Hey guys. A client brought in a Crate amp with what I thought was going to be a simple problem to fix, but... What I thought the problem was isn't the issue.
What the amp does: During normal operation, the amp runs fine for ~2 minutes, then will emit a LOUD pop and the green light beside the solo/rhythm - clean button will go off and sound will die. ~20 seconds later, it'll pop loudly again, run for just a few seconds and pop off again. Rinse and repeat after that. Power light stays on.
I thought, since it is a 120W amp, the issue might be cracked solder joints, since it's a newer amp. This isn't the case that I can see. Mosfets all look good. Heatsink material is well bonded to the mosfets and the heatsink is well greased where it attaches to the chassis of the amp. There's no obvious heat damage and none of the components look blown. All caps are clean and flat topped.
Any ideas?
What the amp does: During normal operation, the amp runs fine for ~2 minutes, then will emit a LOUD pop and the green light beside the solo/rhythm - clean button will go off and sound will die. ~20 seconds later, it'll pop loudly again, run for just a few seconds and pop off again. Rinse and repeat after that. Power light stays on.
I thought, since it is a 120W amp, the issue might be cracked solder joints, since it's a newer amp. This isn't the case that I can see. Mosfets all look good. Heatsink material is well bonded to the mosfets and the heatsink is well greased where it attaches to the chassis of the amp. There's no obvious heat damage and none of the components look blown. All caps are clean and flat topped.
Any ideas?
I'm not familiar with that particular model, I don't think, but most of the solid state Crates are similar with similar problems. Check the Zenered supplies, either +/- 15/16 VDC. Those are hot running parts, both the Zener and the dropping resistor, and can cook the solder joints. After it fails check the DC voltage on the ICs, pins 4 and 8.
Craig
Craig
Thank you!
Hit the nail on the head! The 270 ohm block resistors are pushing 135F+ when the amp is running full bore. AND LOUD!!! Do you think there's a way to mount a small fan inside the electronics tunnel to move some air? I've resoldered the resistor connections and the amp runs flawlessly now, but I'd like to find a way to keep the issue from happening again. Thanks again for pointing me in the right direction. 😀
Hit the nail on the head! The 270 ohm block resistors are pushing 135F+ when the amp is running full bore. AND LOUD!!! Do you think there's a way to mount a small fan inside the electronics tunnel to move some air? I've resoldered the resistor connections and the amp runs flawlessly now, but I'd like to find a way to keep the issue from happening again. Thanks again for pointing me in the right direction. 😀
Mount the new resistor off the PCBoard leaving a gap between it and the board and put a couple of bends in the leads to aid cooling. That normally cures the problem.
I think those circuit boards are wave soldered and don't get enough solder on them to begin with. A good cleaning and resoldering should be good for years to come. Also mounting them with an airgap as Jon suggested will help also. Were the resistors surrounded with silicone? Those resistors are dissipating constant heat no matter what the amp is doing, idle or full blast.
Craig
Craig
You also should squirt some silicone under them to help stabilize them. They are larger parts and vibration makes them crack their solder. Some Crate models even have a "goop chart" to show which parts are siliconed in place. usually large resistors and large caps.
And just a note, this thread belongs in the Instrument amp section rather than up here. Just for the future...
And just a note, this thread belongs in the Instrument amp section rather than up here. Just for the future...
I have ordered new resistors. Sorry about starting the thread in the wrong section. But now I'm getting conflicting replies. Jon says to mount the resistors up high to aid in cooling. To answer Craig, the lead points off the current resistors had PLENTY of solder on them, though I did resolder those connections. Enzo is saying to leave the resistors mounted to the board with silicone beds under them, like they are now. So, which way is better?
Do you want to burn the board? if so leave them on the board. If you want added reliability, mount them 1/4" off the board for air flow with a couple of bends in the leads to cool the wires. Not high off the board as they will break off in use with the vibration.
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