VESTAX VMC-002XL blown - can it be repaired?

[Systembolaget]

Someone plugged a regular 15V DC power supply in, and the device stopped working entirely.

On the smaller brown PCB ASSY IN/OUT, capacitor C138 was visibly blown, and when powered with an original VESTAX power supply with reversed DC-jack polarity (thank you very much for that non-standard trickery), the voltage regulator NJM7812 does not output 12V DC on Vout, and PIN 1 on the CN4J connector does not output 12V DC either, so the main PCB receives no power also, hence the permanently off power LED and everything else not working.

On the larger green PCB ASSY MAIN, the dual op-amp U7 seems to have melted a wee bit.

All other components on both PCBs look good upon careful visual inspection.

If I replace C138, although only required for the headphone part of the circuit on PCB ASSY IN/OUT, and replace the NJM7812 as well as the dual op-amp U7, and the thing does not come alive, what would be the best troubleshooting strategy if visual inspection yields zero clues?

I found the original electronic schematics here https://elektrotanya.com/vestax_vmc-002xlu_sm.pdf/download.html

Thanks in advance!

 

[Nigel Goodwin]

Replace the parts and see what happens.

If that doesn't cure it completely, and you've decided which parts of the unit don't work, then you can use voltage measurements to check for what's wrong - checking the output pins of opamps is a good start.

BTW, there is no 'standard' polarity on DC sockets, which is why you should always use the supplied adaptor - centre negative is not far off as popular as centre positive. It would be nice to have a protection diode though.

 

[Systembolaget]

Thanks, I will replace C138 and the NJM7812 first and see if I get power on PIN 1 on the CN4J connector, and thus power on the main PCB. I hope Digikey ships fast. I also like your idea about modding the device with a protection diode!