I could use some help on finding a replacement Transistor. One of the depth finders on our boat has failed and shows a dead short on the power input lines. The problem may be a shorted Transistor in the power supply section. I haven't removed it yet because i wasn't sure i could get a replacement. It has a Motorola logo on it and the numbers 734 and J31. It is a SOT23(?) surface mount package.

My Internet searches couldn't find the Mot part, but found a FMMT734. I'm not sure that is the correct match. Can anyone give me some information this component and where i might be able to purchase one?


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  • \$\begingroup\$ Are you sure that it's a transistor? \$\endgroup\$ – Roh Jul 13 '14 at 16:23
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    \$\begingroup\$ Although looks can be deceiving, it doesn't look like it overheated or anything. However, that inductor(?) just below your circle looks like it may have some problems. \$\endgroup\$ – sherrellbc Jul 13 '14 at 16:30
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    \$\begingroup\$ Why do you think it's dead? | Is the circled in cyan (above) inductor burned? - it appears to be. \$\endgroup\$ – Russell McMahon Jul 13 '14 at 16:34
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    \$\begingroup\$ Not SOT23. DPak is an OK enough name to give people a guide as to size etc. Very unlikely to be a FMMT734 (100V darlington SOT23 typically). Model brand circuit diagram reasoning measurements all helpful. \$\endgroup\$ – Russell McMahon Jul 13 '14 at 16:39
  • \$\begingroup\$ Since it has a Motorola logo, that means the part is at least 15 years ago since Motorola spun off ON Semi in 1999. \$\endgroup\$ – tcrosley Jul 14 '14 at 6:49

It's an MJD31, which is equivalent to a TIP31 (a popular 3A 40V NPN transistor).

To see if it is causing the short you could just unsolder the Emitter lead. Inductor L3 looks like its insulation has broken down, and will also need to be replaced or rewound.

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