A studio where I work is having recurring problems with the main monitors in one room. The situation is a head-scratcher and maybe someone out there has seen this or can at least give me a sanity check.
The system consists of an SSL console that feeds a Bryston crossover network (2 units that create a 3-way system) that feeds three Crown K2 amplifiers that power three-way Augsperger mains — (horns, single 15" mids, and subs). The system has been up and running (and well-maintained) for around 20 years.
About 4 months ago the K2 amp that powers the mids (single 15-inch for each side) failed during a session. The amp powered off and stayed off. The next day it powered up but powered down again after a few minutes. No error lights (Clip, IOC, Thermal) came on, just the powering down. I swapped in a backup K2 and brought the broken amp to a service center. After a week or two, the second amp failed in the same way.
Both of these amps are about 20 years old, so I wasn't shocked that they failed, but the timing seemed a bit odd. I looked at the signal feeding the crossovers and the signal feeding the amps and I didn't see any strange harmonics or noise on a sine wave (via an oscilloscope and RMS voltmeter). I also figure that this is the mid-frequency amp, so it's probably not receiving any extreme HF or LF signals. To cover any possibilities of noise-induced failure, I swapped out the crossovers for a DSP system, instead of the Brystons. I also changed the power outlet to a different isolated 20-amp circuit to rule out power-related problems. Each amp has always been on its own dedicated circuit, but I went to a different one anyway in case there was a problem with the circuit.
This week the third K2 amp (still powering the midrange drivers) started behaving strangely. Not exactly the same failure, but after a few hours of playing music it starts muting and un-muting every few seconds in both channels. If I shut down the amp for 60 seconds it will play normally again for several more minutes or hours before failing.
In related news, the Crown authorized service center has been scratching its head trying to repair the amps, so I found the service manual for the K2 and also read up on some common faults. I wound up recapping the main board and checking all the resistors in the main power section. Several caps were pretty far out of spec (measured with an ESR meter), but the resistors measured in spec. This unit will now power on for about 10 minutes and then turn off, again with no error lights. I unplugged each Output PWA (one at a time) to see if an output module was taking the system down, but the unit failed with either or both Output PWA unplugged. I haven't removed the input board to test its components yet, but that is next.
Additional info:
1. I have now swapped out the 15-inch speakers and checked the wiring just in case one of them is overheating or shorting out and somehow becoming difficult for the amp to drive, but I don't think that is the issue. We'll see how that goes.
2. The amps are located in a machine room that is reliably kept at 70 degrees Fahrenheit 24/7.
3. The subs are powered by another K2 that has not failed and it has been in service as long as all the other amps in the system.
4. The control room volume pot was replaced about a year ago on this console. It is a passive stacked switched resistor network and it was replaced by an expert SSL tech. The '22 card was tested and calibrated at the time, but I suppose an IC or cap could have failed on that card.
Questions:
1. Could 3 or 4 20-year old K2 amps fail in within such a short time? seems unlikely to me.
2. Could the console output a signal that could damage the amps even though the signal is passing through either an analog or dsp crossover? again, seems unlikely, especially with the DSP crossover.
3. Has anyone serviced K2 amps with this type of failure?
4. As this is intermittent and the failure takes hours or days, how can I monitor the audio signal and/or power to the amps to ensure that no harmful signals are getting to the amps?
I'm happy to replace the aging amps, but I want to be sure I'm not putting new amps into a compromised circuit.
Any advice or prayers are welcome!
- Adam
The system consists of an SSL console that feeds a Bryston crossover network (2 units that create a 3-way system) that feeds three Crown K2 amplifiers that power three-way Augsperger mains — (horns, single 15" mids, and subs). The system has been up and running (and well-maintained) for around 20 years.
About 4 months ago the K2 amp that powers the mids (single 15-inch for each side) failed during a session. The amp powered off and stayed off. The next day it powered up but powered down again after a few minutes. No error lights (Clip, IOC, Thermal) came on, just the powering down. I swapped in a backup K2 and brought the broken amp to a service center. After a week or two, the second amp failed in the same way.
Both of these amps are about 20 years old, so I wasn't shocked that they failed, but the timing seemed a bit odd. I looked at the signal feeding the crossovers and the signal feeding the amps and I didn't see any strange harmonics or noise on a sine wave (via an oscilloscope and RMS voltmeter). I also figure that this is the mid-frequency amp, so it's probably not receiving any extreme HF or LF signals. To cover any possibilities of noise-induced failure, I swapped out the crossovers for a DSP system, instead of the Brystons. I also changed the power outlet to a different isolated 20-amp circuit to rule out power-related problems. Each amp has always been on its own dedicated circuit, but I went to a different one anyway in case there was a problem with the circuit.
This week the third K2 amp (still powering the midrange drivers) started behaving strangely. Not exactly the same failure, but after a few hours of playing music it starts muting and un-muting every few seconds in both channels. If I shut down the amp for 60 seconds it will play normally again for several more minutes or hours before failing.
In related news, the Crown authorized service center has been scratching its head trying to repair the amps, so I found the service manual for the K2 and also read up on some common faults. I wound up recapping the main board and checking all the resistors in the main power section. Several caps were pretty far out of spec (measured with an ESR meter), but the resistors measured in spec. This unit will now power on for about 10 minutes and then turn off, again with no error lights. I unplugged each Output PWA (one at a time) to see if an output module was taking the system down, but the unit failed with either or both Output PWA unplugged. I haven't removed the input board to test its components yet, but that is next.
Additional info:
1. I have now swapped out the 15-inch speakers and checked the wiring just in case one of them is overheating or shorting out and somehow becoming difficult for the amp to drive, but I don't think that is the issue. We'll see how that goes.
2. The amps are located in a machine room that is reliably kept at 70 degrees Fahrenheit 24/7.
3. The subs are powered by another K2 that has not failed and it has been in service as long as all the other amps in the system.
4. The control room volume pot was replaced about a year ago on this console. It is a passive stacked switched resistor network and it was replaced by an expert SSL tech. The '22 card was tested and calibrated at the time, but I suppose an IC or cap could have failed on that card.
Questions:
1. Could 3 or 4 20-year old K2 amps fail in within such a short time? seems unlikely to me.
2. Could the console output a signal that could damage the amps even though the signal is passing through either an analog or dsp crossover? again, seems unlikely, especially with the DSP crossover.
3. Has anyone serviced K2 amps with this type of failure?
4. As this is intermittent and the failure takes hours or days, how can I monitor the audio signal and/or power to the amps to ensure that no harmful signals are getting to the amps?
I'm happy to replace the aging amps, but I want to be sure I'm not putting new amps into a compromised circuit.
Any advice or prayers are welcome!
- Adam