Author Topic: PPC640 System ram failure R540 Very hot  (Read 366 times)

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Offline kolleykibber

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PPC640 System ram failure R540 Very hot
« on: 16:27, 17 September 21 »
I wonder if someone can help a beginner trying to fix a PPC640. It is showing 'SYSTEM RAM FAILURE'.
I've tried switching to 512k, but no joy. I've noticed the large 2W resistor R540 near the power input on the power/memory board is getting very hot. I assume this means there's a short somewhere? Or is this normal? Is the next step to go though each memory chip? Am I wasting my time and should I get another?


Thanks!




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Offline gertk

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Re: PPC640 System ram failure R540 Very hot
« Reply #1 on: 22:10, 20 September 21 »
According to that schematic that resistor is directly connected in parallel to the 12 V supply.Following Ohms law, a 150 Ohm resistor will generate about 1 Watt of heat when conneted to 12V
So it will be quite hot to touch. Not sure what the purpose is of this resistor other than creating a 'dummy' load on the supply.


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Offline Bryce

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Re: PPC640 System ram failure R540 Very hot
« Reply #2 on: 09:49, 21 September 21 »
According to that schematic that resistor is directly connected in parallel to the 12 V supply.Following Ohms law, a 150 Ohm resistor will generate about 1 Watt of heat when conneted to 12V
So it will be quite hot to touch. Not sure what the purpose is of this resistor other than creating a 'dummy' load on the supply.

It's there to ensure that the switchmode PSU starts up correctly and remains stable.

Bryce.
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