No Volume Control on Theremax

Free-for-all discussion about PAiA products and the kinds of audio you produce with them.
Post Reply
Posts: 2
Joined: Sun Jan 03, 2016 4:52 pm
Hometown: Dallas
Favorite topic: science fact and fiction
Super Robot Smashing Puzzle Time!: 12

No Volume Control on Theremax

Post by Mandy »

Hi guys,

A friend gave my husband a Theremax kit about 5 years ago. Since then, it's been sitting in the closet waiting to be built. I decided to give it a try. I'm new to electronics, so I didn't jump right into building it. I started with some beginner soldering projects from Adafruit. All of those went well :)

Last week, I decided to go for it since I had the week off between Christmas and New Years. I have it put together, but the volume isn't changing at all, either with the potentiometer or the arm that connects to the choke coil.

The pitch seems to be working fine.

The first time I turned it on, the power supply got really hot, and the area between + and D1 on the board got hot to the touch. I didn't notice this at first because I jumped to tuning it after not noticing heat for the 1st minute or so it was on. Pitch changed, Volume didn't. The gate light did light up when tuning lug 3 and touching the volume arm.

I checked all the parts that have polarity multiple times, and my husband gave it a look over, too. I really think I have everything in the right place, and in the right direction.

I touched up the solders that looked cloudy on the board, reheated the solders in the potentiometers and jacks to make sure the connections were good, and tried again with a new power supply ( the other one seemed to have gotten fried). The new power supply is adjustable. When set to 12V, it shows 17V on the Voltometer. When dialed back to 9V, it seems to work fine and not overheat. I have it set to 9V and it's reading close to 12V. I'm not sure if this is OK or not? I also made sure the wires weren’t running over the Oscillator circuits. Pitch is still working fine, Volume is still unchanging. The gate light is still lighting up when I touch the Volume arm. I also notice a really quiet click when the gate light illuminates. However, the volume stays steady.

I checked the board again looking for flecks of solder on the traces. I found a couple and removed them. I tried again, and got the same results.

Next, I tried starting with Lug 4 turned in 1 turn, tuning Lug 3 and testing. Then turning Lug 4 down in 1/4 increments and retuning Lug 3. I got Lug 4 down just past 2 full turns and didn't notice any difference.

I have no idea what else to try. Does anyone have suggestions? I have a voltometer, but I don't have an oscilloscope.

User avatar
Posts: 741
Joined: Sun Nov 09, 2008 2:49 pm

Re: No Volume Control on Theremax

Post by PAiA-Scott »

Hello Mandy.

The power supply getting hot points to more current flowing than is usual. Maybe it is as simple as the volume antenna cable having a short between the internal and shield wires. The internal has the Q4 collector voltage which is right off the V+ supply (through a 680 ohm resistor and the i.f. transformer "oscillator coil" L4) and normally has the oscillation from the fourth oscillator circuit-section. A quick check for 7V DC here will confirm whether or not there is a short between the internal voltage carrying wire and the grounded shield wire.

Preparation of the shielded cable as it relates to avoiding shorting the internal and shield wires is detailed here:

For unregulated DC power supplies, such as the wall-mount 12V 100mA power supply to the Theremax, the voltage will measure higher when it is not attached to the circuit being powered -- it is rated to produce 12V when there is 100mA of current flowing and when the power switch is off, no current flows and a measurement of the unloaded power supply yields a peak DC reading. R1 and D1 work to regulate the voltage to 8.2V with D1 shunting or conducting an amount of current that the V+ voltage is steady at 8.2V. If the power supply is rated at much more than 100mA, then D1 has to work harder to keep the voltage at 8.2V (theremax uses about 20mA so D1 must conduct the remainder or the available current that comes in through R1.

This section of PAiA Talk has Theremax trouble-shooting tips:

Following are some general checks and tests that can be made which should help to locate the trouble. First is a check of some control panel connections, then measurements of dc voltages on transistors Q3 and Q4, and then following these checks, others will be made according to the results of these.

Oscillator circuit sections on the panel.

Check the Pitch Trim control connections on the panel. R80 should have the same code marked on back as R79 (the 1k marking is on the front of the brown layer towards the panel. The soldering for the red-vio-brown-gold 270ohm R86 must be solid and for wires C (upper terminal) and D (middle terminal) and the brown-green-red-gold 1500ohm that links down to the ground circuit at J5-S(leeve) below. Nudge each of the bare wire ground links from sleeve terminal to sleeve terminal to be sure one isn't loose. V+ being 8.2v (within 0.1 v or so) is important too for the biasing of the transistors in the oscillator circuits and the Q8 and Q9 amplifier stages.

DC voltage tests.

The 12V wall mount DC supply powers the board via the G and + wires. You can touch the solder lug which has one end of the G wire and the power supply (-) for the DC readings. The 12V going in to wiring point + goes to resistor R1. One side of the resistor should measure about 12V and the other end (the end attached to 8.2V Zener Diode, D1) should measure 8.2V. This voltage is labeled V+ on the schematic and you should find this 8.2V on the components on the board labeled with V+ on the schematic.

The V+ voltage is divided in half for 4.1V by the equal valued series resistors at Rs 22 and 23. This voltage is labeled Vr on the schematic and again, you should be sure that components on the board are getting the voltage as specified on the schematic. Since these voltages go to so many parts (ground/common too) they often go through jumper wires to get there. These are likely spots for an open circuit either as a result of the jumper not reaching all the way through to both ends of the printed circuit, or the tiny solder pad and connecting trace in the printed circuit for the jumper wires breaking.

For now, the main concern is that the V+ is 8.2v, next we'll check for voltages on transistors Q3 and Q4.

Transistor Q3 and Q4 voltage tests.

As for the dc voltage test of V+, touch the ground/circuit-common with one probe and measure the dc voltage on the collector of transistors Q3 and Q4 to confirm there is 7v dc. Looking from the top, component side of the board, the collector is the third or rightmost leg with the flat face of the transistor towards you (EBC is the pin-out, emitter, base, and collector). This 7v results when there is about 2-3v on the base and about 2v on the emitter. Reasons the voltage might be 'off' can be a short-circuit due to a solder bridge or an open-circuit from a joint that didn't flow to both the component lead and the printed-circuit-board soldering pad, or a broken or detached wire inside a I.F. Transformer (oscillator coil).

0v on the collector of transistor Q4 can result if there is contact between the shield and internal wires of the shielded cable for the Volume antenna wiring. The shield is connected to the ground/circuit-common at the board and the internal wire carries the oscillation from the Q4 collector out to the volume antenna mount. Is the shield touching the mount? Has it melted through the insulation of the internal wire where the two separate and solder at the board?

1-2v on the collector of either transistor could be pointing to an open coil winding. If voltage is going in on the end attached to the R13 or R17 680ohm, then it should be coming out the other end attached to the Q3 or Q4 collector, if the soldering has flown at these joints (ie not a cold solder joint).

Voltage comparisons can be made between Pitch and Volume oscillator sections to help locate trouble. Notice these main sections are mirrored on the schematic. There are some variations in values which offset oscillator frequencies, but the dc voltages should be comparable and knowing this, may be useful for locating short-circuits or open-circuits. A methodical test of all points shown connecting on the schematic for these sections can 'prove' there is an open or short, or no-connection where there should be, or, a connection due to a bridge or contact with metal or stray wire.

If these test and checks are all OK, we need to proceed to the section used to make the volume cv. The volume cv, or control voltage, is used to operate a vca (voltage controlled amplifier) section which varies the audio level to the output.

Volume CV section.

As L3 is adjusted for the faint, background tone associated with the volume pair being tuned about an audio range, the volume cv should be a varying dc voltage. When the panel Volume and Velocity controls are advanced, and L3 moves through the audio ranges, the volume cv rises and falls quickly and this should cause a velocity cv, resulting in a gate/trigger action and the LED indication of the same. If the LED goes on an off with the adjustment of L3, the needed volume cv is being made. If not, the volume heterodyne might be too weak for the volume cv to be generated, or, there could be trouble with this volume cv section.

Sometimes the circuit being installed in a different case or with different antennae can result in the oscillator attached to the antenna to have a weaker than usual amplitude (if it's not a wire laying alongside an oscillator component). It's only a small amount that gets to the cv sections so if it is less than usual, it is likely not enough for the cv to result. It might help to try retuning with L4 another half-turn inwards when starting and going through the volume section tuning—the more the slug is into the windings the stronger the oscillation, but the tuning must be to a frequency that is attainable with the adjustment range of L3 so it is not possible to just set L4 all the way in...

The Vr voltage previously described takes a long patch over the printed circuit and through jumper wires (a place it can be lost due to a bad solder or a break of the tiny solder-pad/printed-circuit-trace interface), to the volume cv circuit section resistors R37 and R38. These resistors are about the middle of the board near wiring points E and F and have yellow-violet-orange bands. The ends of the two, nearest each other, should have the 4.1v Vr dc voltage. If not, check to be sure it is getting to, and beyond, the jumper wire by L , R47, R49, and R74 (follow the circuit viewing assembly manual fig.1).

These resistors should hold the inputs of IC1:B, pins 6 and 7 at 4.1v, but pin 7 also gets the audio fluctuations of the volume tuning and this makes the output pulse on and off between 0 and 8.2v with this rate working to vary the charge on C25 which Q6 makes into the Volume CV.
If at this point there is still no Volume CV action with the tuning of the volume pair to an audio/near-audio range, tests and checks need to be made in the volume heterodyne amplifier section.

Volume HetAmp checks/tests

Inspect and confirm the printed-circuit side of the board to be sure the path from the volume pair of oscillators through yellow-violet-black-gold 47 ohm R 21, and brown-black-orange-gold 10k R16 and on to Diodes D7 and D9 is correct. With power switched-off, resistance measurements can be made to confirm the circuit through the i.f. Transformer windings and these resistors. Near zero ohm reading should be found when one probe touches R20 and the other probe touches R21 where they join with L4.

Yellow-violet-red-gold 4700ohm R59 and 221 marked 220pF C34 join the volume pair tuned for an audio range heterodyne over to the Q9 amp stage through capacitor C35. Inspect the solders for this part and it's polarity (these taller parts can work like a lever against joint and printed-circuit). C33 is another, after the Q8 transistor amp stage for the volume heterodyne signal. The power source being stored in the case and banging against these parts could break a connection on the solder side of the board, or, maybe it was compromise while the board was being built and was flipped-over for work on the solder side.

A yellow-violet-yellow-gold 470k at R60 and an orange-white-orange-gold 39k at R62 connect from the V+ supply to ground to bias the transistor so it's collector is a dc level in the range of about 2-6 volts. The voltage for the collector is from yellow-violet-orange-gold 47k R61 and is set by the voltages that junction with the base of the transistor. Equipment to view waves should show audio ranging about 0.3 to 1v when the volume pair of oscillators is tuned just each side of null. Null is equal oscillator frequency and audio results when the difference between the two (at hundreds of kiloHertz) is 20Hz or more. Audio could be injected at the C35 minus pole to confirm the amp and f-v section action.

If you can email ( photos that include the unit in its case/housing, the top and bottom sides of the board and the board to panel wiring connections I might be able to spot trouble and reply with recommendations that can save time spent making the checks and tests.

This is a photo of the soldering on a board which might be useful if you need to confirm the joints and connections: ... dering.JPG
Scott Lee
Posts: 2
Joined: Sun Jan 03, 2016 4:52 pm
Hometown: Dallas
Favorite topic: science fact and fiction
Super Robot Smashing Puzzle Time!: 12

Re: No Volume Control on Theremax

Post by Mandy »

Thanks so much for your reply, Scott! It looks like there are several more things to check. I'll get some photos and email them over in the next couple of days.
Post Reply