Help with Jacar KC5426 Deluxe Theremin kit

Posted: 2/13/2008 3:46:56 AM

From: Melbourne Australia

Joined: 2/13/2008

Hello all!

It's a bit presumptuous of me signing up to a website to get some help but after an extensive search online this looks like the best place to ask it!

I would really genuinely appreciate some help/advice with troubleshooting a Jaycar Deluxe Theremin kit that I built. I took a LOT of care to build it so I am most disappointed that it is not in working order (yet) but I am hoping to find out where I could of possibly gone wrong.

After building the kit, the instructions ask you to measure voltage at a few points and adjust coils so the volts are within a certain area. I check all the ICs and they are getting 9v as stated in the instructions.

I then have to adjust coil T4 to get a reading of 1.5v. I can't get it to come close, it stays at a really low voltage (below 1v from memory), so I am stuck!

Possible issues: When I first built the kit, I soldered the optocoupler with the wrong orientation. I thought this may of damaged the optocoupler, so I replaced it with no change to the voltage reading.

The other strange thing is, when I listen via the headphone jack, the whole thing is oscillating/squealing . Adjusting the volume control doesn't change volume but affects the pitch.

And finally, when I touch the LM386 chip after it's been on and oscillating for 20 secs or more, it is so hot to touch, I have to turn it off. I know that this isn't what's meant to happen!!

I have tried to the best of my abilities to check all soldered joints, and make sure values are correct. I took my time to build it over a few months and so I am surprised I am having this much trouble with it.

I am really hoping someone out there who has built this kit, or has an inkling as to what may be the problem can help - any suggestions are welcome as I am burning to get the thing up and running so I can play it live! :-)

Thanks for taking the time to read my tale of theremin building woe, hopefully there will be a solution somewhere! :-)

Posted: 2/13/2008 9:46:23 AM

From: Colmar, France

Joined: 12/31/2007

I would be glad to help you, but I could not find the schematic on the web. If you want, please scan it, publish it somewhere and post a link.
Posted: 2/14/2008 8:53:37 AM

From: Melbourne Australia

Joined: 2/13/2008

Thank you so much!

I scanned the entire building instruction manual. It's a 26 meg PDF file (hope it's not too large) but it contains all the information on the kit.

I encourage anyone to download and have a look at it, as it is a pretty interesting kit and fairly complex!

I also apologise for using Megaupload to distribute the file. It's a messy way to transmit a file. After you click on the link you will need to enter a code up on the right hand side of the page and then wait 45 secs to download it.

Thanks so much for your offer of help - it is appreciated so much!
Posted: 2/14/2008 3:46:47 PM

From: Escondido, CA

Joined: 2/6/2008

Hi Bags,

I downloaded your "manual", so I can try to give you a hand with that theremin kit.

First off, the really obvious stuff:
1. Did you use tin/lead, rosin core solder? Unless it specifically says everything is "lead-free" or "RoHS compliant", you won't get good connections with lead-free solder.

2. Thoroughly check your circuit board to make sure there are no solder splashes between traces or pads. The article shows "x ray views" of the board, so you can use those to see where things should NOT be connected.

3. Make sure all the polarized capacitors have their plus signs in the right direction according to the board's silkscreen (or the picture in the article).

Ok, now onto troubleshooting ...
If IC4 is getting really hot, there's definitely a problem there. Disconnect the speaker and lift one side of the 220nF capacitor. See if IC4 is still hot. If it is there are a couple of possibilities:
1. The 47nF capacitor or the 10 ohm resistor off of pin 5 are missing or wrong values.
2. IC4 was damaged in some way during your testing and troubleshooting.

It would be helpful if you have an oscilloscope, but all you have is a voltmeter you can probably still troubleshoot this thing.

This design uses a single positive supply rather than split supplies. What this means in terms of troubleshooting is that what you see on pin 6 of IC3, for example, should be half of what you see on pin 7.

Make sure you have something close to +8V coming out of REG2 and something close to +9V coming out of REG1. Everything they are powering is fairly low power so both regulators should not be much more than slightly warm to the touch. If they are hot, you have either a short-circuit or defective part somewhere.

If the regulators are hot and the ICs are socketed, you can try removing them one at a time to see if the temperature drops on the regulators.
The one that causes the reduction in heat is either the culprit or something in it's immediate vicinity is.

If all that checks out, and you get the LM386 to cool down (if not, you can remove it from the board while you continue troubleshooting), Next set VR8, VR9, VR10, VR11 to the middle of their range. Verify that you get around +4.5V on pins 1, 7, 8 and 14 of IC2 (the LM324).

The article has a lot of voltage reading for the oscillator circuits, but the best way to see if the oscillators are working is with an oscilloscope. You might be able to tell by bringing a cheap AM transistor radio in very close proximity to them to see if altering any transformer core settings causes interference with the radio. If the radio is digital radio, you won't have much luck with this method.

Hope some of this helps.

Posted: 2/17/2008 8:19:15 AM

From: Melbourne Australia

Joined: 2/13/2008


Firstly, thank you SO MUCH for giving me a direction on how to fix my theremin.

I am pretty sure that the solder was a 60/40 tin/lead solder that came with the kit. I would be surprised if Jaycar supplied substandard solder with their kits.

I went over all connections, re-tinning any that I suspected could pose an issue. I carefully inspected all solder tracks and used the board layout image as a guide. I am fairly certain that it's all looking ok. I did have a magnifying glass but couldn't find it :-( it would of really helped this process.

I checked orientation of all polarised caps and they were correct.

I applied power, and still there was no change. IC4 still went hot.

I then disconnected the 220nf cap and re-applied power. This time IC4 remained cool, and when I listened to the headphones, I didn't hear a squeal anymore.

I removed IC4, set VR8, VR9, VR10 and VR11 to the middle, and got the following readings on IC2:

Pin 1: 3.43v
Pin 7: 3.64v
Pin 8: 5.63v
Pin 14: 5.68v

Hope this provides some kind of illumination as to what is going wrong?!

Posted: 2/17/2008 8:22:06 AM

From: Melbourne Australia

Joined: 2/13/2008

Sorry, to add to the previous post,

REG2 and REG1 output 8.41 volts.
Posted: 2/19/2008 3:54:15 PM

From: Escondido, CA

Joined: 2/6/2008

Hi Bags,

Ok, so we have the obvious stuff out of the way.

What we know so far:
1. You probably have 2 7808 regulators instead of a 7808 and a 7809 -- no problem there, the point was to have separate regulated supplies and you still do.

2. Since the rail voltages are ok, you don't have any "dead shorts" on the board (i.e. no paths drawing a lot of current).

3. The power amp (IC4) cooling down after down disconnecting the 220nF cap indicates that it was probably being overdriven by previous stages or previous stages are oscillating ... and not the kind we want further back in the oscillator circuits!

So next, solder that 220nF cap back in place and lift one end (the one away from the 100k resistor) of the one going to the attenuator (it would have been so much easier if they put reference designators on this schematic! -- I might just have to redraw the schematic and do that for you!)

Anyway, with the input to the attenuator open, and headphones connected, try touching your finger to the free end of the 220nF cap. You should hear a buzz in the headphones, and, hopefully, IC4 is still remains cool to the touch.

This will branch into 2 possible cases:
1. Things work as I described = the problem is farther back in the circuit.

If this is the case, see if you can't hook something up with clip leads to that cap (an iPod, or CD player line output for example). You should be able to hear that signal in the headphones and if you are lucky the volume loop might be able to control the volume of the CD player or iPod (the SPKR/PHONE VOLUME control should be set to about mid position).

If the Volume loop isn't working, you'll need to go look in front of IC5b. Connect your DMM's negative lead to a convenient ground point and clip it's positive to the cathode side (K) of D3.

Moving your hand close to the volume antenna should change the DC voltage you measure here. If it doesn't, you'll need to trace through the volume loop circuit.

A little theory discussion here ...
Q5, T3, the 68pF and 100k resistor form an oscillator. Q6 is a buffer/amplifier circuit.

If you have a analog-tuned AM radio of some sort you could try bringing it close to T3 to see if you can hear interference somewhere on the dial. I have not tried that myself, but a lot of theremin articles talk about using an AM radio to tune your theremin.

Here's where a 'scope would really come in handy! You could just probe TP1, TP2 and TP3 to see if you get any signals.

2. IC4 gets hot again, indicating there's something wrong with the circuit around IC3. In this case, you should make sure the optocoupler is installed correctly. You should check that there is about 4V at the junction of the 2 10k resistors next to the 100uF cap.

Any chance on getting a hold of an oscilloscope for your troubleshooting?

Let me know what happens with these tests and I'll do what I can to help you find the problem(s).


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