No sound/high voltage readings from theremax

Hi,

I just finished building my PAiA theremax kit. When I first tried it I got no sound, so I turned it off and did a visual check. I found a loose capacitor, so I soldered it on, but still no sound. So I followed the troubleshooting steps and found that the voltage was reading 8.5V at a point where it should read 8.2V (R1 to be specific). I figured I'd better get the voltage down to 8.2, I didn't want to fry anything. So I replaced the resistor that was responsible for this (R1). The original was 100 Ohms, I put in a 200 Ohm resistor and the voltage dropped to 8.15V. I figured that was pretty good.

 

Turned it on, tried again, still no sound. To be clear, as I adjust oscillator coil L3 the gate light does turn on and off as the manual says it should. Also, i can get the gate light to respond to me moving my hand closer or farther away from the volume antenna. I sometimes can hear a faint crackling noise as well. There was never any smoke, or burning smell. I have gone back through the manual and checked every part and connection and I can't find anything wrong with what I did.

 

What should I do? I'm completely stumped. 

Please help

Thanks 

No expert on the Theremax, but the 8.5V you saw isnt going to blow anything up! .. This voltage should be clamped by the zener diode D1, and zener voltage can be higher than specified.. Dropping the voltage to 8.15 by increasing R1 you are probably making D1 redundant - If the voltage across D1 is below its zener voltage then it will never go into conduction and the V+ will effectively be unregulated.

If C3 was the capacitor which was loose, then there is perhaps some possibility that a power-up surge "took out" D1, so failed to regulate the supply - but I think this is unlikely. I dont think that your problems are likely to be related to D1 or V+.. I would put 100R back and replace D1 as a precaution, but really not worry about the Voltage on V+ too much - AFAICS it could be a lot higher (12V?) without causing any smoke!

For the rest - Seems you need to check that you are getting an audio signal from the mixer.. Sounds like your volume circuit is probably working, so you need to see if you get signal at Q8:C..

But others here have built these theremins and will know more about how to debug them.

Thanks for helping.

I put the 100 ohm resistor back in. As for the diode, how close a replacement would I have to get? Which values have to be exactly the same? I was hoping to get it working without having to order anything, and I doubt radioshack will have what I need.

As for checking the audio signal to the mixer, you said to see if I get a signal at Q8:C. I'm not sure what that means. Specifically the ":C" part. I tried taking the voltage at Q8 and got 4.2V. Which reminds me, the troubleshooting guide says the Vr voltage should be 4.1V. My Vr voltage is 4.2V. Is this okay?

Hi Osterac,

Oh, I do hate to be such a grumpy discouraging git, but I cannot help myself! - So please forgive what follows, I know you are not to blame -

You really, IMO, need to have (at least) a little bit of electronics knowledge to undertake a project like the Theremax, and this lack of knowlege on your part is obvious. I do not know what the instructions say (all I have is the schematic) - But its going to be extremely difficult to help you without teaching you electronics right from DC basics such as how potential deviders work right up to AC theory and "advanced" stuff like how heterodyning works.. I do not have the time to do that!

So lets just touch on your points:

"you said to see if I get a signal at Q8:C. I'm not sure what that means. Specifically the ":C" part."

Transistors (or at least bipolar transistors) have a Base (B) an Emitter (E) and a Collector (C).. Thats the "C" part - There are 3 terminals, without specifying which terminal to test, saying "test the voltage on Q8" could (would) be ambiguous!

"the troubleshooting guide says the Vr voltage should be 4.1V. My Vr voltage is 4.2V. Is this okay?"

Vr is a voltage derived from a potential devider (R22 + R23) which are each 1k - If all is well with the circuit then the Vr voltage will be 1/2 the voltage across both resistors (as in, 1/2 V+)

For the rest, I must hand over to some Theremax expert, because they are the only ones who are likely IMO to be able to get you up and running without you having to do a lot of studying  electronics -

IMO, The Theremax is a highly overcomplicated design - certainly no beginners project! Its not (IMO) complicated because it is good or because it needs to be complicated - its complicated because, IMO, it is a terrible design by incompetent designers... This combination of complexity and incompetence makes the Theremax one of the worst projects a beginner could attempt IMO.

Good luck!

Fred.

Wow...

I try to learn by doing. My life situation puts certain limitations on me, and as it is now schooling is not really an option, so the only way I can learn new things is by doing them and soaking up new information from the internet and various sources. If you don't want to help me you don't have to, that's fine. Is the act of simply asking for help such an imposition? I am not demanding anything.

 

That said, I did get a sound out of it. After thoroughly cleaning the leftover flux off both sides of the board, the theremax started making sound. however, I'm not sure it's acting quite the way it should. I can't seem to get the volume antenna to control the loudness in response to my hand motions (even though the gate light does respond), and the timbre knob creates a loud buzz at zero volume (as soon as I turn it up it goes away) and doesn't seem to do much of anything at other volumes.

 

Just FYI, I have installed some mods. I installed the Timbre mod and the classic RCA sound mod as seen on the PAiA FAQ (http://www.paia.com/ProdArticles/therefaq.htm). However, in the case of the timbre mod, instead of just clipping of the capacitor, I soldered some wires to a switch so I could toggle the mod on or off. Similarly, with the RCA sound mod, I soldered wires from the two oscillators to a resistor and switch so I could toggle it on or off. I've been testing with these off.

Thanks in advance for any help any of you can offer.

EDIT: 

Fixed the problem with the timber pot buzzing at zero volume, seems a bit of wire made its way inside the pot. Blew it out with an air compressor. Still can't really hear much of any difference when I turn it, actually seems to turn the volume down a bit.

EDIT 2:

It seems to make a lot of screeching and crackling that come and go. Could there be some kind of interference? 

"If you don't want to help me you don't have to, that's fine. Is the act of simply asking for help such an imposition? I am not demanding anything." - Osterac

Sorry, I think you get me wrong here.. If I could help you, I would..

But I cant - simply because I have no "feel" for the Theremax, so all I am left with is systematic scientific / engineering approach to debugging.. And this only works if I can communicate above a certain level.. Below this level I would need to describe every detailed step and we would probably still get nowhere..

So for both our sakes its best if someone else jumps in (Ideally a Theremax constructor or someone expierienced with this machine) and helps. My big mistake was to post any reply to this thread.. I just hate to see someone being ignored.. But probably its better to be ignored than to be answered by me! LOL ;-)

As I say, Good luck!

Bye,

Cmon Theremax experts (if there are any.. ) - help this guy out!

 

osterac: First, let me point out, I Am Not A Electronics Expert. A couple of years ago I did "Assemble" a Theremax Theremin kit. Check out this link.----

 

http://www.thereminworld.com/Article/14126/sewer-pipe-theremin

 

The only suggestions I can offer is to:

 

1. Remove any switches or components that did not come with the kit.

 

2. Go over the instruction sheet --- Step buy step.

 

3. Do not have any wires running Under or Over the board.

 

4. Do not allow the two shielded wires to cross one another. (pitch & volume).

 

5. You must have a metal plate "Under" the entire board.

 

6. Make certain the plate does not touch the board at all.

 

7. The plate must be "Earth Grounded"

 

8. No wires must lay near any of the 4 square metal cased coils.

 

9. Look at "Each & Every" solder point for bad/loose/cold solder.

 

10. Follow the "Instructions" step by step for adjusting the coils.

 

11. See my "Build" as to how the wires are Not crossing over the board.

 

12. Make certain both antenna are connected solidly.

13. Verify the components that have a "+ & -" are in the proper way.

 

Note: Nothing "Extra" should be added or subtracted, until the Theremin is working to start with. I was fortunate enough that my build worked correctly the first try. Every single detail and step in the "Instruction" sheets "Must" be followed.

 

Sorry I can't be of more help, but as I said to start with, ----- I know "Nothing" about the electronics part.

 

Hope this helps at least a little.

 

In His Service ---- Dana (SewerPipe)

 

                                     

IMHO it was a very bad idea to install modifications before having the basic circuit running and tuned.

If you do not hear anything at the theremin's audio output, that will NOT forcibly say that it is not working, it may be working fine but outputting a very high frequency beyond 20kHz and thus inaudible... That's why you should understand the theory and the principle of operation of a theremin before you start building one. And you should have started first with a relatively simple kit like the Jaycar.

Unfortunately the Theremax has a very female design - over-complicated, unpredictable behavior, difficult to understand and to handle. 

For systematic diagnosing and for a coarse first tuning of the theremax, you should have an oscilloscope with minimum 5MHz bandwidth and a 1MHz frequency counter with at least 10Hz resolution (I've built two and fixed another 4 or 5 for other people, thus I know what I'm talking about). Due to the unusual high oscillators' frequencies, you have an enormous tuning range of almost +/- 80kHz for each of the 4 oscillators, which makes it almost impossible to tune it correctly "by ear", especially because the ranges of the pitch and volume oscillators overlap which can lead you to tune accidentally on false interferences when you do not use a frequency counter to make sure that both oscillator pairs have close frequencies "inside" and a huge difference in frequency "outside". The free-run frequency of both volume oscillators should be at least 60kHz above both pitch oscillators.

Please tell when your workbench will adequately be equipped, so that we can systematically check and tune all sub-circuits of your theremax one after the other.

Send your board to PAiA. For a nominal fee they will check it over and get it working for you.    No worries.  If you are indeed trying to learn electronics, there are easier places to start.

LPKaster

 

[quote]Unfortunately the Theremax has a very female design - over-complicated, unpredictable behavior, difficult to understand and to handle. [/quote]

 

Really? Is that supposed to be a joke? Maybe this is why there aren't many females on this forum.

Anyway, I have to say a big THANK YOU to sewerpipe, seems the main problem was that I had my wiring running over the top of my board, when it needed to run around. I have now tuned it and it works pretty well. There are some oddities, I have to keep the pitch cv all the way up or I get random screeching and distortion noises, and my timbre mod doesn't seem to toggle, but the timbre control seems to work well enough. Now I just need to build a case!