deezaloza

Scope : 
I have been inspired by @Steve87 to embark on repairing , as he facilitated  3 x faulty MAX7.2 machines  for which I am very grateful . We repaired one machine , but that was really just repairing on module level (faulty  MPPT) . The second machine has two AC side IGBT devices shorted , but the rest of the main board seems ok , it gives fault code 09 (soft start fail). The third one was as dead as a doornail and this is the one that prompted me to repair and document my progress in order to help myself and others , because already I have so much to thank to @Coulomb and @maxo for some powerful partial / full schematics and in general the documentation available on the AUS forum of Coulomb and Weber .   So this machine  I am going to use to  repair in stages , as you will see it has some spectacular nasty faults/failures . It would probably not be economically viable to repair this board , but the focus is on learning how to repair other boards. In the process it will probably take me weeks to repair this one , if it can be repaired at all , but the end goal is learning and to possibly attract more gurus and expertise on repair/support.
The journey started when I switched on the Machine 3 for the last time  ,  then smoke escaped from the machine itself . I decided to disassemble it . The Machine still looks very neat inside , I believe its not an old machine as there is no dust layers etc etc and really looks new .   game on.
 
Chapter 1a :  The Baby Face Assassin 
So I tried to check for visual burns and damage , and finally saw a power diode that had burn marks . This is the diode that feeds the ac side igbt driver transformer and is fed via the SPS 12V secondary transformer . so I reckon this could be a shorted transformer primary causing the diode to burn. So I de-soldered the diode , and switched  the main board on via the SPS module , in anticipation that I have relieved the main power supply from a short circuit.  But , instead , all hell broke loose when I switched on again  ,  devices just started to explode and stink and smoke . Something on this board was just shooting from the hip for fun to make a fool of me. So I remember that old Eagles lyrics : ' .... I had to find the passage back to he place I had been before . Goodnight says the night man , you are programmed to receive , you can check out any time you like , but you can never leave ..."
The blown devices were all electrolytic capacitors on the 5V , 12V ,15V and -12V rails. So it became apparent the the SPS power supply on he main board  was a fault. Before of all of this I had removed the control board and as many removable modules , as I possibly could. Fortunately no damage o the boards themselves . Even the capacitors did not open up their shells , although the explosions at times had been horrendous. 
So I had to drill down and study the partial schematics . I redraw the sps power supply as shown below :

 
The SPS+ primary supply comes from three sources really , that is mains , battery or PV , they are wired OR together , that is not shown on the schematic. I limited my damage by introducing a variable 60V bench PSU courtesy of @Steve87. Current limited it to 500mA and set dc to 40V.   removed faulty capacitors and tried to isolate load circuits as far as i can , e.g. removing filter inductor to mosfet drive supply , cutting a track carefully to the bus soft start circuit etc.  
Switched on again , and long story short  , the SPS flyback converter around TX9 and U10 was just shoveling out as much DC on the 12V output as it received from the primary .  I measured the duty cycle of U10 and found it to be at 50% (50% by the way is the max limited by UC3845 type ). The 12V output was roughly as high as the supply DC input. So there is the cause of our explosions , 12VDC probably went as high as 50V !!!!. To limit this I merely turned down the DC input to about 16V in order to minimize my losses further. Note that the U10 chip will only bootstrap at 35V , so you must have that initially then you can ramp down again . In the future I will wire OR a separate 12V supply to the U10 VCC to bypass the bootstrapping.
So I studied the voltage closed loop feedback circuit . It basically samples the 12V output (R209/R210 on the schematic) and then controls a precision shunt regulator U9 , which in turn   controls the diode current thru opto-isolator U8 . U8 transistor is connected to the COMP input of the PWN controller , which in layman's terms will decrease duty cycle the more the COMP pin is pulled towards ground (GND) . Initially I checked if the COMP input works by shorting U9 output to ground. Indeed the PWM controller switched off. So at least some of the feedback circuitry is working . Then I discover that the VREF input of U9 is fixed at 0.00V . Which is shouldn't because of the R209,R210 divider across the 12v output . But it is just a simple divider , so I removed U9 in the hope that it has failed and pulled down the divider . Again I was made a fool of , as that divider midpoint stayed at 0V . In other words there was no feedback voltage provided to  decrease the duty cycle of the PWM . I tested both R209 and R210 . R209 showed 10K , R210 showed 10.5K in-circuit. The resistance of 12V down to GND is about 500 ohms , so this was a giveaway that R210 is open circuit. But how the hell, this is a 38.3K resistor how can it go open circuit .  Out of desperation I soldered a 39K resistor across R210 pads , with the original one still in situ . 
Switch on again , and the SPS was regulating at exactly 12.15VDC . I varied the input and even took it all the way up to 56VDC . Perfect . Short circuited the 12v output , limited perfect.
Can you believe a tiny 38.3K resistor R210 going open circuit !!!!!!!!!!!!!! . This resistor will henceforth be known as the Baby-Face Assassin.
Need to do a lot of cleanup around the SPS  before I am going to address the load circuits and repairing each section bit by bit . 
 
 Why such a big margin on the open loop transfer ? . Also there is no transzorb or voltage crowbar . I am going to breadboard a programmable crowbar around the TL431 and a triac and have it onto the 12V output as long as  am repairing the rest of the circuits. Below you can see my point , pulse width is 800ns to regulate at 12V from 56V input. That is about one fifth of 50% pulse width , which will indeed equate to around 60V with max duty cycle.
 

You can't connect 2000Amps to a 700w inverter. There is definitely a current limit. And if that current is beyond it's overcurrent protection range it will blow or blow a fuse.
Like I said, the UPS I have will take a single battery but not 2 batteries in parallel. Luckily that was still in the range where it could still beep and turn off. If I had connected 16 batteries in parallel it would have been toast.

@Dawid1Yes you can add a external charger. You can also add a extra MPPT to the charge the batteries. 
 

This image below reminds me of a very old Afrikaans song : 
 twee-mosfets-met-een-skoot-middeldeur-geskiet-fontein
 

Fault code 09 means Bus Soft Start failure. Usually this means that the 400V bus is shorted, usually because two or more IGBTs have failed shorted. As Jadav mentioned above, this often damages gate driver components like resistors and diodes, sometimes also the isolating driver chips, which are 8 pin devices, often white in colour, but can also be black. 
If you're getting repeated catastrophic failures like this, then it might not be worth repairing a second time. There has been a recent spate of VM II clones, and some seem to be very poor quality.
Switching power electronics like this is something of an art to design well, keeping inductance low, knowing where to place snubbers and what values to use. These circuits get hot, so it's important to choose components with long lifetimes, which will not be the cheapest parts.