Pearl help - Calling all cars!

[Boban]

Hi to all,
The last week I completed my Pearl boards and tried them outside of the enclosure. No noise of any kind, just music and if I can say nice music. I spent some time listening and enjoying the new piece.

So, I was eager to place the boards in the metal enclosure in order to complete the whole project and that’s what I did late last night. I plugged the power supply (as before) and the famous white smoke filled the room. I promise I will never try anything like this very late in the evening (after midnight) on a full moon. Anyway, I learned my lesson and end-up with the big problem.

After I disconnected everything again here are the readings of my left board. As you can see, the voltages are way out. Don't know where to start, some part(s) is fried, but which one. My actual readings are red in color. Can anyone point me to a right direction just from looking at the schematic and my readings? Any help (or suggestion) is greatly appreciated.

Regards
Boban

[Peter Daniel]

I would start with checking Q4 and Q5.

[Boban]

Thanks Peter,
I'll replace them tonight, any other suggestion?

Boban

[Peter Daniel]

Those are the ones that voltages are really off. You didn't show the voltages on Q6 (?) 2SK389, maybe you should check it too?

[Boban]

I did check them. Same as all others, way off at 29V. Something went wrong big time.

[moamps]

Hi, Boban

If all your readings are around 29V, this means that you somehow lost ground connection of R21, R28, and FETs. Check this ground connection to PS gnd.

Regards

[vpharris]

The top half of the circuit, above c15, is all power supply regulation. That seems to be working. The voltages wouldn't be exact, and they are close enough.

Look for cooked parts. If there was smoke, you can probably find one or more. That's always a good start.

Then start looking at the voltages. Remember about V=IR. That will stand you in good stead.

For example, how about R18 & R21? If it should be 24V at the top of them and ground, 0V at the bottom, then it should be 12V in the middle. They're both the same value. [That despite the fact that it says 11.6V. That may be just what Wayne measured. Things aren't exact.] If you have 29V at the top of them and 28.9 (=29) between them, then I bet you have 29V at the bottom of them too, meaning that there's an open circuit somewhere. So that asks the question: where's the ground? Or is R21 open? Start proceeding like that.

If I recall, the grounds to the power supply regulator and the rest of the circuit were separate. You have to make some decision how and where to connect them. Have you addressed that?

[Boban]

Peter, Vince,

thanks for the leads. I'll check them on Monday. no time for this tomorrow and I need a cool head. One way or another, I will post my findings (and more questions I guess).
btw. I connected both grounds to the common spot on the aluminum chasis.

Regards
Boban

[Boban]

Here is the update.
After I replaced burned resistor (R2) and fixed cold solder joint between two grounds the voltages are now in order. Please see the enclosed picture and my new voltage readings in blue color.
The only thing bit off is on Q5. I have 9.7V instead 11.6 Should I be concerned or how to fix that while the board is out of metal enclosure?

Tomorrow I will check the other board, don't know the condition yet. Thanks for the help.

[vpharris]

Bob-

Check those two resistors at Q5 as I mentioned before. That should be just a simple voltage divider. What little current that flows at the junction of the resistors should flow into the base of that transistor and then out the emitter toward ground. And that should be small, not affecting the voltage drops across the resistors significantly. BUT, you see that in your case, the voltage at the emitter of Q5 is more positive than the voltage at the base. That should keep the transistor turned-off, which is what you don't want. There is a problem with the resistors, or perhaps that transistor (thought I don't really understand its failure modes).