Main monitor amplifier problem

[kags]

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

 

[pucho812]

Amps like any gear will need servicing,
It sucks that it comes like this but most likely aging amps at the 30 years of age mark need a recap. Failed caps can cause such behavior.

 

[kags]

Amps like any gear will need servicing,
It sucks that it comes like this but most likely aging amps at the 30 years of age mark need a recap. Failed caps can cause such behavior.

I know you’re right about that and I’m happy to maintain the gear. I spend at least one day a week maintaining gear- usually recapping or replacing opamps in the consoles at our studio. I just have that spider-sense that something else might be going on with these amps. Maybe it is just a coincidence that all these amps are failing in a short time.

 

[pucho812]

I say coincidence. I was working a desk weeks ago, a digital broadcast desk. What should have routine fader replacement from age turned bad real quick. The desk’s dsp cards, main and backup, both failed upon reboot.
Luckily it’s fixable and easy to do.
It’s back working and healthy.

 

[kags]

As much as I don’t like that answer, you’re probably right!

 

[kags]

Just a follow-up. 2 of the amps came back to life after recapping the main board and the rectifier behind the face plate and touching up all the solder points. Another one still isn’t staying on, which might be an output section issue. I’m not gonna go deeper into that because the o/p section has too many hard to source components for time investment.

 

[ct.waveform]

This strikes me as far too coincidental to be unrelated. The common factor is the speaker, and though I can hardly imagine a 15" midrange will have had a hard life (or indeed moved very far at all) I'm wondering whether it might have a passive crossover in there as well. I realise that's unlikely but an unsoldered (through heat cycles across two decades) or blown resistor might mean the amps are being presented with too much of a load. Designers can sometimes not think through the failure paths of their products and think that if the client is stupid enough to have blown something then they deserve what they get and it'll be sorted out by the service department anyway. I came across a problem similar to this in some Bose bass bins where they hadn't considered what would happen if the satellite speakers (connected by a CL high pass) weren't attached. It meant that the amp saw less than half an ohm at 100Hz, which in this instance meant that it had melted some underrated cables but hadn't actually blown up.

This might sound a touch unlikely but an impedance sweep at a range of voltages would be one of the things I would do first. And we all have experiences of under-specced resistors burning PCBs, or creating dry joints, or lifting track. Just having a look might be all that's needed.

 

[RoadrunnerOZ]

Did you have any luck getting it sorted?
If not it’s maybe worth checking the sleep circuit - this may cause problems - relies on a couple of segments of an LM339 U5 and two transistors Q2 a 2N4125 and Q3 a 2N4123 on the input board I believe.

 

[kags]

Two of the amps came back to life after a thorough recapping- including the difficult to get to board hidden behind the faceplate by the power switch. One is still intermittent and in the “eventually/maybe service” pile. One amp has been recapped and is still dead, which could be the output stage, but we probably won’t go far enough to troubleshoot it due to shortage of parts and documentation.

We replaced the K series amps with some new Crowns and a spare older Bryston and have had no problems in the last 6 months.

There are no passive components between the amps and drivers. The 15s are the new “Augsperger” clones sold by Makelpour (sp?), which seem to be rebranded or possibly modified B&C drivers.

So far so good. The old K series were 20 years old and run hot typically 80 hours a week, so they needed a recap any way you slice it…

 

[RoadrunnerOZ]

All good then. Seems a shame to junk pile an amp but sometimes you just move on. I’ve had similar issues with the older recording consoles and their elephantine power supplies - hot, heavy and hard to source parts with ripple you could surf on. Replace with new and don’t look back.

 

[kags]

Yeah - there is a time to cut bait - Our studios are busy with paying clients, so having working and reliable gear is foremost. We will eventually repair the old amps and either sell them or put them into our "floater" inventory.

Mission-critical systems often get updated on a time scheduled whether or not things have failed, just to raise the odds that failures will not occur during important times. We all do that with our main hard drives, right? ;-)