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My shop is 14 miles from the U.S. and Mexico border. I have a lot of clients who play in bands on both sides of the border or back and forth and a lot of the times running on an overloaded and overworked generator to power their impromptu outdoor festivals (social gatherings). Sometimes they stick the 120 into the 240 or visa versa and a brief light show usually ensues causing their equipment to show up at my door.
Here, I got a Mark Bass 600 something or another amp with a blown mains fuse. First thing was to replace the fuse and bring it up on the limiter… the limiter glowed brightly… yeah, it's a dead short, so as Australia's Dave Jones would say: "Let's take it apart!". Here's a shot of the amplifier/power supply board or, main board once I got it out of the chassis:
On the left is the power amplifier and on the right is the power supply, nicely segregated. My first suspect for the short was the power MOSFET's in the amplifier but a quick diode mode check of their leads didn't show anything out of the ordinary, no shorts, no opens just what you would expect from a good working amplifier. I think that part of this board is O.K. so that leaves the power supply side. I can see two big MOSFET's on a heatsink in the power supply so this leads me to believe this is a split bridge resonator type of supply, not a traditional feedback coupled SMPS. Both MOSFET's were dead shorts. There's not a lot to this supply so I began the task of removing all the big silicon, heatsinks and a couple banks of filter capacitors that were just in the way. Now I could check the little things and even see them, also give the board a good close inspection:
One problem here is that I don't have a schematic for this particular Mark Bass (Parsek) amplifier, I do have a closely related Mark Bass amplifier service manual but it's not exactly the same, mine is older for one thing. The manuals schematic for the power supply is nearly identical to the one before me, some of the reference designators are different and a few minor areas of the circuit have changed for the better or worse, I don't know. It's a good guide none the less as far as I can see. Here I have marked with red "X"s the shorted semiconductors I have found. I have not found any passive components that are bad or damaged so far but I do have reservations about IC5, the IR21531D split bridge driver. The one on the board has it's markings filed off (nice!!), anyway, it's timing circuit sets it to 100Khz and all the surrounding circuitry support it being an IR21531D… not an S.
As always I get to wondering what happened to cause this? The split bridge driver is rated to handle up to 600 volts so if it was a power surge then that was some power surge, was it that? I don't trust that chip anymore and plan to replace it. All the front end mains circuitry appears to be unaffected by whatever the event was but that stuff is very overbuilt and can probably take a bit of a beating.
Anyone have prior experience or things I should watch out for, I'm working with an unsettling amount of guesswork here.
Here, I got a Mark Bass 600 something or another amp with a blown mains fuse. First thing was to replace the fuse and bring it up on the limiter… the limiter glowed brightly… yeah, it's a dead short, so as Australia's Dave Jones would say: "Let's take it apart!". Here's a shot of the amplifier/power supply board or, main board once I got it out of the chassis:
On the left is the power amplifier and on the right is the power supply, nicely segregated. My first suspect for the short was the power MOSFET's in the amplifier but a quick diode mode check of their leads didn't show anything out of the ordinary, no shorts, no opens just what you would expect from a good working amplifier. I think that part of this board is O.K. so that leaves the power supply side. I can see two big MOSFET's on a heatsink in the power supply so this leads me to believe this is a split bridge resonator type of supply, not a traditional feedback coupled SMPS. Both MOSFET's were dead shorts. There's not a lot to this supply so I began the task of removing all the big silicon, heatsinks and a couple banks of filter capacitors that were just in the way. Now I could check the little things and even see them, also give the board a good close inspection:
One problem here is that I don't have a schematic for this particular Mark Bass (Parsek) amplifier, I do have a closely related Mark Bass amplifier service manual but it's not exactly the same, mine is older for one thing. The manuals schematic for the power supply is nearly identical to the one before me, some of the reference designators are different and a few minor areas of the circuit have changed for the better or worse, I don't know. It's a good guide none the less as far as I can see. Here I have marked with red "X"s the shorted semiconductors I have found. I have not found any passive components that are bad or damaged so far but I do have reservations about IC5, the IR21531D split bridge driver. The one on the board has it's markings filed off (nice!!), anyway, it's timing circuit sets it to 100Khz and all the surrounding circuitry support it being an IR21531D… not an S.
As always I get to wondering what happened to cause this? The split bridge driver is rated to handle up to 600 volts so if it was a power surge then that was some power surge, was it that? I don't trust that chip anymore and plan to replace it. All the front end mains circuitry appears to be unaffected by whatever the event was but that stuff is very overbuilt and can probably take a bit of a beating.
Anyone have prior experience or things I should watch out for, I'm working with an unsettling amount of guesswork here.
..... who eventually found out anyway.
.
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