820-2327 short to ground on pin 2 of L7000

[Cocobill]

Let's have a look at another 820-2327 while we're at it. This one also has a green light on the charger but no fan spin. I got to the point where I found out that there is a short to ground on pin 2 of L7000. I used a bench power supply to inject some current on pin 2 and it does not get above 685 mA and 0.4V. Any ideas?

 

[dukefawks]

What is the resistance to GND on L7000? Check all other rails for low resistance to PPBUS, measure from PPBUS to all other rails to check for shorted "high side" FETs.

 

[Cocobill]

Resistance to GND on L7000 pin 1 is 35.31 kOms and pin 2 is 21.9 Ohms. The other two tests you ask me to do, I have not done before. Can you give an example of two points to check for low resistance to PPBUS, also an example of two points for shorted "high side" FETs? Thank you.

 

[smiba]

A measurement of 21.9 ohm on L7000 is normal AFAIK, your issue is probably somewhere else.

How are all the S5, S3 and S0 lines doing? You might be missing one thats causing the system not to boot

 

[dukefawks]

How can there be a short on pin 2 and 35K on pin 1? Please don't tell me you are measuring with power applied.

 

[Cocobill]

Power is not applied. In diode mode I measure on L7000 and pin 1 is 0.496, pin 2 is 0,012. In resistance to GND mode I measure on L7000 and pin 1 is 7.64 MOhms and pin 2 is 20.8 Ohms. Does that make sense?

 

[dukefawks]

Nope, makes no sense at all unless L7000 is open circuit. L7000 should normally be pretty much 0 Ohm of course, hence identical readings on pin 1 or 2.

 

[Cocobill]

Sorry, guys. As stated in the beginning, I removed the inductor L7000 to inject some voltage and find the short. I didn't know I had to put the inductor back on in order to measure... So, with L7000 back in place I measure in diode mode and both pins show 0.009. In resistance to GND mode I measure on L7000 and both pins measure 20.6 Ohms.

 

[dukefawks]

Measure from L7000 to all other coils on the board and see if any measurement is 20.6 Ohm or less. This would indicate a shorted high side FET on this rail.

 

[Cocobill]

Okay!! It was a shorted high side FET, replaced Q7402 and short is gone. All S5 rails seem OK, I still get no fan spin. What next?

 

[aprendiz]

Dead CPU..... That´s next.....