820-2330-A Green light, no fan

[sws]

Hello,

Working on an 820-2330-A that was easily the dirtiest computer that I've ever worked on. Airflow from one fan was completely blocked. The computer was brought to me as "no power". Magsafe 85W goes green, so one-wire seems ok but no fan spin. After extensive cleaning with no apparent corrosion found, I've been troubleshooting. I jumped the gun on a replacement of C7771 that was reporting 0.321V and after replacement nothing changed. Don't know why I did that.

In any event, Q7770 pin 5 (PPBUS_G3H) is 12.57V but then pins 1-5 are all 0.321V. Q7220 also has 12.57V on pin 5 but then P5VS5_LL (pins 1-3) and P5VS5_DRVH are all 0.00V. Am I misunderstanding the schematic or should those be present when the board is plugged in?

Looking towards U7201, PP5VS5_ENTRIP is 0.00V and that's where I lose the plot. Resistance in Ohms of R7200 is 71.5k (specified as 75k). Q7210 pin 2 P5VS3_EN_L is 3.412V, R7210 resistance is 9.97k (close enough to 10k, I think).

Possible for multiple things to be wrong with the board, so I'm guessing that I'll be in for some work. Hoping for some suggestions as to what to test next to find the fault source(s) and also thoughts on where I did redundant or unnecessary troubleshooting, besides the obvious unnecessary replacement of C7771.

Steve

 

[2informaticos]

Did you replace C7771 with new, low ESR too?
Do not use hot air there.

 

[sws]

Yes it was low ESR - Panasonic 2TPE330MFB that I ordered from Digikey - arrived earlier today. I used the iron on it also based on what I read in another post here about hot air for that.

I've had good success with both Digikey and Mouser but I'm always open to suggestions on other component sources too.

Steve

 

[2informaticos]

Board cannot start with 0.3V on C7771; try another cap.
Check R7751/80/81.
Post U7750 voltages, pins 1, 2, 4, 12; use surrounding components as test points.

 

[sws]

I have no problem replacing C7771 but is the problem upstream or before C7771 though?

R7751 (pin 2, which corresponds to pin 12 of U7750) is 5.055V. Resistance to ground in ohms is 4.2k.
R7780 (P1V05S5_VSNS) is 0.322V. Resistance to ground at R7780 is only 490 ohms. That voltage matches what I'm seeing from C7771.
R7781 appears to be 0.00V and I can't seem to get a good resistance reading for it.

U7750, pin 1 is 12.57V, pin 2 is 5.055V, pin 4 is 3.326V, and pin 12 5.055V.

 

[2informaticos]

Sorry, I meant 0.3V on C7771 in last post; corrected now...

You should get an ohm reading for R7781 too; change it, if not.
You may need to change U7750 too.
Be sure C7770 is good also.

 

[sws]

Yes, I assumed as much on the reference to the voltage for C7771, no worries.

Resistance to ground of R7781 is 0.4 ohm which seems quite low.

I'm looking for components to remove from old boards but I fear grabbing an already dead cap. I can't seem to find an exact match for the specs of C7770 (0.1UF 10% 25V X5R spec in a 402 package). As a general rule, when that happens is it ok to substitute with slightly different specifications such as different voltage rating (higher is ok?) or different tolerance? I think I'm asking generally, are there certain specs on a capacitor that should never be compromised or are there "close enough" values so I don't have to keep searching for an elusive cap? Same question for resistors too: If I can't find an exact match for a 4.42K 1% 1/16W 402, which (if any) of the specifications can be compromised?

Steve

 

[2informaticos]

"Resistance to ground of R7781 is 0.4 ohm which seems quite low."
Depends which pin you've checked; one is connected to ground!

For capacitors must find same value and voltage can be higher.
Higher voltage means bigger case sometime; you may have problem to solder it in place.

Tolerance no matters, you must choose the closest value.
In case of R7780/81 important is to maintain the raport Ra/Rb as close as possible, so you can change both accordingly.

Resistor size can import for lower ohm values; lower size, less current supported.
You can solder 0402 in place of 0201, if the traces allow that.

 

[sws]

Thank you much for the information on capacitor and resistor substitutions. Do you happen to know if U7750 is ISL6269A or ISL6269B? The datasheet for the "B" version is "with audio clamp" but appears to have the same specifications and pinout.

Steve

 

[2informaticos]

The code should be visible on the chip.
I don't think to be important here...