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Centris 650 with smoking Cap C34

Posted by: ChaosRom on 2020-05-20 02:21:04 Hello, I'm an retro computer fan from Germany. One of my main topics is 68k Macs. Yesterday I tested a Centris 650 which I had waiting for inspection for a while. I started up fine. It was nearly ready booting, when I recognised a smoking part on the mainboard. (Fortunately I opened the case before powering up) I immediately unplugged it. The smoke came from a part called C34 near to the Nubus slot which is nearest to the CPU. It is a (now?) black thing looking like a Tantalum cap. Has anyone come across this already? A will be fising this part help or did it die because something else which will also kill the replacement? Thanks for any hint

Posted by: Bolle on 2020-05-20 02:42:16 I'd say that's failure due to age. From looking at the pictures C34 is connected to the +12V rail that's going to the op-amp that's part of the amplifier circuit for the internal speaker. It's possible they used a 16V tantalum capacitor there. Replace it with one rated for 25V and you will be good to go again. You can probably leave it out as well to see if everything else is still working. Also it wouldn't hurt to check if all the voltage rails are still within spec.

Posted by: ChaosRom on 2020-05-20 07:26:54 OK, thanks. I have the PSU of Astec (often called Aztech), which I read is known for being not that reliable. I will check the Voltage.

Posted by: ChaosRom on 2020-05-20 10:53:46 I removed the Cap and checked the PSU: 12.04V and 5.1V Sounds good. BTW: Between the pins of the former Cap I get 11.9V. The board is booting and even has sound. Can someone please tell me the capacity of C34? The similar looking C36 is 47nF with 6.3V, but that would work there.

Posted by: Fizzbinn on 2020-05-20 12:57:31 C34 = 10µF 16v (pic attached so you can double check me!)

Posted by: Bolle on 2020-05-20 13:30:59 Rule of thumb for tatalums is to go at least for twice the maximum voltage rating that the cap is going to see in circuit, so as I said earlier I would opt for 10uF/25V in this case.

Posted by: ChaosRom on 2020-05-21 04:09:48 Thanks @Fizzbinn and @Bolle I will order some tantalums next time. And to be sure I will replace the other black one, too - with a 47µ/10V I think.

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