No sound from theremax
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Author:  osterac [ Fri Mar 15, 2013 10:30 am ]
Post subject:  No sound from theremax


I just finished building my 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 R1. There is no smoke, can the theremax handle this? The Vr voltage is 4.2V instead of the listed 4.1V. Is this okay? I get 7.37V from Q1-Q4.

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. 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


Author:  PAiA-Scott [ Mon Mar 18, 2013 2:46 pm ]
Post subject:  Re: No sound from theremax

The voltage readings you list are close enough and the described volume action makes it seem you're not that far off...

I wonder if that loose capacitor might be one that couples the "Pitch" audio signal (C29, C30, C36, C38), and it being loose was the result of a lifted/broken printed-circuit pad/trace and it although it was soldered again, the path is open where the trace meets the pad.

Capacitors are usually taller parts on the board and force against them translates to the solder joint on the bottom of the board. If no-lead solder was used in the assembly of the kit, more heat is needed to flow the solder to the joint and this might have resulted in the pad lifting or its adhesion being compromised and force from the topside being enough to break the printed circuit.

Breaks like this can be fixed by scraping away some of the green colored solder-mask coating of the connecting trace, exposing copper, flowing some solder to this copper, then, bridging the gap with a fine piece of solid wire (a component lead clipping, or even a single strand from left-over insulated, stranded hook-up wire).

This topic details the procedure:


Or, maybe it isn't a matter of a lifted pad due to too much heat during soldering, but a lifted joint -- a ball of solder that had flown to the component leg but not the solder pad due to insufficient heating of all metal surfaces in the joint (the solder, the component lead, and the printed-circuit solder pad).

If is not something preventing the audio from getting through, maybe it's the pitch pair tuned to a phantom "pitch" that's much weaker than the normal one which might lie a bit further inwards on the L1 slug adjustment.

Author:  osterac [ Mon Mar 25, 2013 11:50 am ]
Post subject:  Re: No sound from theremax

After much tweaking and a little help from the guys at thereminworld, I have the theremin working. I do have some questions though. The issue (the main one), by the way, was that I did not have the wiring from the board wired out of the way of the oscillators. I must have missed that page of the illustrations supplement, but I see it now.

The first question I have is regarding some repairs I had to do. 3 solder joints came loose in the area around R1 cutting off the power to the rest of the board. Since all 3 joints connected up to another joint immediately adjacent, I decided to just make a jumper wire and solder the two joints together. Since then though, it has become significantly harder to tune the theremin from scratch with the procedure outlined in the manual. For one thing, L1 and L4 no longer work at one turn, I need to tweak them a lot. Also, the volume antenna seems to want to act as a pitch antenna, and it takes a lot of fooling to get it working. Most of the time it just seems like luck when I do.

I also have a question about the pitch CV. I find that if I have it anywhere lower than 100% I sometimes get screeching and static at seemingly random intervals, especially when tuning.

I have some mods attached from the FAQ that I wanted help with too. I did the RCA sound mod, where you hook up a 10k resistor to R4 and R9. I made it toggle-able, however, and I was wondering if this might have messed things up. I wired a switch into the circuit. When I switch it on, I have to re-tune and the pitch trim is much more sensitive, but I don't notice much of a difference in sound. The wire leads I used are maybe 5-6" long.

The other mod I did was the timbre mod, I made this toggle-able but I'm not really sure if that was necessary. Can you give me some more detail on this mod (the one where you remove C44)? Will the theremin still sound the same with the timbre dial in 0% position, but just have more timbre in the 100% position? The toggle I set up doesn't seem to do anything.

I have been experimenting with battery power as well. I notice that the power adapter is 100mA. The first thing I tried was a 9v and a couple of AAs. This worked fine, but it would be ideal to use a single battery. I picked up a little (about half the size of a AA battery) automotive 12v battery at a hardware store and tried that but it seems that was under-kill as I didn't get a peep out of the theremin and the voltage dropped immediately. I was looking at something like this (if you know of something rechargeable that is better/smaller, let me know):
It's 5 amps, so that should be no problem, but the real issue is integrating it. I would have to have a way to charge it too. It's a lead acid, so it would trickle charge. I wonder if there is a way to set up the Theremax power brick to do this without wearing out the battery? Or could I connect a beefier adapter that would allow me to play while it was charging? If it all gets super complicated it would probably be best to just put a power jack on the case to connect to a charger, and I suppose you could use a diode to prevent power from flowing backwards from the theremin's adapter to the battery? If so, what kind of diode?

Sorry about the barrage of questions. I thank you for your help. Have a good day.

Author:  PAiA-Scott [ Tue Mar 26, 2013 1:04 pm ]
Post subject:  9505kfpa , Theremax, kit and front-panel/antennae project

9505kfpa , Theremax, kit and front-panel/antennae project troubleshooting:

First, I think it might be helpful to you and other builders to reference these pages: ... artsID.pdf

The printed-circuit area which lifted/detached were likely due to excessive heat during soldering of the joints and/or stress from dull cutters tugging or sharp cutters in a bit too close, or, a combination of these.

It is always best, in terms of the results being as intended, with respect to the details presented in the assembly instruction manual, then, perhaps make changes when satisfied all is in working order. Since Theremax is offered not only as a complete package, 9505kc (kit and case), but also as a board and parts kit only, 9505k, or this with “special” pieces from the case (finished control panel and antennae/mounting-hardware set, 9505fpa, much of the completion of the project is user determined and provided. The Theremax category of the Technical Bulletins and Assembly Notes section of PAiA Talk is a good place to find tips useful in custom builds:


When an overall photo and then maybe one of board soldering and board-to-panel wiring and soldering connections can be included in emails to <>, I can sometimes see things that prevent our “barking up a wrong tree”.

I am glad the routing/dressing of the wiring was recognized and that with this in order, your kit is working better. We sent kits for many years without providing detail about the importance of avoiding letting the wires lay against or too close to parts in the four rf oscillator circuit sections. As an aside, the first and fourth sections extend to the control panel, Volume and Pitch Trim potentiometers and associated/attached fixed resistors to ground/circuit-common linked point-to-point across the panel connector (S)leeve terminals.

The repairs you made to the printed-circuit board, linking legs extending through common pads, should be sufficient as long as the parts/wires involved don't move much or receive external forces which could result in more breaks/opens or further compromise of the part's attachment to the board

The screeching / static noises you observed with adjustment of the panel Pitch CV (amount appearing at the panel output connector) are likely the result of a combination of one or more situations. When the panel Volume (cv amount to the board “vca” (voltage-controlled-amplifier) circuit section) is advanced beyond about a 3 o'clock setting, the audio output section is more sensitive to energies in the f-v sections for volume and pitch cvs and if wires are bundled, or, a seemingly unassociated circuit-section is out of order, a “feedback” sort of condition can occur.. It might be worth double-checking the wiring to the Pitch CV control, R82 and the soldering for it and its links. You did notice the variety of panel potentiometers, right? Volume and Pitch Trim – 1K, Timbre – 100K, Pitch, Velocity, and Volume (cv amounts) – 10K. It is particularly important to the health of the oscillations in the first and fourth oscillator sections (and the resulting heterodynes used for Volume and Pitch CVs (and the theremin audio) that the potentiometers at Rs 80 (vol trim) and 79 (pit trim) are 1K and not 10K or 100K.

I'd say for now, at least, put things back so the modifications you've added aren't in place...

Adding the 10k resistor to couple the pitch oscillator pair affects the sound by mainly reducing the amplitude a bit and partly because the pitch het. is pulled out of shape for a more saw/ramp wave type sound (mostly just at the lowest frequencies); however, as with the gimmick capacitor add-on, these changes take from the bottom-end. The pair snaps out of null and to a higher frequency than was possible with things original. Linearity of pitch is best when the pair is “idling” at the lowest anticipated “pitch”, without hand to antenna control range wasted with movement from null to the musical range. Wiring to a switch for remote on/off control of the coupling extends nodes of the first and second (pitch pair) oscillator circuit-sections and even when “off” there will be some coupling, loading, and entry points for external energies.

The “timbre- and control-response- improving parts changes, 100pF at Cs 39, 44, and 37 / added 6800ohms on R81-3 to R81-2 are now standard. These make the waves at the Timbre control extremes more as originally intended, Sine, Square, and a Mix with intermediate settings). Sine is not really a sine like is available from a signal or function generator, but smooth and pure sounding... When Timbre is set fully ccw for this “clear” sound, introducing Velocity Control (advance R84) works to add some “edge” to the sound when hand to volume antenna variation is extremely rapid.

About substitute sources of power for Theremax (batteries, wall-mounted DC power supplies, etc.), it must be considered there is on-board regulation of the voltage to the various circuit-sections by Zener Diode D1 and associated current limiting resistor R1 and nearby capacitors. D1 works to shunt an amount of current needed to maintain a steady and clean voltage on the “theremax” side of R1. R1 and D1 have to change extra power from the external source to heat. If there is too much extra from the external supply, the two parts will have extra heat and maybe so mulch they'll burn (discolor, smoke) and worse, fail. The 8.2V V+ local power supply has a 20mA current flow. R1, D1, and Theremax and a 12V, 100mA power source balance. When choosing a substitute power source, try to keep the ratings about the same, or also substitute heftier/greater resistance parts at R1/D1. Or connect a regulated 8.2 volt supply without R1/D1. The battery you mention is for devices that don't need so much current, so the load presented to it by Theremax and its voltage regulation resulted in a V+ less the needed 8.2V. Eight rechargable AA batteries would be suitable as a power source to Theremax.

If it is your intent to have a battery powered Theremax, and amp/speaker, for outdoor (unplugged) use, you might find it works best when there is some electrical contact between one of the panel connector Sleeve terminals which run back over wire SG to Ground/circuit-common/DC-power-supply-minus-pole, and the floor, or metal in contact with the floor.

Author:  osterac [ Wed Mar 27, 2013 1:06 am ]
Post subject:  Re: No sound from theremax

OK, I will de-mod my theremin. The page you linked to says not to use paste flux. Can I use other types of flux? I find soldering to be a pain without flux. I used paste flux to assemble my theremax, but a lot of cleaning was needed to get it all off afterwards because the flux was acting as a weak conductor and preventing the theremin from working properly.

You say 8 rechargeable AA batteries will work? So the theremin doesn't actually need 12v then, the diode is stepping that down to 8.2v and anything at or above 8.2v will work? Because 8 rechargeable AA batteries would provide 9.6v. I'll give it a try, I have an RC car battery pack that should do the trick. It's essentially 8 AAs in shrink wrap.

Author:  PAiA-Scott [ Wed Mar 27, 2013 12:59 pm ]
Post subject:  paste flux

With the Kester "44", or equivalent, the solder is hollow with rosin in the core which works as a flux-- it is only necessary to feed it into the heated joint. It's almost like magic the way it flows to the wire or component lead extending through the printed-circuit solder pad or control/connector terminal. It is not necessary to clean this rosin flux from the board for PAiA kits.

Some paste flux is suitable for printed circuit work, but it just complicates the process and is a mess if not cleaned. Cleaning can lead to other troubles when it gets inside pots, trims, switches, etc...

9.6v from rechargeable AAs. If this is connected to the 8.2V Zener through the 100ohm R1, then with 20mA of current flow, 2V will drop -- so it might be best to use a lower value resistor for input to D1 via the battery pack. A 33 ohm will drop about 0.6V.

Author:  osterac [ Wed Mar 27, 2013 1:20 pm ]
Post subject:  Re: No sound from theremax

Hmm, so I guess I would need 10 AA batteries then? There's this on amazon:
Also, about soldering, sometimes I apply the solder to the tip of the iron and then let it flow to whatever I am soldering. When I use this method, I usually need flux because the rosin will burn off before I start soldering anything. I use this method when I need one hand free.

Also, in the tuning section of the theremax manual, it says "We are after the post-audio edge." What is post-audio? I haven't been able to find a reference to it anywhere. I know "post" means after, but "after-audio?" What does that mean? I ask because I am having a little trouble getting the high pitch sound (from the volume het. I think?) to go away.

Un-bundling a wire coming from the pitch CV pot solved the problem with the squealing/noise, so thanks for that!

Author:  PAiA-Scott [ Thu Mar 28, 2013 12:28 pm ]
Post subject:  Soldering, battery power, tuning

This is inviting cold joints... It is better to hold the iron tip to the joint, heating it to the melting temperature of the solder, and feeding it in so the pad or lug fills and flows to all metal surfaces. Just dabbing some in on top of paste flux is likely to leave flux insulating the joint and if it's not trouble right away, it can be later.

Use a finger not holding the solder to steady a wire or component as it is soldered in place. If the wire or part isn't positioned just right, re-melt the joint and re-position. Often, wires can be formed or trained so they stay at the desired position as a joint is made.

Yes, the battery pack you show will work as a power source for the project.

Yes, tuning the volume pair for "post-audio"-frequency is when the faint, background "pitch" is set just beyond the highest audible pitch. As the slug for L3 is turned inwards the heterodyne frequency resulting from the difference between frequency in the third and fourth oscillator circuit-sections will drop down into audio range. As this occurs the volume control voltage will increase and with the velocity control clockwise, the trigger activates as indicated by the Gate/Trigger LED -- a visual cue indicating L3 is about right. Moving the hand to the volume antenna has the same effect as backing L3 out, increasing the volume het. frequency even more post audio (actually it is lowering the frequency of the fourth oscillator resulting in a higher volume het. frequency.

As shown in the image linked below, it is best to not have any of the wires bundled, but just smoothed/formed to curve around the side of the board. Notice too, the volume and pitch trim wires take a more direct path from the board to the panel and are not along with the others. ... Repair.JPG

This is the soldering on this same kit that was through here for service: ... dering.JPG

A few areas needed some touch-up and the rosin-flux darkened with the additional heating, but again, it isn't a problem. To make it look pretty, this can be gently scraped away using a pointed tool and wiped with an alcohol-wetted paper towel or cotton swab.

Author:  osterac [ Thu Mar 28, 2013 2:30 pm ]
Post subject:  Re: No sound from theremax

I will be sure to stop soldering like that, thanks for the tip!

I looked at the image, and arranged (and de-bundled) my wires to match it as closely as I could. I also removed my mods. Theremin sounds pretty good, only problem is an occasional popping - like footsteps on a tile floor. Every once in a while there will be a series of them. I wonder if it could be some electrical device interfering? I was wearing my cell phone at the time, and my computer was adjacent.

Also, you say the timbre mod has been integrated into the design. Perhaps that should be mentioned in the FAQ?

Author:  PAiA-Scott [ Mon Apr 01, 2013 10:09 am ]
Post subject:  Re: No sound from theremax

It might be the cell phone. One I used to have did that on nearby sound generating devices and I have heard this over PA systems, car radio, etc... Not sure of a specific solution here, but a quality lead to the amp/speaker input might help.

Yes, I would like to update the FAQ and will put that on the to-do list. Much of what is presented there is from when the kit was introduced in 1995. The Technical Bulletins and Assembly Notes category for Theremax contains more recent issues:


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