Roomba 551 missing +5VDC

[TechGuy]

I am working on a completely dead Roomba 551 - no light, no sound and no movement...

The +5VREG is missing at +ve pin of C287 which is the output of the Buck Converter SMPS,

551 Measurements
================
VBAT = 15.43V
MC34063 pin 6 = 5.12V
C287 +ve = 0.08V
R138 = 11.4 ohm

Good Roomba Measurement
========================
VBAT = 16.51V
MC34063 pin 6 = 15.01V (There is 1.5V drop across the Q17 and Q18)
C287 +ve = 5.03V
R138 = 1.5 ohm

I replaced the R138 with an 1.4 ohm resistor
======================================
VBAT = 15.43V
MC34063 pin 6 = 13.02V
C287 +ve = 0.81V
R138 = 1.4 ohm
Since the +5VREG is 0.81V, this Roomba 551 is still sleeping...

I reviewed Gordon Q17/Q18 charging/standby mode diagrams. The note #3 says that there are 2 resistors in series to the D47 - a 1 ohm(R138) and a 0.27 ohm(R??). I was thinking the value of the 0.27 ohm resistor may have changed too. After tracing through the motherboard, I cannot find the 0.27 ohm resistor.

Gordon, do you mind to go over your notes and let me know where is the 0.27 ohm resistor. Any suggestion on what to check next will be appreciated.

[Gordon]

I am working on a completely dead Roomba 551 - no light, no sound and no movement...
The +5VREG is missing at +ve pin of C287 which is the output of the Buck Converter SMPS,

551 Measurements
================
VBAT = 15.43V
MC34063 pin 6 = 5.12V
C287 +ve = 0.08V
R138 = 11.4 ohm ... I replaced the R138 with an 1.4 ohm resistor
======================================
VBAT = 15.43V
MC34063 pin 6 = 13.02V
C287 +ve = 0.81V
R138 = 1.4 ohm
Since the +5VREG is 0.81V, this Roomba 551 is still sleeping...I reviewed Gordon Q17/Q18 charging/standby mode diagrams. The note #3 says that there are 2 resistors in series to the D47 - a 1 ohm(R138) and a 0.27 ohm(R??). ...

"in series" would be incorrect for me to have written. I'll have to check that and fix as needed. See the buck convertor schematic to see how R111, 0.25 ohms is wired. One of its ends connects to U3-VCC and the opposite end connects to a U3 pin. It seems to serve as a current sensor element.

...do you mind to go over your notes and let me know where is the 0.27 ohm resistor.

It is about 1cm away from U3. My notes say the SMD chip is 117 mm forward of the Clean SW center, and 17 mm to starboard. Chip width & length are same as R138's.

Any suggestion on what to check next will be appreciated.

Sorry, no. I think it may be necessary to compare O-scope measurements on each circuit node with same nodes in a working converter.

[Gordon]

Thanks for the error catch TechGuy. I have entered corrective notations in the post at
viewtopic.php?p=81792#p81792
about note #3 in each diagram. That will have to suffice until I can steal enough time to correct the artwork.