6k8 on the board, or an external 10k) then the audio out will be at / near maximum, and one does not have the added confusion of amplitude variations when debugging the pitch circuit.
Fred.

IF all the oscillators are functioning, I suspect you need to radically re-tune both the pitch and reference oscillators..

In order to get an audio beat frequency from changes to the volume oscillator frequency, the volume oscillator must be beating with something.. The most likely is the Reference oscillator.. If the pitch oscillator is way out of tune, its beat frequency (against the reference oscillator) could be far beyond the audio range - so would be filtered out - the result would be that even if the pitch oscillator frequency varies as a result of hand movement near the pitch antenna, you will hear no change in output pitch.

Try tuning the pitch oscillator first.. I would disable the volume oscillator (disconnect the drain resistor) and tie the bias resistor (the 6k8 connected to the bias pin on the multiplier) up to 6V... you should be able to get audio by tuning pitch and reference oscillators. Then enable the volume oscillator, but leave the bias tied to 6V.. tune the
volume oscillator so that movement near the volume antenna does not alter the pitch and does not cause 'birdies' or other distortions.. Then put the bias resistor back, and tune the volume filter so that the bias voltage increases to about 4.5V with the hand away from the volume antenna, and drops as the hand aproaches the antenna.

Fred.