Matador
Well-known member
I recently debugged a failing Mesa Boogie preamp, the Studio Preamp, as it had low, weak, distorted outputs. This preamp looks like most Mesa guitar preamps (topology wise), although with an additional tube stage(s) to drive "line level" outputs.
Here is (one of) the output stages:
After pulling the tube, I checked the socket voltages. To my surprise, pin 1 of the tube socket was showing about 50V, whereas the B+ supply (at the top of the 121K) was about 300V. So there was about 2mA flowing through the 121K resistor (even with no tube). I then looked at the center of the output level pot, and it had 20V sitting on it. Clearly the 0.1u coupling capacitor had failed. I removed it, and tested it, and it reads as a dead short (0 ohms) across the two terminals.
After replacing it, the output level was restored, however it was a scratchy, oscillating mess. On a whim, I checked all of the front panel controls, and most had significant DC voltages sitting on them.
To fast forward to the punch line, this preamp has 19 orange drop coupling capacitors in the signal chain. After removing all of them and testing individually:
1) 6 of them (all 0.1u/400V) had failed dead short
2) 7 of them measured significant leakage, like with 10V DC across them, they all flowed over 1mA (showing up to my DMM as ~10K resistors)
3) 2 measured as high value resistors (with 10V, measured as 500K to 1M resistors).
4) 4 measured correctly (out-of-range on the DMM, and rated capacitance on an LCR meter)
I have seen coupling caps fail before when subject to over-voltage stress. However this preamp, even with 20% over line voltages, doesn't approach the VDC rating of these caps. I've replaced individual ones inside an amp, however I've never had an entire amp full of failed orange drops in this manner, to the point where essentially every single one needed to be removed and replaced.
Any idea what could have caused this? The person who asked me to fix it said it was working normally, then just suddenly the output just became low and distorted.
Here is (one of) the output stages:
After pulling the tube, I checked the socket voltages. To my surprise, pin 1 of the tube socket was showing about 50V, whereas the B+ supply (at the top of the 121K) was about 300V. So there was about 2mA flowing through the 121K resistor (even with no tube). I then looked at the center of the output level pot, and it had 20V sitting on it. Clearly the 0.1u coupling capacitor had failed. I removed it, and tested it, and it reads as a dead short (0 ohms) across the two terminals.
After replacing it, the output level was restored, however it was a scratchy, oscillating mess. On a whim, I checked all of the front panel controls, and most had significant DC voltages sitting on them.
To fast forward to the punch line, this preamp has 19 orange drop coupling capacitors in the signal chain. After removing all of them and testing individually:
1) 6 of them (all 0.1u/400V) had failed dead short
2) 7 of them measured significant leakage, like with 10V DC across them, they all flowed over 1mA (showing up to my DMM as ~10K resistors)
3) 2 measured as high value resistors (with 10V, measured as 500K to 1M resistors).
4) 4 measured correctly (out-of-range on the DMM, and rated capacitance on an LCR meter)
I have seen coupling caps fail before when subject to over-voltage stress. However this preamp, even with 20% over line voltages, doesn't approach the VDC rating of these caps. I've replaced individual ones inside an amp, however I've never had an entire amp full of failed orange drops in this manner, to the point where essentially every single one needed to be removed and replaced.
Any idea what could have caused this? The person who asked me to fix it said it was working normally, then just suddenly the output just became low and distorted.
