Reply 20 of 137, by snufkin
PC@LIVE wrote on 2021-11-17, 19:11:
Thanks for your valuable suggestions, I'll try to do other tests, the difficult part of the SMDs is to remove them without damaging the tracks. Today I removed the capacitor that seemed to be shorted, in reality by testing it it was found to be working, I did not do further checks, since once removed it is not shorted.
Removing it you can see the tracks below, the - goes to ground, while the + goes to the PIN2 of the AMS 1085CM, two SMDs are connected to the same track, and both seem to be shorted, being more problematic for me to remove them, I check the + and the - of the capacitor removed, and again surprise it is shorted.
Yeah, the problem will be figuring which component is causing the short. Simple example, imagine there are 4 capacitors between Vout and Gnd, but one has failed short (C3 in this example):
__________________________ Vout __________________________
| | | | | | |
=C1 =C2 |C3 =C4 =C2 |C3 =C4
| | | | | | |
-------------------------- GND --------------------------
If you measure the resistance across any of the capacitors it will show as a short because C3 has failed. If you removed C1 and tested it you'd find that it was ok. If you then checked the pads where it had been fitted, the board would still show a short because the actual problem (C3) was still there. You'd only find the actual fault when you removed C3, and all of a sudden the short would disappear.
So I check the track of the - and the PINs of the power supply collector (20 PIN), surprisingly I find the + 3.3V shorted, doing further investigations, I find why, in JP1 there are bridges between the PINs 1-2 3-4 and 5-6, basically the - of the capacitor is connected to + 3.3V, this of course should be normal, since bridges have always been like this.
A further check performed with the diode test of the digital tester, shows a reading of about 20 between PIN 1 and 2 of the AMS 1085CM, even by inverting the leads, I don't know if it is an anomalous value, but most likely there is a fault, unfortunately I don't have another one with which to check or compare.
Hmm, that's odd. Best I can tell is that the copper area by JP1 should be 3.3V, supplying pins B113, B117 & B121 on the slot 1 connector. The copper area by the capacitor you've removed should be Gnd the supplies some pins on the A side of the connector. So the layout looks right, but they definitely shouldn't be shorted together. Are they actually short, i.e. they measure close to 0 ohms (certainly <2ohms)?
20 ohms between Vadj and Vout of the 1085 sounds a bit on the low side, but that'd depend on the values of the resistors that actually control what the output voltage should be. Can you take a close up photo of around the 1085? What's the resistance from Vadj (pin 1) to GND?