[sdiy] faulty SoundLab

[Michael Bacich]

Hello David,

I'm not sure if anyone mentioned this yet, but if the circuit  
includes any electrolytic or tantalum (polarized) capacitors used as  
power supply bypass caps, then it's also very likely that they were  
damaged by the backwards power incident.  In case you don't know,  
bypass caps are capacitors that are wired across the power supply  
lines, with one side of the cap connected to the supply line and the  
other side connected to ground.  If polarized caps (caps that have a  
plus and/or minus marking) are used for this purpose, then it's  
extremely important that the plus and minus terminals be connected  
correctly.

Connecting the cap to the power supply backwards (or wiring your  
power connections backwards) will surely destroy the cap -- sometimes  
quite spectacularly, with lots of smoke and maybe even an explosion.   
Even if the cap doesn't visibly destruct, it can still be damaged  
inside.  When caps get damaged this way, they usually short out  
inside, and the short can be identified by simply reading the cap's  
resistance with an Ohmmeter -- but it usually has to be done with the  
cap taken out of the circuit, so other resistances on the circuit  
board don't confuse your meter (especially other blown caps!).  A  
good cap will read with a fairly high resistance, and a bad one will  
read with a low resistance (or zero Ohms).

I haven't looked at Ray's circuit, so I don't know if it uses any  
polarized caps in this way.  If it does, there may just be a couple  
of them, or there may be several in various locations.  Look on the  
schematic for any capacitors that connect one side of the cap to the  
power supply and the other side to ground.  If you find non-polarized  
caps that are connected in that way, don't worry about them --  
they're probably OK.  It's only the polarized ones (tantalum or  
electrolytic types) you need concern yourself with.

BTW, the circuit may also use polarized caps for purposes other than  
power supply bypass.  For instance, it may use them as "coupling  
caps" that connect the output of one sub-circuit (such as a filter)  
to another (such as a VCA), or as timing caps in an LFO or envelope  
circuit.  It is far less likely that those coupling or timings caps  
would have been damaged by the backwards power (although not  
impossible, depending on circumstances).  Check all the supply bypass  
caps first -- they are the usual culprits.

While you're checking all those power bypass caps, make sure that  
they are all correctly oriented on the circuit board -- that is, with  
their plus and minus terminals connected where Ray's parts layout and  
schematic say they should be connected.  You don't want to remove  
them to check them, and then reinstall them (or any new ones) backwards!

Of course, you will also need to test all the IC's and replace any of  
them that were damaged during the mishap.  Once you have identified  
and replaced all bad caps and IC's, you will get a relatively high  
resistance reading when you measure the resistance between ground and  
your power supply terminal(s).  If you get a relatively low reading,  
then you probably still have a bad part (or parts) to replace.

Don't get discouraged -- any damage that may have occurred is  
definitely fixable.  You may indeed need to replace nearly every IC  
on the board, but you will end up with a working and healthy synth.   
You will also learn quite a lot as you go through this  
troubleshooting process.  Go slowly and methodically, follow the good  
advice you've been given, and you will get to the top of the mountain.

Michael Bacich