820-3476 RAM Beep. Missing PP1V35_S3

[jb23]

Hi Guys,

Got an 820-3476 that was liquid damaged. Fixed this and was working fine but it's come back a month later with a beeping RAM issue. Checked voltages and I get all main power rails and machine in S0 state except I'm missing PP1V35_S3 (0V).

My U7400 readings are as follows and can't see anything obvious blown there but I've never diagnosed any problems on this circuit yet:
[TABLE="border: 0, cellpadding: 0, cellspacing: 0"]
[TR]
[TD="width: 64"]Pin[/TD]
[TD="width: 91"]Voltage[/TD]
[TD="width: 281"]Power Rail[/TD]
[/TR]
[TR]
[TD]1[/TD]
[TD]0[/TD]
[TD]DDREG_VTTSNS[/TD]
[/TR]
[TR]
[TD]2[/TD]
[TD]0[/TD]
[TD]PP1V53_S3[/TD]
[/TR]
[TR]
[TD]3[/TD]
[TD]0[/TD]
[TD]PP0V675_S0_DDRVTT[/TD]
[/TR]
[TR]
[TD]4[/TD]
[TD]0[/TD]
[TD]GND[/TD]
[/TR]
[TR]
[TD]5[/TD]
[TD]0[/TD]
[TD]PPVTTRRD3_S3[/TD]
[/TR]
[TR]
[TD]6[/TD]
[TD]1.8[/TD]
[TD]DDREG_1V8_VREF[/TD]
[/TR]
[TR]
[TD]7[/TD]
[TD]0[/TD]
[TD]GND[/TD]
[/TR]
[TR]
[TD]8[/TD]
[TD]1.3[/TD]
[TD]DDRREG_FB[/TD]
[/TR]
[TR]
[TD]9[/TD]
[TD]0[/TD]
[TD]DDRREG_VDDQSNS[/TD]
[/TR]
[TR]
[TD]10[/TD]
[TD]0[/TD]
[TD]GND[/TD]
[/TR]
[TR]
[TD]11[/TD]
[TD]0[/TD]
[TD]DDREG_DRVL[/TD]
[/TR]
[TR]
[TD]12[/TD]
[TD]5[/TD]
[TD]PP5V_S4[/TD]
[/TR]
[TR]
[TD]13[/TD]
[TD]0[/TD]
[TD]DDRREG_LL[/TD]
[/TR]
[TR]
[TD]14[/TD]
[TD]0[/TD]
[TD]DDRREG_DRVH[/TD]
[/TR]
[TR]
[TD]15[/TD]
[TD]5[/TD]
[TD]DDRREG_VBST[/TD]
[/TR]
[TR]
[TD]16[/TD]
[TD]3.3[/TD]
[TD]DDRREG_EN[/TD]
[/TR]
[TR]
[TD]17[/TD]
[TD]3.2[/TD]
[TD]MEMVTT_PWR_EN[/TD]
[/TR]
[TR]
[TD]18[/TD]
[TD]0.5[/TD]
[TD]DDRREG_TRIP[/TD]
[/TR]
[TR]
[TD]19[/TD]
[TD]0[/TD]
[TD]DDRREG_MODE[/TD]
[/TR]
[TR]
[TD]20[/TD]
[TD]0.027[/TD]
[TD]DDRREG_PGOOD[/TD]
[/TR]
[/TABLE]


Any help would be appreciated on whether I'm missing any signals to the chip (I don't have a known good board to check against) and how to best troubleshoot further?

Thanks,

Jon

 

[dukefawks]

Most likely corroded trace or resistor around U7400. Most likely are R7425+traces and pin 2 and 12 on U7400. Could be dead U7400 too, but I doubt it would have worked at all.

 

[jb23]

U7400 area looks good but saw slight red area indicating burn on pin 1 of R7425, U7400 traces look good but will check all of them with the multimeter to be sure. Replaced U7400, R7425 & C7425 - no change. Resistors in that area all measure good. Could it be something on the 1.35V side of the circuit - bad RAM chip or capacitor etc?

 

[dukefawks]

Unless there is a short on the 1V35 line it should come high. The enable is present so it must work. Checked traces to R7425/C7425?

 

[Ciki]

Have same issue with PP1v35_s3 ,I think is shorted somewhere but nothing get hot