Big Theremax Problem

hey,

i have a big problem with my own made theremax.

when i switch power on, R1 is getting hot. And after a while it dies. 

behind R1 i get about 1,4 V. When I take of IC1 everything is fine. About 8 V at R1.

All measure points of the circuit get the V they need.....

I tried a new IC but doesn´t help ... :(... Any suggestions? 

Thanks a lot

greets 

boerker

"when i switch power on, R1 is getting hot. And after a while it dies."

Excessive current drawn on the V+ will do this

"behind R1 i get about 1,4 V."

Something is really pulling the supply down. 

"When I take of IC1 everything is fine. About 8 V at R1 All measure points of the circuit get the V they need....."

It looks therefore like IC1 is pulling the supply down.

"I tried a new IC but doesn´t help ... :(... Any suggestions? "

Did IC1 get hot? ..

Most likely reason I can see is that you put IC1 in the wrong way 'round.. If you did, both ICs will now be dead (in fact, whatever the cause, if the current has been via IC1, I would expect any IC you put in the position of IC1 will now be dead). The other possibility could be related to the components connected on any output of IC1 - If there is a short across say C40 or C41 or pin 13 or 14's connections ..

I would start by removing IC1 and testing with an ohm meter to check if any output has low resistance to V+. (ps - do not power the theremax when you do this - any resistance lower than 1k is suspect, I would expect you to find a resistance below 20 ohms to account for the voltage drop you are seeing across R1)

Dont put any other (new) IC's into IC1 until you have found and fixed the fault - you cannot gain any information on the fault with IC1 in place, all you will do is kill it.. This fault should be easy to locate and fix.

If you think this is a "Big" Theremax problem, think again! ;-) .. Its when one wants to turn the Theremax into a musical instrument that your problems will really start! ;o)

Fred.

boerker,

Fred gave you the same advice I would have and probably stated it better than I could have as well.  This is also the kind of question that Scott at PAiA would have a lot of Theremax specific debugging knowledge for.

Good luck!

Greg

Hey,

thanks so much guys! i´ll try this later at home.

@fred: after hours and hours searching for the mistake i felt/feel like having a really big problem....but you´re right!! playing the theremin is the real big deal!!

i sign up later and tell what i found...

boerker

"hello again"

at pin 1 and 2 of IC1 i get 14kOhm and at pin 14 2,2kOhm.

Seems to be normal?!

pin 13 goes to one jack without other connections so there´s no resistance-problem...

i just don´t get it. is there a way to check C40,C41 etc? 

thx

boerker

"i just don´t get it. is there a way to check C40,C41 etc? "

Yes -

Remove IC1, take your multimeter, set it to ohms, if you can select the range set the meter to measure low resistance. Disconnect power from the Theremax, connect one lead of your meter to the +8V somewhere on the board, then with the other lead check all the output pins of IC1 (confirm wour meter is correctly connected by testing the +V pin on IC1 - This pin, and ONLY this pin, should show low resistance when probed - This is pin 3 on the LM339) - If the meter shows low resistance to any of  pin other than the +V pin, you have probably found the problem.

Doing the above will not actually test C40, C41 etc, but it will show if there is a short across these or (extremely unlikely) that these components are faulty.

Now - the biggest problem you are likely to have is if, when removing IC1, you unknowingly removed the fault (solder bridge or whatever).. This wont be likely if you have IC1 in a socket - If you dont have IC1 in a socket, put a good quality (turned pin) socket in!

So, you have the socket in... Now test all the output pins as described above - If you dont find the fault, test every pin on IC1 - Only one pin should be low resistance (<1 ohm) to the +V supply, all the others should be >= 2k2 (2200) ohms, so any that are less than this value are suspect, and any below 100 ohms is likely to be the pin connected to a faulty component or solder bridge.

If the fault was because you put the IC in the wrong way 'round, then you will not find any shorts - I am not sure which IC IC1 is - If its an LM339 then pin 3 should be +V and pin 12 0V - if put in wrong, pins 5 and pins 10 are connected across the supply, and this could cause problems (all the other pins are also wrongly assigned) -

If you cannot work out the fault by doing the above, write down your reading of the ohms for each pin, and post it here.

Fred.

Thanks! I tried this and a get at every pin more than 2k. Only at pin 3. But this is fine....

:-(

Ouch!

You SURE you had the IC in the right way 'round ?

Yes. I tried also a new one....