Opamp in theremin

Here is what I would do if I had your 'problem' board:

1.) Remove C30
2.) Remove R42
3.) R81 Wiper in position F
4.) Check waveforms at Q8:C and adjust R26 value for best sine purity.
*There should be no 'nasty' shapes or spikes or transients of any kind - if there are, something is wrong at an earlier stage.
5.) Check the audio output waveform over the full volume range, ensure the waveform is a reasonable sine shape over this range, If there is any swquaring or clipping etc, try increasing the value of R39 to get a clean sine wave - then double this value.
6.) Replace C30
7.) check that the waveforms are ok - that adding C30 has not caused any problems.
*If adding C30 causes distortion of any kind on the waveform at Q8:C, then something really wierd is going on!
8.) Check waveform on IC1:A Pin 2 - this should be a square-like waveform and should switch almost at the mid-value of the sine at pin 5.
9.) fit R42
10.) Move R81 wiper to position E
11.) Check audio output as per (5) and adjust value of R40 to obtain same levels as you had for the sine - then double this resistor value
* if you cannot get signal low enough, you can also increase the value of R42

Fred.

That waveshape was caused by my PC (I wish I had a scope). Output was connected to mic, now it's to line in and it looks 'better'. [img]http://img402.imageshack.us/img402/9397/screenhunter1509.jpg[/img] (http://img402.imageshack.us/i/screenhunter1509.jpg/) Wave is off-centered and clipped. Increased R26 - sine looks nice, but still off-centered.

It seems that my multimeter isn't good for reading low Ac levels, lowest I can set is 200~, I can see 4.6mV when on DC.

I will try this software, thanks.

Thanks, that makes me even more sure that I need a scope :(

EDIT:
Increased R26 to 22k - sine looks nice, but still off-centered.

That's a thing which I don't stop repeating since 3 years:
In order to diagnose theremins, you need a frequency counter and an oscilloscope!

I use a small and cheap combined pocket-multimeter and scope whose sampling rate is largely sufficient for theremins: Uni-Trend UT81B (http://www.uni-trend.com/UT81B.html) which can be bought new on ebay.com for only about 180US$ (some will show still lower prices but adding more shipping, so it will be the same).

Out of that I use a small pocket frequency counter which goes from 10Hz to 50MHz (don't actually remember the manufacturer) and which I bought on ebay for around 40US$ 3 years ago.

[i]"sine looks nice, but still off-centered.."[/i]

You dont say where you took the signal from, or what the waveshape setting was..

If the signal was taken from the audio output, and was at the 'sine' setting, then you have a problem somewhere in the audio path - this is a 'soft' square wave, not a sine! .. If this was taken at the square wave setting, then it looks great - and if you can adjust from this to sine, then it looks like you have fixed the problem.

If signal was taken from the audio output, then the offset is not 'real' - it is due to your "scope".. The output is capacitively coupled (C38) and the waveform therefore MUST cross the mid value which should be the 0V / Gnd reference of your 'scope.. The only exception to this (extremely unlikely) is if there was a big DC offset on IC2:6 AND C38 was 'leaking' this DC to the 'scope input.

You do need a 'scope.. The one Thierry recommends is good value and, as he says, perfectly suitable to this type of application.. Without a 'scope you cannot take accurate measurements of signal levels, offsets etc..

A lot depends on how much you are willing / able to spend on test equipment, and on how much electronics 'work' you intend to do. If you plan to do a lot, and can afford it, I would recommend the Hantek DSO1060 (http://www.hantek.com.cn/english/news_list.asp?unid=14) – this instrument combines everything you need (High resolution frequency counter, High resolution DMM which can display simultaneously with waveform, and simultaneous readout of a large number of waveform parameters, 60MHz Dual Chan ‘scope) in one compact unit.

The DSO1060 can display FFT, but it is not a good spectrum analyzer (8 bits vertical resolution is not good enough to resolve low-level harmonics.. Useful for quick checking for gross distortion, but no good for evaluating ‘Hi-Fi’ circuits). My only quibble re the quality of this instrument was the ‘scope probes – their compensation is poor compared to other probes – but a good pair of probes costs nearly half the price of this instrument, and they are good enough – so I am just being silly!

I paid about £350 for my DSO1060, but have seen them on sale now for about $500 .. You can probably get 3 or 4 UT81B’s for this money – If you bought a UT81B, and a high res Frq counter, you will still be paying a lot less.. but if you intend to go further in electronics, my advice is to get good equipment at the start - The DSO1060 is the only 'scope/multimeter/frequency counter you will need - add an adjustable PSU and signal generator, and you have all the test equipment required for almost anything.

Fred.

Signals were taken from audio output. Soft square wave was when R26=10k. After I changed R26 to 22k I got a sine - I haven't posted it. Now I can't see same wave probably because volume part tuning.

I'm a student so I don't have much money. Electronics is my hobby. You showed great equipment, if I could afford it I would buy it. Now I have to choose the cheapest way. I managed to change my old computer into oscilloscope, now I use better software (I had been using audio recording software). Other option is to borrow old soviet oscilloscope from my friend.

Measures done while R26=22k R56=4k7. I can't tune volume to make it inaudible :/ Scale set with potentiometer (max 5V soundcard input), so it may vary.

All without R40

http://s1123.photobucket.com/albums/l557/Arsimantur/Theremax/ - see description
it shows that most damage is caused by volume part, I'll see when I wake up what is on C36.R39.R40 with/without pin2, c30, r42 as it's hard to see anything on audio output.

[i]"Other option is to borrow old soviet oscilloscope from my friend"[/i]

I suggest that this is probably the best option - most of the old Soviet 'scopes are good instruments (I have a soft spot for them - the first 'scope I ever bought was a Russian 'tube 'scope back in the '70's.. It was huge and needed a trolley to move it - but it worked without ever giving a monents trouble, and was still working when I gave it away in the mid 90's)

Wish you the best with your explorations! You will find that you will need to buy equipment at some time - If you have space for chunkier equipment, buying old used equipment (particularly old military gear) can save a lot of money.

If you want to continue this thread, perhaps we could do so Here (http://www.thereminworld.com/forum.asp?cmd=p&T=4819&F=3) as the S/C has a slightly similar topology, and its easier to tap an existing (dying) thread than to create a new one!

Fred.