J99 - What would cause C20 to smoke?

[Randyman...]

Hello,

I have a pair of J99's that I built a while ago (along with a pair of N72's and A12's ). I have only used the J99's for about 20 or 30 hours total (they were my most recent addition). Yesterday, I smelled the lovely silicon burning smell, and determined that C20 had completely burned to a char on one module. That module still works, and the other J99 is 100% fine from what I can tell/hear. What would cause this to suddenly happen after a good bit of normal useage? This happened with a pair of KM184's on the J99's into a RME ADI-8. Nothing out of the ordinary there.

Secondary Q - My 990c's seem to run pretty hot, but I understand this is normal. Is there any benefit to backing off the voltage a bit for the sake of slightly cooler opamps?

Thanks for your help.

[tpryan]

C20 is a tantalum bypass capacitor. When they fail, it's usually as a short instead of an open. This causes a lot of current to flow where it shouldn't, and you get blackened caps. It doesn't have anything to do with what's connected to the preamp. Snip it out and replace it, let me know if you need a replacement part.

Unless you really need the headroom, running the 990Cs at +/-20v or so will cool them down a fair amount. Generally speaking, the cooler they run, the longer they'll last.

[Randyman...]

Thanks for the speedy reply, Tim! I can probably pick one up at the store on Friday. What is the voltage for this 10uF tantalum Capacitor, or what other specs are important, because I doubt the store will have an exact replacement.

What would cause this to happen in the first place?

I think I will back off to 20 Volts as well.

[tpryan]

10uF, 25v or higher. You can use a larger capacitance or a higher voltage, but definitely not a lower voltage.

[Randyman...]

Just picked up a 35Volt one. About to test it, and re-set the voltage to ~20 V...


Should I test anything else while I have the modules pulled out? Could I have toasted anything else?

[Randyman...]

I got the new C20 put in. So far, so good (no more smoke). I compromised the Opamp voltage to 22 Volts. Hopefully, the modules will work indefinitely now Any clue on why C20 would have toasted in the first place? Maybe just a bad Cap?

Awesome

[tele_player]

Tantalums just do that sometimes.