[Aerobat]

I just finished a Piccolo, and after working for a minute or two, it made a couple loud thumps and is now silent.  The LED still lights up.

I emailed Jim about it, and he told me to check voltages along the power rails.  So I followed the schematic and recorded the following voltages with the gain knob in the 12 db position.  I could use some guidance as to which components to replace.

Thanks.

C20   14.63v
U2
Pin 1  1.3v
Pin 2  0
Pin 3  0
Pin 4  14.52
Pin5  5.57

R22  1.3v
C21  5.58v
C3  200mv
U1 pins 3 and 6 -- zero
C6  70mv
C7  70mv
C8  70mv
C9  14.5v
C19  79mv
C18  78mv
C10  78mv
C11  78mv
C12  78mv
C13  78mv
J2R  83mv
J2L  74mv
R9 L and R  zero
R4 L and R  zero
R1  zero
J1 R  63 mv and increasing
J1L  10mv
C14  84mv
C15  84mv
C16  84mv
C17  84mv

[poty]

You either made error in the process of measurements or have rather big mistakes in stuffing the PCB. I assume you power the Piccolo from a DC PS 15V. Lets look at your measurements:

C20   14.63v; U2 Pin 4  14.52

is actually the same points - OK.

U2 Pin 2  0; Pin 3  0; U1 pins 3 and 6 -- zero

All this points tied to the ground - OK.

U2 Pin 1  1.3v; R22  1.3v

is actually (5.5/4.3)*1=1.27V - OK

Pin5  5.57; C21  5.58v

the same points - OK.

C3  200mv

This is already wrong. It seems the R12, S2 or D4 could be out of service, but...

C7  70mv and C9  14.5v and C18  78mv

totally puzzled me - as soon as it is on the same rail I can't imagine where the 14.5V on C9 and the voltage for...

The LED still lights up.

comes from?
The rest of the measurements doesn't really of any need because of that.
I think you should carefully check your PCB and find maybe solder bridge or mistakes, bare wires, shorting a circuit...
Then, please explain what exactly was the sound (and where the sound comes from - the Piccolo, your acustic systems or...)?

[Aerobat]

Here are some photos.  I suppose I could replace R12 and D4 and see what happens. I checked everything multiple times. 

When I touched the output jacks it made a very loud thump.

Thanks for the help.

[poty]

Firstly - obvious things: the jumper on S2 should be bettween bottom-right two pins, not upper-right (in the manual the jumper is shown from the components side of the board).
Then - there are some suspicious joints, for example - C5 (lower pin) and several joints (mostly on S1 and S2) which have rather fat solder amount.
About your actions. I'd better turn off the power supply and trace all resistances from the point where you get 14,5V (C9) to the neighbored parts (R16, R14, R17, C18, C7, C10). You can trace the parts on your PCB's traces and the schematic from your manual.

[Aerobat]

OK, I moved the jumper to the correct pins on S2 and rechecked voltages.  Up to C3 voltages are OK, then things seem to go wrong at U1.  Here are the U1 voltages:

Pin 1  0v
Pin 2  2.57v
Pin 3  0v
Pin 4  low voltage
Pin 5  low voltage
Pin 6  0v
Pin 7  1.58v
Pin 8  5.3v

It looks like there should be 5v at pin 5.  C6, C8, C19, and C13 are showing low voltage as well.  Should I replace U1?

[poty]

Maybe... But you haven't said anything about positive rail (C7, C9...) - it is also get power from U1.
Please check C5, R13 and C6. The capacitors - for the right polarity and breakdown, the resistor - for the right value and continuity.

[Aerobat]

C5 and C6 have good continuity to U1, R13, and the ground, and the polarity is correct.  R13 also has continuity to U1 and C4 and shows 10 ohms on my multimeter.  The other caps show these voltages:

C7  5v
C9  5v
C4  2.4v
C12 5v

It looks like U1 is bad.

[poty]

It seems the U1 is not working, but the source of the situation may be not in the chip itself. You can try to replace U1, but if the original problem is not solved, then either you'll have the same state or U1 will be broken agan.
Replacing U1 will give you the opportunty to measure the C5, C6, R13, D2, C3, D3, C4 without desoldering them from the board.
Carefully reading the specs for ICL7662 I also got to the following remark:
"2. Do not connect the LV terminal to GND for supply
voltages greater than 3.5 volts."
LV - means pin 6. The supply voltage is 5 V. Maybe I missed something... That way - only JH can comment on this.

[hagtech]

I don't see anything obvious in the photos.  I'll try to go through this post a bit more carefully when I get some time.

jh

[Aerobat]

I wrote this one off and built another successfully.  I'm not sure what happened.

[poty]

Have you tried to replace the U1?

[Aerobat]

I did replace the U1.  It now works, but somehow now the gain switch is wrong.  In the far left position, the gain is maxed out when it should be zero.

[poty]

Check if you put the stoppers for the switch in the right position.

[Aerobat]

I didn't realize they mattered - I assume you mean the metal rings?  You can see the position in the pic above.

[poty]

Yes, the rings have lugs to stop the knobs in the edge positions. I'm sorry I can't really help you with the right position - I do not have the switches. But as soon as you have another working Piccolo you can easily copy the right position from it.