Murphy's Law and Theremins - Electronics Guru Help Needed

If things can go wrong, they certainly will!

Firstly, thanks Gordon and Thierry for your helpful suggestions in my other post, I managed to do a temp fix to get the antenna connected.

To say the least, the small defective nut was the least of my problems. When I powered on the instrument, after a short period it seemed to develop ethereal hiccups. And it just goes on hiccuping.

It's pretty hard to describe, please listen to the sound file:

etherwavehiccup.wav (http://www.hotlinkfiles.com/files/2182329_snn8p/etherwavehiccup.wav)

The start of the file is what it sounds like when nothing is near it. It just hiccups on it's own. Later on I just about managed to reach over to get a tone, you can hear it hiccup while I am holding a tone.

Can anyone tell me why it's doing that? Is it someathing I can fix? I should be able to handle some soldering if it's a dying capacitor or a loose connection.

Many thanks in advance for any help and suggestions.

[i]fixed long url tw-staff[/i]

I hear 2 things, that are probably related, going on there.

First off there is excessive 60Hz (50Hz if you are in one of those other-frequency places :) ) in the audio.

Secondly, when that "hiccup" occurs, it has the feeling of a DC pop that happens with a leaky coupling capacitor. If that is the case, the excessive power supply noise might be because DC is being passed to the next stage.

Start with the power supply. If you have an oscilloscope, connect it to the output of the power supply and set it to AC-coupling. Crank the Volts/Div control way down to its lowest setting (or to where you see the power supply ripple big enough to measure). I'm not familiar with the Etherwave's power supply parameters, but I would guess that if you have more than about 150mVp-p of ripple, the power supply filter capacitor is bad.

If the power supply is clean, move on to the detector stage. You will want to first look at the audio output of the detector with the 'scope AC coupled. When it hiccups, you will probably see a big jump in the 'scope trace. If not, you might have to select DC-coupled. If you see the hiccup there, try on the other side of the coupling cap you are monitoring. You might see a smaller jump or no jump on that side.

If it is not there, you will need to check other coupling caps in the same manner.

If, by chance, none of the coupling caps seem to be at fault, you will need to dive into the oscillator circuits. There you might find cold solder joints, broken coil wires or leaky transistors.

If all this sounds too complicated, you can probably find someone near to you who can do the troubleshooting.

Don

djpb, I have to add: The Etherwave's power supply is a transformer only AC type. Rectifying, capacitive buffering and stabilizing is done on the inner circuit board.

I had already a similar hick up effect on my Etherwave, after using the power chord many times as a muting item. The DIN plug on the rear side had gone half out and this loose connection gave me such sounds.

Another aspirant is the wiring of the pitch trim pot. If this voltage is not stable, the working point of the fixed pitch oscillator will hop with similar audible results.

Hello again,

Thank you Don and Thierry for your help.

I have checked the power supply output, as Thierry pointed out it is a simple "wall wart" plugged into source and connects to the Etherwave via a 3 pin DIN jack.

I have verified with a multimeter that it outputs a stable 13.8V to the instrument. The DIN jack is not loose and plugs in securely.

Further observations so far:

1. The instrument behaves well when it is first powered on. After a few short minutes it will start "hiccuping" like mad. Very very frequently at this point.

2. The "hiccups" become a little less after several more minutes. But at this point, if I turn the Volume knob clockwise the "hiccups" increase (it is normally at 12 o'clock). If I turn the volume knob all the way anti-clockwise the "hiccups" become very much less.

3. Turning the Pitch control knob, does not seem to affect the hiccups.

4. After an hour or more, maybe around 90 minutes after power on, the instrument then seems to behave well. It becomes stable enough to play.

5. I have left the unit on for over 24 hours and it seems stable apart from the very very occasional hiccup.

6. If I switch it off after this long while, and after several minutes I switch it on again, it starts the hiccuping behavior again. And then stabilizes a bit quicker, maybe after 15 minutes to 1/2 hour or so.

7. If it is switched off for a long while, several hours, then when switched on again the behavior is the same as before and needs over an hour to stabilize.

I have yet to open the unit up to check further as I am wondering if it will be beyond my abilities to fix. I have very basic electronics and soldering skills but I have no access to an oscilloscope nor a qualified technician nearby, unfortunately.

I am hoping however that it can be fixed on my own without having to return the unit. It would cost too much (with my limited resources) to ship it back for a replacement.

I really love the instrument, and I really hope that with all your help we may be able to pinpoint the fault and fix it.

Seems to be a problem with the volume oscillator whose hick ups interfere with the pitch circuit (Similar phenomenon as the "half-legal" tchirps when touching the volume loop). Ouch. I have to go deeper into schematics, but let me a few hours. It is 10 AM here in Western Europe and I have first to go out for work.

Thierry, thank you.

I would like to add that on my Etherwave, it does not "chirp" when I touch the volume antenna.

If you know how to solder, I would recommend trying to touch up the connections to the "volume control".

Not being familiar with this schematic, I don't know if they use a potentiometer or a variable capacitor there (we're talking about the volume loop tuning control, right?).

As for the power supply, I was not speaking about the wallwort. Inside the unit, at the filter capacitor was what I had in mind. But I don't think your problem is power supply-related after reading your post today.

Now I would be leaning towards leaky capacitors, leaky transistors or erratic trimmer caps in the volume oscillator stages. It could still be a cold soldering connection though. Those can do strange things, including temperature related faults!

Not having any equipment and the knowledge to do the troubleshooting, I'd say you have a high probability of doing more harm than good in trying to fix this yourself.

Don

The volume adjust circuit works by modifying the working point of the both push-pull oscillator transistors via the volume pot and a third transistor.

I would now open the case and check with a multimeter if there are stable 0V at one end of the volume pot and stable -12V at the other end, even during the hick-ups and when playing wildly around with the volume knob. After that, if these voltages are and stay OK, I would check if I get a stable voltage at the middle pin of the volume pot, varying between 0V and -12V, depending on the setting.

If all this is OK, there may be a problem with the volume oscillator which tends to "jump" when the inductance L11 is too high. You could try to turn the slug carefully with the red plastic tool a twentieth of a turn counterclockwise.

Please share your results here so that we may continue in detecting your problem. Perhaps one day this discussion will also be helpful for others.

Thanks for the warning Don, well noted. I know the hazards of mucking about in murky electronic waters but I do always tread very carefully, I haven't fried anything thus far in all my electronics adventures (touch wood) and usually walk away learning something new.

And thanks Thierry, I have gone ahead and tested as you have suggested.

The voltages are stable. I am getting a stable 0V at one end of the volume pot and stable -11.14V (it's not 12.00V though) at the other end, even during the hiccups and when playing wildly around with the volume knob. Should there be cause for concern that it is not 12V?

I also get a stable voltage at the middle pin of the volume pot, varying between -0.00V (fully counter-clockwise) and -11.09V (fully clockwise - should it not be -11.14V?), and stable voltages in-between depending on the setting.

Turning the slug of variable oscillator "L11" counterclockwise does not improve anything. The manual states that the voltage reading between pin 12 of IC "U3" (LM13600) and ground should be around zero with the volume knob at 12 o'clock, although on mine it is around +9V.

The chip on my board is an LM13700 instead of the LM13600 indicated in the manual. I also note that the component layout drawing in the manual is different from the actual board. I suppose they never updated the manual when they revised the circuit.

Checking the entire circuit board visually from the top, there does not seem to be any anomalies. I suppose the next step would be to remove the board and check the solder side for any bridges or cold solder joints.

Unless there's anything else I could do at this point before I remove the board?

Yeah. The fact of having +9V at the VCA control input lets me conclude that your volume osc might be totally de-tuned. Re-adjust it as described in the Moog service manual (http://www.arbiter.co.uk/moog/user_manuals/customising_theremins.pdf) starting on page 7. If this will not help, we may still disassemble your Etherwave.

BTW: The LM13700 and the LM13600 are pin-compatible. The LM13700 is newer and has better audio properties.