Theremax; No Output, help wanted!

Hey all,

My name is Eric (if you couldn't tell) and I am new here.

I have a question about the Thermax circuitry. The Transistors (Q1-Q4), they are all supposed to read the same voltage, correct? I tested their voltage and all of them read about 5.5v except Q4, which read 0v. Now, as the subject stated, I am not getting any output at all, with the exception of the humming noise when you touch the base of Q10. Is this reason because of a dead transistor (Q4) or is there something else I should be looking for?

ANY information would be of great assistance, Thanks!
- Eric

No idea why is posted it twice, sorry.

Hi, Eric_ ,

The hum is a good thing ... it means that your output section is working.

For starters, you want to be sure that the rf oscillators are working. One quick way to check is to place an AM radio near the Theremax and see if you can tune the radio to the oscillators (it should be low on the dial and you should hear a "whooshing" sound.

Or, you may want to check the coils with a frequency counter to be sure they are oscillating (as I recall, around 500-700khz).

Are you following the calibration procedure in the Paia manual?

So... make sure the oscillators are working -- then work forward from there.

Also, check the other threads here on Thereminworld that deal with the Theremax... you will find a lot of information on troubleshooting, etc.

Good luck!

I just spent a few hours working on the Tmax, it turns out that the humming I heard was not from the base of Q10, rather another lead from Q10 except the base. =\

I still am not sure about the Oscillators; I have searched for the answers/help on this site with little luck, unless I used the search feature wrong. Anyways, is there any way to defiantly tell whether or not an Oscillator Coil is defective? I did try the radio; I got a pretty strong "swish" from L1, but not so much on any of the rest.

I apologize for probably asking the same question every newbie asks, but I have tried searching and most people had a 'scope to use, which I do not, I do not even have any Frequency Counter, I went to a local Electronics store today, they said the only thing they have that could read anywhere near 500+kHz was $400, I'm on a pretty tight budget until the end of the month...

Thanks for reading! Any info would be awesome!
- Eric

Well, that is good news that L1 is working.

The coils are easily damaged when soldering. The problem is that the heat from the soldering operation can travel up the lug and desolder the pigtail lead(s) from the coil. They can be resoldered however you must remove the metal can from the coil, use a magnifying glass, and work with "surgeon's hands".

I own a digital VOM from Radio Shack... nothing fancy however it does have a frequency counter. It didn't cost anywhere near $400. Anyway, you don't have to have a frequency counter however you will need a VOM (Volt-Ohm meter).

Another way to check your coils is to do a continuity check -- that is, a coil should offer little resistance to a dc voltage. If you don't have continuity, then suspect a desoldered lead.

If the coils are ok, then check for cold or weak solder joints, solder bridges, and parts that are in the wrong place or in the wrong orientation.

Don't feel bad -- when I first assembled my Theremax I spent three evenings to find a weak solder joint! I had also inadvertantly overheated a coil when I soldered it.

There should be a a message in bold print on the Theremax instruction manual: [b]"The coils are easily overheated when soldering. Solder the lugs to the circuit board as quickly as possible to minimize the heat!"[/b]

If you suspect that you messed up your coils, contact Paia and they should be able to send you a replacement.

Also, Scott at Paia is very helpful -- once you have checked all the "obvious" things you may want to contact him -- you might be able to troubleshoot with him over the phone or he might want you to send him the unit for him to troubleshoot.

Good luck!

Thanks for all the help Kkissing! After going over all my solder joints, polarities and what not and not finding anything obviously wrong, I did the continuity check. All L1-L3 worked, but L4 did not, so I opened her up and indeed, one of the leads had been disconnected. I attempted to resolder, but the strand of wire was to short to reach to the coil, hence ending in a failed attempt.

I will contact paia and see what they can do about replacing the coil.

No what will I do in my freetime while I wait for the coil? =\

Thanks Again!

[i]"what will I do in my freetime while I wait for the coil?"[/i]

Well, actually, there are a few things you could do.

You can go ahead and start with the calibration procedure according to Paia's manual. You can go ahead and put the jumper in place to force the volume circuit to the "high volume" state and you can then tune your pitch oscillators. You can also verify that the tone control works. Also, you can verify that the pitch cv works.

When you do all this, then you will know that both pitch oscillators are working, that the output vca is working, and that the quad comparator chip is working.

Then, when you install your new L4, all that should remain is to tune the volume oscillators (L3 and L4).

Enjoy!

Finally some pitch response!

I took out L4, I re-jumped from D1 to R83 Lug 3 and to my amazement, I could actually get an audible tone after adjusting L2. However my little mini amp was all the way up (volume) and the tone wasn't very loud, I am still happy to just get some sort of response!

The Pitch was "backwards" too, as in when I go closer it got lower and the further away I got the higher pitched it became. Except about 4 inches away, it worked like it should have. Perhaps a tunning issue?
Or is that due to the volume oscillator missing from the circuit all together?

Anyways, I am ecstatic that I finally got a sound out of it.

Excellent news, Eric!

You need to adjust L2 to get the proper pitch response -- L1 is probaby ok as is.

You may be heterodyning with a harmonic rather than the fundmental.. or you could be getting weak interaction with L3.

Anyway, experiment by tuning L2 and tune for the loudest signal. Within the range of L2 you will likely find a weak signal and strong one. You want the strong one.

You are on your way!

KKissinger should work for PAiA.
He knows the T-Max in and out.
teslatheremin