Hot U5, and no -B voltage

[YourFrndBen]

originally, when I initially powered up, I got a reading of +/-14v on +B and -B U1 measurements. soon after, the on-board LED turned off, and I realized both U4 and U5 were hot. I let them cool down, turned the unit back on, and the LED turned on briefly, then off. no voltages were found at either +B or -B at U1. after some forum searching and trouble-shooting, I realized C4 and C5 were both backwards (face-palm)! I corrected their polarity, and now the LED stays on. I get a -14v between pins 4 and 5 of U1, but no reading at 4 and 3. U5 is still hot, but U4 isnt. anyone have any suggestions? I checked all other component orientation... maybe I fried a component with having C4 and C5 wired incorrectly? no resistors look burnt, and no caps burst...

[YourFrndBen]

I just finished building a second A12 pre (I ordered 2), after stuffing the board, I compared it to the suspect pre, and everything checked out. I fired up the new build, and it calibrated fine, no egg-frying on the U5, and had voltage between pins 4 and 3 of U1...

my theory is I fried something while having c4 and c5 placed backwards?

anybody know how to trouble-shoot components?

nothing looks burnt out or bulging...


Help?

[tpryan]

Replace C4 and C5, at least. You may have killed the regulators as well.

[YourFrndBen]

*update*

Ok, I got new caps for c4 and c5 and installed them (proper orientation) and I get good readings between both pin 3&4 and 4&5 of U1, and calibrated both of those to 18v (for SC25), no heat at U4 or U5. Good.

BUT, when I plug in the op-amp, then U5 gets hot, and U4, a little as well.

is there any measurements I could do on the regulators to see if I toasted them, or should I just snip them off and replace with new ones?

are the ones in the kit the same as the Digi-key ones, or should I worry about different ones affecting the sound compared to my working A12 (im hoping to use as a stereo pair, so I'd like to get both pres sounding as similar as possible)?

Thanks for the help Tim!

*on a good note, both the N72's I've built since this little fumble work and sound amazing! I can't wait to do shootouts between both pairs!

[tpryan]

Sounds like the op-amp was damaged, send an e-mail to rma@seventhcircleaudio.com for return instructions.