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Liquid Damaged 820-3115 Cyclical Fan Spin, PPBUS_G3H Voltage Jumping

computercaverva

computercaverva

Member
We got a board into the shop yesterday that had pretty gnarly water damage, full glass spilled on the keyboard. After looking at the board under the microscope and notating which areas and surrounding components were damaged, it went through the Crest. After removing the board and completing the cleaning ritual the board got a Green light and fan spin. The fan will spin up and then stop, off and on as long as it's plugged in. I measured PPBUS_G3H and following the changes in the fan, the voltage sits between ~11.5v and 12.6V with the fan running and when the fan stops it falls to around 3v.

I replaced several nasty looking components around the LVDS connector, including Q4262. After doing so the fan was the same and Q7085 was getting hot. I replaced Q7085 and the fan problem persists, however the PPBUS_G3H voltage is now too high sitting fluctuating around 15.8v-17.1v.

Anybody had an issue like this?
 
computercaverva

computercaverva

Member
Will do! Im out of the shop now but will check those in the morning! If those resistance readings are off replace the resistors of course and also the ISL6259 or is there a way to see if the chip is good on the board?
 
computercaverva

computercaverva

Member
Sounds good! Just got in the new Amscope so we got back up and running with our board repairs. The resistance between pins 17/18 is 3.1 ohms which is higher than the 2.21 it should be the 0 ohm (R7052) is reading 0.2 ohms and R7051 is reading 2.4 ohms which should be good as well the only one that reads a little high is the 4 pin resistor R7050 thats reading 0.2 instead of 0.01 the resistor itself looks to be in good condition would it be worth replacing or is the resistance difference not high enough to cause problems?
 
TCRScircuit

TCRScircuit

Super Moderator
Staff member
Well PPBUS_G3H generation is simple... Pulsing is usually current sense but can be the ISL telling the FETs to do funny things and sometimes it's a combo of all three.. If current sesne is OK replace your 6259 and the FET's controlled by it Q7030 and Q7035 I believe.
 
computercaverva

computercaverva

Member
I replaced all of the current sensing resistors as they looked nasty and probably burnt, also replace the 6259 and the board powered on green light, fan stayed running however no post beeps with ram removed. The board then cut off and now has no signs of life, no more green light 0V on G3H and PP3V42. Is it possible that replacing U700 without replacing the FET's blew the 6259 and the ISL and FET's should be replaced together?
 
TCRScircuit

TCRScircuit

Super Moderator
Staff member
Always replace the FETs and ISL at the same time. And yes.. The fets screwed up the ISL and that's why it's not working. New ISL and new fets and try again.
 
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