EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Watcher on February 08, 2018, 10:22:27 am
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Hi all
I am trying to repair a Speakercraft RSA-1.0 RS232 interface module which failed after some kind of power surge.
I have already replaced a blown 100ohm resistor which links the shell of the DB9 with the system ground ( R11 at the bottom of the foto ) and the MAX222 chip that seemed suspicious.
The device still doesn't communicate over RS232 and at the same time the on-board MCU 89C669 gets hot. The total current drawn is also high at 350mA.
The 5v supply rail looks ok and I cant see any other obvious problems.
Is this a case of MCU failure ? In which case it would be beyond repair as I couldn't replace the MCU without the firmware.
Any ideas welcome!
Thanks
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Hot microcontroller almost certainly == dead microcontroller, I'm afraid.
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Hot microcontroller almost certainly == dead microcontroller, I'm afraid.
That is bad news!
I don't suppose anyone knows where to get the firmware for these devices from...?
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I'd say it depends. Sure, generally a hot microcontroller is bad news, but it could also be that one of its output pins is in short circuit - i.e. it outputs 0V but something connects it with 3.3 or 5V, or it outputs 3.3V and something shorts it to GND. Finding out which pin it is could be hard and from my point of view, the only choice you have is to examine it out-of-board using a PLCC socket. You can see that there's space for such a socket on the PCB - in the prototypes, there was a PLCC socket there so they could easily remove, reflash and replace the microcontroller. In the customer version, the socket was left out and the MCU was soldered to the PCB directly.
I suggest the following:
-) Order some matching PLCC sockets (at least 2)
-) De-solder the MCU (e.g. hot air gun with the board upside down until the MCU falls off)
-) Use one of the PLCC sockets and wire VCC and GND up according to data sheet
-) Place MCU in this socket and power it up - if it still gets hot, it's fried
-) Wire up the rest of the MCU that's needed to at least make it run: reset circuitry, xtal, etc. - if it still doesn't get hot, the MCU is okay
-) Try to figure out if you can read out the EEPROM/flash over some debug interface and dump the contents if you can
-) Look at all the GPIOs on the PCB with a multimeter to find out if one or more of them have a low resistance to GND or VCC - if one of them does, it's possibly guilty
-) Try to find the root cause on the PCB
Once that's all done and fixed, you can use another PLCC socket, solder it to the PCB and place the MCU in it. If all is well, it should power up and work.
Indeed that's the optimistic version of the situation but honestly, as long as the MCU is sitting in the circuit, you really can't draw any final conclusions.
P.S.: Can you upload a proper picture of the board? The one you posted unfortunately has a very low resolution.
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Thanks for the input.
I will do as suggested. .will also post a kther pic tomorrow
By the way, the mcu oscillator still runs. Ive checked it with a scope.
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Better photo quality uploaded.
(https://www.eevblog.com/forum/repair/mictrocontoller-getting-hot/?action=dlattach;attach=393682)