[sdiy] puff of smoke! (MB-808 build)

[David G. Dixon]

I don't know if this is relevent, but I plugged a TL074 into my ADSR circuit
backwards (why did the b^$&*#$s put the power pins on quad opamps
upsidedown, anyway?).  Of course, the circuit didn't work, but I only
realized my mistake when I put my finger on the chip and it was stinkin'
hot.  I pulled it out and reversed it, and the circuit worked fine -- for a
while.  However, the next day, the chip was hot again, and so was the 4066
next to it.  I replaced both chips and the circuit was (and continues to be)
fine.

The moral of this story: a fried opamp can wreak havoc on the circuitry
around it (obviously).

The other moral: if you suspect a chip is bad, particularly if it is a cheap
one like TL074, just bin it and replace it.  Trying to salvage it may do
more damage to other components in the circuit.  (I think I got off pretty
lucky.)

My advice: replace the offending opamp, and the chips connected directly to
its outputs, and keep testing the circuit.  (You used sockets, right?)  You
should be able to get to the problem through a process of elimination.   And
don't forget about the fingertip test.
 

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Danjel van Tijn
> Sent: Sunday, June 28, 2009 2:02 AM
> To: synthdiy DIY
> Subject: Re: [sdiy] puff of smoke! (MB-808 build)
> 
> thanks I looked at the traces carefully and tested continuity but
> nothing seems to be wrong. Next step is pulling the TL074 out and
> measuring the voltages at all the pins.
> 
> After that I am pretty much going to have to rebuild the whole circuit
> (except for the resistors) if I can't isolate the bad cap/tranny
> 
> 
> On Fri, Jun 26, 2009 at 4:44 PM, Kevin<kevingeo at gmail.com> wrote:
> > Did you look carefully on the traces of the board?  Sometimes traces
> > may be damaged.
> >
> > On Fri, Jun 26, 2009 at 2:11 PM, Danjel van Tijn<danjel at gmail.com>
> wrote:
> >> So just finished building an MB-808 clone. I have gone through and
> >> tested each section and everything works except for one problem:
> >> At one point I put in the ICs and pots to test the HT section. When I
> >> applied power I saw smoke! I inspected the ICs and it looks like the
> >> TL074 had a crack midway through it. I am not sure if this crack was
> >> there before (but didn't see it) or it was a result of some other
> >> problem.
> >> After replacing the IC there was no more smoke or shorts but that
> >> section of the circuit is simply not working. I have looked at a lot
> >> of the suggested test points and I am only getting sound at one point
> >> (marked in red):
> >> http://eight-oh-eight.org/phpbb2/viewtopic.php?t=141
> >>
> >> I can't hear anything from the noise source and nothing from the opamp.
> >>
> >> The circuit it actually fairly simple if you look at the schematic.
> >>
> >> Any tips for fixing this? I am wondering what could have been
> damaged....
> >>
> >> thanks,
> >>
> >> Danjel
> >> _______________________________________________
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> >>
> >
> 
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