Jaycar MKII Problem- Test Voltage too high

Hi all, 

I am currently building the Jaycar theremin as my first-ever foray into electronics and theremins and after lots of happy soldering I've now hit a bit of a roadblock! Hopefully someone out there can help me out. 

I am using the MKII Kit with a 12V AC 500mA adaptor which is also from Jaycar.

The instructions say my first testing point should be TP GND to TP1 and the reading should be around 9V. My reading here is 19V. 

When I test from TP GND to REG 1 I get a reading of 42.5V in to REG 1, and 17.5V out.

When I test from TP GND to the power input it reads 21.5V. 

Any ideas how I can get the voltage down to 9V between TP GND and TP 1? 

Thanks for any and all help :) 

Shannon

Hi -

Sorry, but I cannot really help you at all - You are either reading your voltages wrong, or you probably have an irreparable project IMO..

42.5V ?  Where could this be coming from if you are supplying the board with 12V AC ? .. I dont have the schematic, but if you have 42V on the input to a regulator, it is likely to be fried..

If you could link to the schematic, that would help  - But it does sound like you might have a real problem, and I would contact Jaycar.

Fred.

in fact, thinking bout it - no SC / Jaycar theremin I have seen has on-board rectifier - so, for starters, you need a DC supply.. Doesnt explain the 42.5V, and doesnt explain a load of other things either.....

Hi,

Thanks for that. Here's the front page with the specifications: http://img834.imageshack.us/img834/8688/thereminfrontpage.jpg

And here's the schematic:

http://img59.imageshack.us/img59/343/thereminschematic.jpg

And just for fun a photo of the power supply. http://img841.imageshack.us/img841/5941/powersupplyi.jpg

As for reading voltages- I touch the black wire of the multimeter to the ground and the red one to whatever point I would like to test while the multimeter is set to AC Volts. Happy to be corrected if that's wrong- as I said I am a complete newbie!

Cheers :)

Hi there Cupcake,

Your list of issues was so scary most would not touch it.

My only conclusion is you have a wall wart transformer that has a switch on the plug side for 115/230 and it is in the 115 position. Also I think your 7809 voltage regulator is soldered in backward if not fried.

You may now have a can of worms. )-'

Did I smell smoke!

Good Luck,

Christopher

Haha! Maybe thats why I can't get a date! ;)

Thanks for your help. I don't see any sort of switch anywhere on this transformer. And I've pointed to the 7809 here http://img38.imageshack.us/img38/363/pcboard.jpg which I am pretty sure is correctly soldered in... If it was facing the other way the heatsink would not be able to attach to it with a screw. (I have removed the heatsink to more clearly show the orientation of the regulator). Also there's a photo of the assembled theremin board in my instructions and it looks the same as mine.

It's possible it's fried, but how? Dodgy adapter? Crappy soldering skills?

Just a thought that maybe I am supposed to switch the multimeter to DC when testing points after the diodes?

Good Morning Shannon,

What is the voltage reading with the meter probes at opposite ends of the two diodes D1 & D3 together in series.

Yes, have the meter on DC.

A view of the solder side of the board would be very revealing.

I must go scratch my head.

Christopher

ok -

Have not seen this one before - A SC with bridge rectifier!

So, heres my (confused) understanding..

"The instructions say my first testing point should be TP GND to TP1 and the reading should be around 9V. My reading here is 19V. "

This is the output from the regulator to GND - this result says that, if the regulator is correctly connected, it is dead.

"When I test from TP GND to REG 1 I get a reading of 42.5V in to REG 1, and 17.5V out."

Now the above is incomprehensible to me - beacause you say "and 17.5V out" - But if by "out" you mean the output of the regulator - well, this should be connected to TP1, which you just said was 19V...

As for the "I get a reading of 42.5V in to REG 1" - If this is true, then you have destroyed all the electrolytics before the regulator..

"When I test from TP GND to the power input it reads 21.5V." 

Well, if you are seeing 21.5V AC input, This should give you a little more than 30V DC .. still doesnt explain the 42.5V..

So:

1.) Dont connect power to the board again!  ;-)

2.) Replace the electrolytic (470uF 25V)

3.) Ideally, check the diodes (use ohms - they should conduct one way only - Google "testing diodes with ohm meter")

4.) Replace the regulator.

You might have blown the whole board up - It may be that the regulator failed short-circuit - it may be that some other components are pulling the output down... Or, you may be lucky.

Oh - Nearly forgot.. Replace the transformer ;-).. But check its output voltage BEFORE you connect it to the circuit! No more than about 16V AC should ever be put into the supply socket.

To be honest, this is by far the crapiest SC theremin schematic I have seen .. 12V AC is way too high for safe design anyway - You need 12V DC into the regulator, 12V AC rectified gives about 17V, but tolerance on (particularly unloaded) transformers is abysmal - You can easily get 16V, which cranks up to about 23V DC - and with capacitors rated at 25V that is way too close!

My advice would be - Pull the diodes, and feed the theremin with a good DC 12V from a regulated wall-wart.

"Any ideas how I can get the voltage down to 9V between TP GND and TP 1? "

With luck, if you follow my advice, you will see 9V (from a new regulator).

Fred.

"Just a thought that maybe I am supposed to switch the multimeter to DC when testing points after the diodes?"

Yeah - thats usually a good idea!

This is the problem - You (probably) wouldnt dream of replacing the piston rings in an engine if you had never seen a car.. But for some reason....

;-)

Actually Shannon,

Not wanting to be rude or discouraging, but the best thing you could do right now is a bit of reading..

You need to learn the difference between AC and DC, you need to know how to use a multimeter, you need the core basics - not saying you need to be an engineer - but the power supply is the first step - not even a minor hurdle (except perhaps when designed by Silicon Chip, LOL ;-)

The SC is probably the simplest "full" theremin, but it is FAR from the simplest electronics construction project..

If you have never before touched a soldering iron or used a multimeter, well - without suplimenting this lack of basics with reading, I give you little better odds than a snowman in a heated swimming pool...

Fred. 

Over to you, Christopher...  Im done here!