Big Theremax Problem

I cannot see any way that IC1 could draw sufficient current to blow up R1 and drop V+ to 1.4V, through any normal fault mechanism, with the component values given..

There is, as far as I can see, only three possible mechanisms - (1.) Low resistance wrongly connected to a pin (particularly an output) or (2.) Excessive current drain through the supply pins of the IC (3.) Supply connected to wrong pins.

I suggest some unlikely "possibilities" below - but really doubt that they will be the problem:

Sometimes ICs can go unstable and draw excessive current - but to draw this much current would require major instability and I would have expected you to smell the IC burning up (unless you were distracted by the smell of R1 burning up).

Sometimes one can get duff IC's - Particularly if bought from some ebay type vendor.. If socketed, and you have confirmed that everything seems to measure correctly (power up the Theremax without IC1 fitted, and check the voltages on each pin - most should be easy to work out... Get a 2k2 resistor, connect this to 0V, take the other end to a wire connected to a probe .. when checking a pin, probe with this resistor - you should see the voltage at that pin drop when the resistor is connected - if it doesnt drop, this is suspect!

sorry, sorry, sorry!!!

at pin 12 (gnd) i only get 1k..... 

so here we are..this means i only have 1k between V+ and ground...at all points..

but what next? replace the C´s and R´s connected to pin 12?

thanks so much!!!

u really help me!!

boerker

Actually boerker, Sorry - This really didnt help much!

Measuring pin 12 to V+ gives the total resistance across the whole circuit - and 1k is probably about right for the passive resistances.. it only represents <10mA.

This is not the problem. (if it was the problem, the fault would be present whether IC1 was in or out - we are looking for a problem which only occurs with IC1 in circuit, which means looking for something that pulls the supply down as a concequence of fitting IC1)

We are left with trying to find the real problem..

So, lets go over the facts or clues:

1.) Without IC1 fitted, voltages on the board seem about right, and R1 doesnt overheat or blow up..

2.) Put IC1 in, and the voltage drop across R1 is at least 8.5V - 1.4V = 7.1V = 71mA = 0.5W ... But this is likely to be a lot higher, because the Vdrop is actually the input voltage - 1.4V as the zener will not be having any effect when such a large current is drawn.

3.) It looks like IC1, or something directly connected to IC1 must be the problem... Most likely causes being wrongly inserted IC1 or short between an output and V+..

4.) If the above are confirmed not to be the problem, then things get a lot more complicated.. Insertion of IC1 leads to excessive current - But IC1 drives a number of active components which could possibly, when turned on, and if connected to some faulty component or short, cause the problem.

5.) I would try to simulate the fault by (with IC1 out of the circuit) connecting a wire to 0V, and monitoring the voltage across C3, and probing each output of IC1 in turn (putting 0V onto these pins) - If when probing a pin, you see the voltage across C3 drop substantially, then you have probably found the output causing the problem - and then come back, tell us which output, and we can move to the next tests..

Or perhaps now is time to contact PAiA - Who knows, there may well be some design problem which can cause excessive current - oscillating circuits whatever, their designs are (IMO) so crap anything is possible!

IMO, This is CERTAINLY not a theremin a beginner should try to build - But they are probably the ones who build these most, as they are the ones who wont notice what a crap design this theremin is!

Fred.

hey,

i tried 5.)

only at pin 3 the voltage drops to 0. this is the Vcc pin....

Ok ..

Cannot see how anything could cause the problem you are seeing.. Its looking more like a faulty IC than anything else - Or that, when removing IC1 (particularly if this was not in a socket) you fixed the problem.

1.) Do you have a socket in IC1 ? - If no, put one in, test the circuit as described in the last postings, if all checks out ok..

2.) Get a new IC from some reputable supplier (get a couple, LOL ;-) and insert this ..

Hopefully you have, unknowingly, fixed the fault and everything will work (or at least, R1 wont get hot .. Dare say there may be more debugging required for other, as yet undiscovered problems).

Good luck.

Fred.

Ps - if you shorted V+ to Gnd (0V) you may want to replace R1 - shorting the supply aint a good idea, as this will certainly stress your supply and R1 !

thanks fred

i have the ic1 in a socket...the socket is in all the time....

i buyed a couple ics. the problem is still there..i´m confused..

i buyed a couple ics. the problem is still there..i´m confused..

Yeah - so am I !

Best advice is probably to contact PAiA

If the ICs you bought are ok (havent been plugged into a faulty circuit, came from reputable supplier, have been handled with care - although this particular IC is quite tough, so static is unlikely to be a killer) then I dont really know where to go next..

If, for whatever reason, you dont want to contact PAiA..

Try bending the output pins of the IC, so that they dont go into the socket, then power up and see what happens.. If the supply still gets pulled down, try bending the +V pin and see what happens.. If the supply doesnt get pulled down, bend the output pins back one at a time.. you get the idea...

Fred.

Just one other thing - check that the IC is actually the right IC! - as in, read the IC itself and dont trust the packet it came in! - This IC has unusual pinouts - if you have some other part with different pinouts, well....

Also - Particularly check pins 2 and 3.. If there was a short across these pins at any time that you plugged the IC's in, it would probably kill the IC - the mere act of plugging /unplugging an IC from its socket could make/break any dodgy connection - so I would clean up any solder joints particularly in that area, re-test the board with IC unplugged, then put a brand new IC in. Also check that there are no solder splashes or bits of wire etc under the IC socket (as in, on the component side - these can be a real pain as the socket can hide them, and they can intermittantly make or break contact, leading one to utter frustration and despair!)

The other action you could take if things get desperate, is to replace R1 with a 1k resistor - this should limit the current to non-destructive levels (the circuit wont work like this, and voltages will be wrong.. but..

Monitoring the supply voltage, you should probably get at least 4V (you measured 1k ohms between pins 3 and 12, so the input voltage should be devided by 2 until it reached 8.2V when the zener kicks in).. If the voltage is at the 1.4 volts or less that you saw before, then you should be able to probe the circuit looking for whats pulling the supply down, without causing damage....

Then, the final ultimate desperate move is to do destructive testing, where one applies a 8V supply with high current capability directly to V+, and let whatever is pulling the supply down "cook" itself to death - Remove R1 before you do this, and dont go above 8V or you will F*** D1.. If you really want smoke, disconnect D1 and put a 12V car battery across the circuit! ;-) (wicked grin!) - ideally providing a glowing trace and lots of smoke to help you to find the fault! ... Be ready to do some major repair of the circuit board (as in, hand wiring the burned out track/s) if you do this - and NEVER do this with a multilayer* (as in one with internal layers - double sided is ok, as you can get to both sides)  PCB!!

Caution: The above advise on destructive testing can be potentially dangerous - high current can cause components to explode and enough heat to cause burns, also, although less likely with RoHS, toxic fumes may be released.

(I had many hours of fun as a kid, connecting surplus components across the mains supply - even had a corner unit I turned into a "blast chamber" complete with protective glass and a remote "fire" button! ;-) - Perhaps im not the best person to be taking advice from, LOL. ;-)

One other thing - (I am digging into my memories of decades of debugging faults here, LOL) - get a bit rough with the circuit board.. With the IC out, push hard on the socket, flex the board a bit, tap the socket with a screwdriver handle, be as physically violent to the board as you can without breaking it! ... All the time monitoring the V+ (connect your meter well so you dont get spurios readings because the probes disconnect while you bash the board - solder a couple of wires across D1 and connect your meter to these) - then watch like a hawk for any drop in the voltage as the board cries out in pain!

If you see any drop, try to repeat whatever you did to cause the drop if it doesnt stay permanent... Its quite an art, board bashing! ;-) You need to keep tabs on what actions you are taking, so that if you do get the fault to show, you know aproximately what action you did to make it show! - But the real pleasure is if you can get the fault to appear, and then repeat the action to turn the fault 'on' or 'off' at will.. exclaiming "GOTCHA ye B*****R (or) *****RD !"

If you are a real sadist ;-) there are even more horrific torments you can subject a board to - Hair dryer followed by freezer spray, dunking the board in Trichlorethelene (although you cant do that now, cause trike has been banned by someone more interested in preventing cancer than in having good clean fun - same applies to Benzine.. )-:

Funny - I saw a program recently where a small bottle of benzine was dropped in a lab, and guys in space suits were called to clean it up... I had bottles of the stuff in my chemistry set when I was a kid - I used it for cleaning my test tubes..

Back to the "big problem"..

I have really exhausted everything I can say, and covered every thing I can think of.. Time for me to back off - I dont know this theremin, have never played with one, never even built any of the circuits in it - All I have is a schematic - no component overlay or board details etc.. never even seen one, let alone seen inside one!

Perhaps Thierry or someone who has actual expierience can jump in and help if you get nowhere with the above. my gut feeling is that it will turn out to be something quite obvious (possibly a bit embarrasing for both of us) when you find it.. But even if this is the case, please let us know your progress -

If you have new data and I think I can help , I will be back - Otherwise I wish you well in your quest!

Fred.

Hey Fred,

again: thanks so much!!!! i really appreciate your help.

i needed a bit distance to the project and a bit more social life so i didn´t try anything the last days. 

I´ll work on it tonight or tomorrow so´ll i be back soon. 

there´s still a part of me thinking that´s a really stupid small overlooked mistake although i checked the board many times....but maybe there´s a point at the board i do not recognize...

i keep you informed.

boerker