Figuring out why a simple pre-amp would go incendiary!

Pickups, wiring schemes, switch techniques and onboard active electronics for guitars
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Prostheta
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Post by Prostheta »

Nope. Had a look at the Molex KKs and there isn't any way of inserting them upside down. Left/right transposition perhaps, however the pre-amp is very very rarely removed from circuit for any reason therefore I am inclined to discount this theory. I think I mentioned the connector setup incorrectly earlier. It is 4/6 going into a 10-way header with the "keys" for the connectors clipped at the factory to allow them to fit into a 10-way header rather than their intended size headers.
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Post by DrNomis »

Well, I'm pretty much out of ideas now.... :thumbsup
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Post by deltafred »

Is there any chance that a battery could be inserted incorrectly with the switch made. This would reverse the polarity on one of the tantalums, or both if you managed to put them both in wrong?
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Post by Prostheta »

I can see how that might be a feasible mistake. Perhaps protecting the caps with a pair of Zeners would be an improvement on the existing design. The batteries were straight clips on the original Aria basses rather than boxes like the new "Arias". If Tantalums are really that fractuous, a little slap from reverse polarity won't exactly help keep them un-leaky (ummm....a-leaky? contra-leaky?).

I originally though the 4558 might be the culprit, but scouring the datasheets showed that it hasn't been implemented out of spec. This is - on first inspection - one of the simplest O-level circuits imaginable really. I'll PM MyGuy™ and see if I can source another dead BB. I think that if it has an incendiary cap I should be able to see that as a burn, trackside. I doubt I'll get it degooped on the top side well enough to investigate the cause unless it is really obvious....
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Post by deltafred »

Yes, it's a bit of a puzzler, not really a complicated circuit as you say.

A couple of 1N4001s across the supplies would limit the reverse voltage if the batteries were connected wrongly. If you can stand the volts drop put them in the supply leads, then way they would never see any reverse voltage.

Other than that I am clean out of ideas.
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Post by snofla1900 »

Just my 2cents:

I've once seen a tantalum cap go up in smoke after years of duty without any problems.
So I don't trust them anymore as I used to.

It was in a mixingdesk with a symetric 24 volt psu.


Alf

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