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

[Danjel van Tijn]

The problem with my situation is that I did not plug in anything
backwards, and since the incident nothing has been getting hot or
looks physically damaged. I also replaced the TL074 right away. So far
I have tested most of the the traces and diodes. I am going to try
replacing some of the transistors and test the capacitors. I also need
to more carefully measure signals and voltages at key test points in
the circuit.



On Sun, Jun 28, 2009 at 6:57 AM, David G. Dixon<dixon at interchange.ubc.ca> wrote:
> 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
>> >> _______________________________________________
>> >> Synth-diy mailing list
>> >> Synth-diy at dropmix.xs4all.nl
>> >> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>> >>
>> >
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>