EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: adeser on February 25, 2022, 09:43:59 am
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I got this 25 keys midi controller as not working for realty cheap as a puzzle to solve during cold days :).
(http://[attach=1])
It can be powered by 4 C batteries, 6V negative tip power or USB. There is a push switch for Batt./DC or USB. As a bonus I got 4 very rusted and leaked batteries from the other decade. As it came, it didn't powered up the 16x2 display or LEDs. But the computer sees it as Unknown USB device (it is class complied (no drivers), so the TAS1020B is not getting it's info from the 24C memory).
After some good hours of investigating and replacing all the electrolitics, I managed to pin out that the two dual Mosfets that are fed with 5V from the USB and 3.3V from the 3.3 voltage regulator are not passing out these voltages on to other boards (the Audio board and main LCD MCU controller board). So I bypassed them with good old wires and managed to power up the MCU and got the Alesis logo on the LCD, but not to boot properly and the device is still Unknown to the PC.
(http://[attach=3])
An angel from Alesis support sent me the full schematic of the thing, and I found out the the mosfets are no longer in stock anywhere. The thing that intrigates me the most is how the Q5 works in the power-on stage. The Audio board receives the 5V passed by the mosfets from the usb or battery and regulates it to 5V back to the main board and mcu board. All mosfets receive 5v and 3.3v on Sources and also there is 5V on all gates, and also 3.3V on the Full PWR gate from the TAS1020B. Can you give me a clue to propper power and boot up nicely? (I tried to boot up with all boards and wires connected). Does the power-on delay in the mosfets specification can influence the boot sequence? I can provide the full service pdf.
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U2 and U15 are P channel mosfet so in order to turn on the Vgs has to be negative. If full_pwr_en is low U2B turn on that turn on Q5 that turn on U2A and U15. That means that U1 that generate the full_pwr_on signal is not happy. You have to find why. I think it's doing more that usb interface.
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Yeap. The U1 is not happy and shoots 3V on the mosfets gate. If I disconect the Full_Pwr_On, every Drain shoots 5V and 3.3 as normal. So all mosfets work. Next week I’ll get hold of an oscilloscope and try to find out if there is any communication between U1 and memmory. Can the 24C memory be erased by some short circuit or any static?