This charger is a lead-acid version 1.8 control board that was reprogrammed for 57 Lithium cells at 1Vpc.
The board has none of the black conformal coating we have seen in most chargers--this is actually good for troubleshooting since we don't have to scrape it off.
There is a bit of arc flash debris inside the mains relay cover but the parallel 150R resistors don't appear overheated, and the diode bridge appears okay.
On power up the mains relay doesn't engage, so there is no 12V supply from the ViPer circuit.
There appears to be no output fuse--or it has blown off at the leads right at the board. Didn't see any fuse residue floating around inside.
Will reserve some space to edit and add photos as this problem gets resolved.
[EDIT]
1. Pulled the filter inductor on the +12(+15V) volt supply near the Viper, L10, it was good.
2. Pulled the D11 diode, also good and short was still present downstream of the capacitors C22,C42, but caps measured ok.
3. Pulled L1 on the control board, now the short was 330 Ohms to ground on the main board, but was ~1.8 Ohms to ground in the central section of the control board (across the cap near the FET drivers). The caps on either side of L1 didn't appear bulging (C1 and C2 100uF, 50V).
The 330 Ohms to ground is likely the ac relay coil plus the inline resistor, R29.
So the short is in the filtered 15_VDC_PS supply for the FET driver circuit on the control board.
4. The 2 output pins of U14, the SMPS chip are showing a low impedance short to both the 15V and ground.
5. De-soldered the control board from the main board. Removed C2 (50V, 100uF) it appears okay. Removed U14 and the A and B output pins are not shorted on the chip but they still are on the board. Didn't see any blown chip residue in the vicinity of the FET drivers, but the short has to be in that section. Need to check all the ceramic chip capacitors.
6. Pulled the steel heatsink clamp and inspected the pfc and boost FETS on the main board--didn't see any evidence of damage. Checked for shorts and found none so i think the FETS and pfc diode are okay--that's good news.
7. Pulled the U15 and U16 fet driver chips, and found that pins 1,2,3, were all shorted, while 6,7,8 were shorted on U15. The short was measuring ~ 3 Ohms on these triplets. So U15 and 16 are blown.
8. Haven't pulled U12 yet, but nearly all the pins are shorted to ground and to the solder pads of U14 for the A and B output. U12 is blown.
Maybe the popping sound was the output fuse being blown off the main board--i'm not seeing any damage so far that we would expect to see with a "pop"
more later...