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I'm an untrained hobbyist trying to fix this motor driver circuit and was looking for some guidance or help with troubleshooting, or reconstruction. The circuit is from a handheld vacuum cleaner that stopped working.

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The device itself isn't very old, the battery pack and charging function seem to be working fine: the batteries measure at 19.00 V and are keeping their charge steadily after days. The charging indicator LED (on the other side) lights up to indicate charging when plugged in. The mechanical switch works fine (verified by continuity test and proper voltage measurement when switched) and the motor itself works fine when the circuit is bypassed and is directly powered.

My understanding of the circuit is that it's a MC96P0202 microcontroller-based PWM driver (8051 clone?), using the CED83A3G N-channel field effect transistor.

My best theory is that the C6 capacitor (top, near the FET) exploded. There is what looks like debris all around where C6 should probably have a surface-mounted capacitor. There are no schematics of the proprietary board and I can't see any traces on the other side that would provide additional information.

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R23 measured at 99.4 Kiloohms, R24 at 0.478 Megaohms, R25 at 0.465 Megaohms. There was continuity between the upper FET lead (source?) and bottom of R25. I'm assuming this board uses a common FET-PWM circuit that I'm obviously not familiar with but had hoped someone would recognize.

My question is both specific and yet open-ended. First, whether this is possibly the right cause of malfunction (a missing C6)? If so, how would I estimate values for a suitable replacement candidate?

And of course, if not, what should I be looking into to determine cause of the problem? Any pointers, help or knowledge are very much appreciated.

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    \$\begingroup\$ Just to add to Tim's answer : the "muck" looks like solder paste left over after a very poor wash, after assembly. And there's not much you can do. Very occasionally you get lucky : check if D6 or D7 have failed dead short circuit. If so, there's a chance replacing them will work (though something else may also be wrong) \$\endgroup\$ – Brian Drummond Aug 29 '17 at 11:42
  • \$\begingroup\$ Thanks much - I'll check the rectifiers and work my way up. The microcontroller can't be doing too much since the charging function is wired directly to batteries, and this seems like could be replaced with a 555 (or PIC uc), and a LMD-18200 H-bridge I have spares of. \$\endgroup\$ – pp19dd Aug 29 '17 at 16:26
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No, judging from the solder flow on the pads of C6, there was never a part placed there. If there once was a part placed there, then you would see broken ends of the component still in the solder, or a badly mangled solder ball/pad.

Intentionally unpopulated components are common in manufacturing. Multiple products may be manufactured using the same board, or the pads for a component were included in the design so a component could be soldered in place for evaluating circuit performance.

Beyond that, without a circuit diagram there really isn't anything we can tell about what the circuit was intended to do let alone what went wrong.

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  • \$\begingroup\$ Much appreciated. Think it's time for me to invest in an oscilloscope. \$\endgroup\$ – pp19dd Aug 29 '17 at 16:17

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