C20 and C27

[Philocor]

Ok, so after fixing the resistor problems, all seemed well, until I left the preamps on for 45 minutes or so. First, C27 burned out on one preamp, and C20 started to expand, but I shut it down. I disconnected that preamp and then about 20 minutes later, both C20 and C27 popped on the other preamp. They've been sounding fine and seeming to have no problems, but I never had them on for more than 20-30 minutes. What have I done wrong?

[tpryan]

You almost certainly have them in backwards.

[Philocor]

I've got the + sign on the silk screen corresponding to the side that does not have the stripe on the side.

[Philocor]

I took all the popped caps off this morning, and all of them were installed with the + sign on the board corresponding to the side with no stripe. Any other suggestions?

[tpryan]

I'd replace the caps taking great care to observe the correct polarity. Make sure the voltage directly across the caps does not exceed 24V. If they blow up again, we'll need to see it. Note that C20 and C27 face the opposite direction than C22 and C29.

[Philocor]

I will try that an measure across the cap. If its more than 24V, would it likely be the regulator IC's?

[Philocor]

Well, I replaced the caps C20, C22, C27 and C29, making sure to align the negative colored strip opposite the + sign. I measured the voltage across the caps and got readings of +/- 24V except for C29 which was at -14V. C29 promptly exploded after the preamp had been on about 60 seconds. I replaced the damaged cap on my other C84, C27. I measure +/- 24V at all four of the caps, yet I now no longer get signal. The second preamp was only on for about a minute and nothing has popped, but given that its not passing signal, I haven't turned it on again.

[tpryan]

Well, I replaced the caps C20, C22, C27 and C29, making sure to align the negative colored strip opposite the + sign. I measured the voltage across the caps and got readings of +/- 24V except for C29 which was at -14V.

As you can see on the schematic, C22 and C29 are both connected to the -B supply. You can't have -14V on one and -24V on the other, so I'm suspicious of your measurements.

C29 promptly exploded after the preamp had been on about 60 seconds.

Again, there's just no way to blow up a 35V electrolytic with 14V unless it's connected backwards or damaged in some way.