R25 - final test problem

[Sonic Idiot]

I've built two A12s. One tested out fine. The second has this problem:

After first hiting it with power, I tested the U1 pins per the instructions - the op-amp has not yet been insterted. Pin 3 was around +14 and 5 was around -14. So I began adjusting R25. Then, no signal reading with the multi meter set to 200V! No mater where I set R25, (including turning it clockwise to the soft clicks) the reading doesn't change. Pin 5 continues to read negative 14V, however.

Now, I just tested it and it reads 3.4V. And again, adjusting R25 makes no change in this reading. What could be the cause of this. Thank you so much for the help.

[Sonic Idiot]

To be extra clear, I should mention that the 3.4V reading is at Pin 3. Pin 5 is testing just fine.

[Sonic Idiot]

I powered down and back up - same reading on pin 3 BUT I'm smelling (and feeling SERIOUS heat coming off the pre. I've powered down. *cry*.

[Sonic Idiot]

So I checked my work and sure enough a pretty obvious mistake somehow snuck through my seemingly airtight carefulness - C4 was soldered in backwards. I had a nervous breakdown getting it out and re-soldering it in correctly, but the problem is solved - or is it? Did I damage the unit by turning it on with that mistake in place??

[craigmorris74]

Could you identify which part was getting hot and smelling?

[tpryan]

There's a very good chance you damaged C4. I'd replace it.

[Sonic Idiot]

Well, in the big book of newbie mistakes, this is a good one. I'm hesitant to remove C4 again, as I'm worried about the board itself being affected by another round. The corrected C4 tests out perfectly fine. I haven't plugged a mic in yet, but my question now has to do with what entails damage. If it's damaged, will it simply not function? If so, of course I replace it. If damage means the audio suffers negatively any number of "points" on a sliding scale, I also must replace it. But if working, period, means an ok C4, I'm inclined to let it stay put. Your thoughts? Thanks for the feedback. I've very much enjoying the kit.

[tpryan]

It may work for a long time, but someday when you have a mysterious short on that card it'll be the first thing to check.