Double Horse 7009 aka Shuang Ma 7009 RC Boat no reverse problem
I've got this boat as a present for my kid, and she is fun to play with, as it runs pretty fast on stock everything and is quite easy to handle. The boat itself has been reviewed number of times, there's an excellent thread on RC Groups with all the demise and glory described in detail.
However, everything has its end, and after running it for about ten times, she lost the reverse and when going ahead it seemed like she wasn't running at full power. After a few standard checks - like changing radio battery and making sure that the boat receives reverse signal - rudder servo constantly moves sideways a bit whenever the boat goes astern or ahead - it was clear that the ESC is at fault.
So, what we've got is a small ESC and receiver combo, hidden in the small plastic box with a strange airplane logo on it:
Opening it up is not really easy, as it is glued together with silicone compound, but with some patience and effort we could reveal its internals (picture displays the board already repaired, but nonetheless):
On the original board, in the area marked with the red box there were two small IPAK-packaged MOSFET transistors, similar to one on the lower side of the box. In the blue box are two original large TO-220 packed FETs the board already had. These four form an H-bridge that drives a motor, but for some reason, probably to account for reverse power limit, both high and low side of the bridge are built using two different FETs, so the left side of the bridge is in the blue box, and the right side is inside the red one.
A quick voltage check revealed that without the motor both forward and reverse inputs produce a full battery voltage on the motor terminals, whilst attaching the motor and commanding reverse yielded 0 Volts. So it was clear that FETs are to blame, and after some careful examination here we go:
You can see a tiny solder bead in the blue box above, right at the joint between metal and plastic parts of the FET, a clear sign of the breakdown happened inside..
The broken FET shown above is from the high-side, and it is AO U417 P-channel in IPAK package from Alpha & Omega Semiconductor, here's its datasheet; next to it was its right-side counterpart, GEP14241, a strange Chinese-origin brand for which no information could be found. However, as it is on the low side, it is clear that this should be N-channel, and given that we know high-side parameters its not a big deal to find the replacement.
As broken FET was rated for 18 Amps and died, it is clear that we need more than that for it not to happen again. However, there's a little choice of high-current FETs in IPAK package and that's not surprising, as heat dissipation is an issue for such a small footprint.
At the same time, there's an abundance of standard TO-220 packaged MOSFETs for any voltage/current combination, and our Chinese brothers might have noticed that already, as some pictures of the same board from Google suggest. Granted, soldering TO-220 instead of an IPAK requires some careful sanding and bending, but its not a rocket science anyway.
So, I've grabbed what I had available, and these were IRF4905PBF P-channel for the high-side (rated for 64A/55V) and IRL2203NPBF N-channel for the low side (116A is sure an overkill, but that's what I already had).
Having these two MOSFETs replaced, the boat got its forward and reverse capabilities back, and running it on the table for a full battery charge all FETs are ice cold. The motor itself, however, is a crap, as its bearings and brushes sometimes produce an awful noise, and I think that this is a root cause of high current in the H-bridge that destroyed the FET in the first place. Applying some lube to the bearings rectified the problem somewhat, but given visible side movements of the shaft sure its not for long.
Anyway, I think this could be useful for someone looking to fix the boat's no reverse problem while brushless conversion and radio upgrade are still on its way from China. Stay tuned for the updates!
However, everything has its end, and after running it for about ten times, she lost the reverse and when going ahead it seemed like she wasn't running at full power. After a few standard checks - like changing radio battery and making sure that the boat receives reverse signal - rudder servo constantly moves sideways a bit whenever the boat goes astern or ahead - it was clear that the ESC is at fault.
So, what we've got is a small ESC and receiver combo, hidden in the small plastic box with a strange airplane logo on it:
Opening it up is not really easy, as it is glued together with silicone compound, but with some patience and effort we could reveal its internals (picture displays the board already repaired, but nonetheless):
On the original board, in the area marked with the red box there were two small IPAK-packaged MOSFET transistors, similar to one on the lower side of the box. In the blue box are two original large TO-220 packed FETs the board already had. These four form an H-bridge that drives a motor, but for some reason, probably to account for reverse power limit, both high and low side of the bridge are built using two different FETs, so the left side of the bridge is in the blue box, and the right side is inside the red one.
A quick voltage check revealed that without the motor both forward and reverse inputs produce a full battery voltage on the motor terminals, whilst attaching the motor and commanding reverse yielded 0 Volts. So it was clear that FETs are to blame, and after some careful examination here we go:
You can see a tiny solder bead in the blue box above, right at the joint between metal and plastic parts of the FET, a clear sign of the breakdown happened inside..
The broken FET shown above is from the high-side, and it is AO U417 P-channel in IPAK package from Alpha & Omega Semiconductor, here's its datasheet; next to it was its right-side counterpart, GEP14241, a strange Chinese-origin brand for which no information could be found. However, as it is on the low side, it is clear that this should be N-channel, and given that we know high-side parameters its not a big deal to find the replacement.
As broken FET was rated for 18 Amps and died, it is clear that we need more than that for it not to happen again. However, there's a little choice of high-current FETs in IPAK package and that's not surprising, as heat dissipation is an issue for such a small footprint.
At the same time, there's an abundance of standard TO-220 packaged MOSFETs for any voltage/current combination, and our Chinese brothers might have noticed that already, as some pictures of the same board from Google suggest. Granted, soldering TO-220 instead of an IPAK requires some careful sanding and bending, but its not a rocket science anyway.
So, I've grabbed what I had available, and these were IRF4905PBF P-channel for the high-side (rated for 64A/55V) and IRL2203NPBF N-channel for the low side (116A is sure an overkill, but that's what I already had).
Having these two MOSFETs replaced, the boat got its forward and reverse capabilities back, and running it on the table for a full battery charge all FETs are ice cold. The motor itself, however, is a crap, as its bearings and brushes sometimes produce an awful noise, and I think that this is a root cause of high current in the H-bridge that destroyed the FET in the first place. Applying some lube to the bearings rectified the problem somewhat, but given visible side movements of the shaft sure its not for long.
Anyway, I think this could be useful for someone looking to fix the boat's no reverse problem while brushless conversion and radio upgrade are still on its way from China. Stay tuned for the updates!
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