Tuning problem

We attempted the tuning instructions on a TMax but were unable to null the pitch. Got no heterodyne tones at all using suggested settings nor any others we tried. Checked solder joints then went to trouble shooting and measured emitter voltages on Q1-Q4. Q1 and Q4 show no voltage, the others ~ 1.5 volts. No o-scope is available only VOM.

What's the best next move?

Hey all,

We bought some replacement transistors...If no one has any ideas, we may just install these. But would it be more likely that the problem is in the wiring rather than the transistors?

Thanks,
-Stacy

Since you are reading no voltages, you want to check the voltage coming off of the power supply.

Most Theremax problems are due to cold/weak solder joints, solder bridges, and/or incorrect polarities. For instance, check the diodes on and other parts on the power supply section... make sure they are correctly installed.

Check your soldering for solder bridges (that is, where you inadvertantly got solder across adjacent traces on the board.

Good luck!

Well, we are getting voltage (about 1.5) on the other two transistors, so I think the power supply probably isn't the problem. I will check the wiring around there, and of course check for bridges/cold solder joints. Is there anything else you could suggest that would tell us if the transistors are the problem or if we need to be looking more carefully at the wiring leading to them?

Thanks much for the reply!
-Stacy

If you have visually checked everything and you still can't get a pitch, you need to determine if your pitch oscillators are functioning.

If you don't have an oscilloscope, you could check the coils with a digital VOM that has a frequency counter. If you don't have that, then place an AM radio next to the Theremax, tune the radio to a low place on the radio dial (off station) then adjust your coils in the Theremax. You should hear a whoosing or some kind of noise from your radio speaker as you tune the coils.

The coils are very very sensitive to heat. The tiny leads from the coils are soldered to the posts... when you solder the posts to the circuit board, the post heats up and melts the solder. It is reparable.. you have to pull the can off of the coil and solder the lead back to the post. Unless your vision is superb you will need a magnifying glass to see what you are doing.

You may want to troubleshoot your Theremax with Paia. They are very good about troubleshooting over the phone and, if worse comes to worse, you could send your kit to them and they will get it working.

Pretty much the troubleshooting is from one end of the circuit to the other.... you have to be sure the oscillators are all working, then tune them, and trace the signal.

You may want to check some of the Theremax threads... a lot has been written about troubleshooting Tmax's.

btw -- it took me four intense evenings of troubleshooting to get my Tmax working, so don't feel bad if it doesn't work at first. Hang in there, you will get it working eventually. :)

Thanks for your reply.

We tried placing an AM radio next to the theremax as we tuned and got whooshing sounds only on Q2 (Are we supposed to get anything from the others at this point?)

Also, in playing around, we discoved something else. The instructions say to attach a clip lead from D1 to lug 3 of R83, and I think we were previously testing everything with this attached. When we take it off, we also get voltage on Q1, leaving only Q4 with 0 volts.

Thanks for any suggestions you may have. I'm off to check for similar posts.

-Stacy

Progress! We took the case off of L1 and found nothing wrong. Put it back in, and the pitch was working. (Yay!)

Still nothing when we try and tune volume. (No response when turning the coil or placing hands near the antenna. The pitch changes if you touch the antenna.) We're also still getting 0 volts on Q4.

Earlier, I neglected to mention that my casing is an old violin. It's pretty crowded in there and we're wondering if this might cause any problems. A couple of pics: http://www.deviantart.com/view/31504166/

[Hee hee...even without the volume, I had loads of fun playing around with it!]

-Stacy

Hey - neat case! Are you going to mount it on a mic stand or are you planning to hold it upright somehow while you play?

Lol; That would be quite a feat. No, I'll probably just mount it on something. For now, I have it on a tabletop while I mess around. (Still haven't gotten around to looking at the volume...)

*cough*
Actually...it would be kind of cool to set up something like that, if your body didn't interfere too much. If the pitch antenna were mounted in the center of the violin, you could hold it on your shoulder like you would normally, and control pitch with the right hand (even better if you could rig up some sort of "bow" to play with!) The left thumb would rest on the neck of the violin, also as you would normally, and you could control volume with the rest of your hand. Hmm...