MEP-831a Inverter Overhaul

[kloppk]

Part 2...


Remove this bolt and spacer. (mine is missing it)



Remove the screws attaching the fan and bottom panel. There are spacers where the bolt passes thru the fan housing.



Pull the fan and base panel away.
The bracket shown in the pic attached to the base panel would be loose by the heat sink. Just slide it out and set aside.


Almost there...


Remove these 2 bolts and spacers


Remove the screws attaching the top of the capacitor board to the top board vis the "L" brackets.
Remove the 2 screws attaching the bottom of the board.



Tilt the board away and remove these two nuts. You may need to use and allen wrench on the inside to prevent the bolts from turning.
(Pic taken before removing the cap boards mounting screws.)



Remove the various "L" brackets that used to be bolted to the top of the frame. Keep track of were they go. Some are slightly different.


After that it's just a matter of removing the spacers and screws that make up the stack of boards.
There are many washers used in the spacer stack up. Be sure to remove them from the PCB's as they sometimes are stuck to them.
!!Very Important!! Be sure to re-install them properly to maintain the correct spacings.

Easy peezy

 

[Light in the Dark]

OR you can throw it in your closest river to start a new underwater fish home. Whichever is easier.

 

[kloppk]

831 haters
When they work they are great. Perfect 60 Hz output frequency & AC power plus stable VAC output. No droop like 802's and 803's.

When the inverter fails it sometimes burns out one or more of the fuses.
Check each of these 8 fuses by the PMA power input connectors to be sure they are all still good.
There are 4 for each PMA power input.
It's a bear to get thru the C.C. to meter them.

 

[marcjs14]

kloppk, that was very helpful - thanks!

I've managed to break it down to individual boards, with the exception of the board holding all the electrolytic caps.

The board holding the electrolytic caps also holds the output transistors, which are attached to the heatsink by socket-head cap screws. I take it those are accessible via the access holes in said board? I haven't yet separated the heatsink, since it was time for a fresh cup of coffee.
The scorching of a pair of coils (one large/one small) is evident in this pic...

Lastly, as far as evidence of capacitor bulge, this is the only mark vaguely indicating damage to the can's top...all others appear fine - at least on the surface. Hopefully they've just dried out. Once pulled, I'll test each individually in order to determine whether cap failure was a likely root-cause of the inverter failure.

 

[kloppk]

kloppk
The board holding the electrolytic caps also holds the output transistors, which are attached to the heatsink by socket-head cap screws. I take it those are accessible via the access holes in said board?

Correct

 

[marcjs14]

Separated power board from heatsink...

Clear evidence of source of smoke (roasted disc capacitor - other side shown for reference)...


Pretty clear which transistors have been damaged also (look bottom-right)...

 

[kloppk]

Interesting. I've torn apart dead inverters and never saw one with a toasted disc capacitor.

The 12 semiconductors on the left and right are diodes. They comprise the AC to DC rectification of the 3 phase AC power from the PMA.

 

[peapvp]

Separated power board from heatsink...
View attachment 876612
Clear evidence of source of smoke (roasted disc capacitor - other side shown for reference)...
View attachment 876613
View attachment 876614
Pretty clear which transistors have been damaged also (look bottom-right)...

View attachment 876615

this is probably a variastor and is used as inrush current limiter.
This burning up of the Varistor indicates that the high voltage side has a severe short(s) to ground.
At this point, any semi conductor, any capacitor in the HV part of this circuit is suspect.

That’s pretty much a major repair and the other question which has to be checked are PCB’s for burnt traces, looking at the amps involved in this failure.

One missed defective component or open track May cause this whole thing to go up in smoke again.

So, be careful and very thorough. Question every measurement you take. As they say, measure thrice, cut once.

Peter

 

[peapvp]

A metal oxide varistor has usually a bulge on both sides when compared to a ceramic disc capacitor .

your picture is not that clear, so we will find out if it is a ceramic capacitor or a varistor once you remove the part in question

 

[peapvp]

Is also used as transient voltage suppressor

 

[peapvp]

Inrush Current Limiters look the same as Varistors but are a NTC.
Thought I had to clarify this

 

[marcjs14]

Ugh...not going to give up just yet, but the thought of tossing it into Lake Norman as point-of-interest for young carp seems quite appealing right now .

 

[Light in the Dark]

This whole thing sounds like going through childbirth, but for men.

 

[marcjs14]

this is probably a variastor and is used as inrush current limiter.
This burning up of the Varistor indicates that the high voltage side has a severe short(s) to ground.
At this point, any semi conductor, any capacitor in the HV part of this circuit is suspect...


View attachment 876621

Peter,
should I be checking the alternator first before going any further with the inverter? Could the problem have originated there? Certainly just prior to the smoke pouring out I heard something which could best be described as some sort of mechanical rubbing sound.

Thx,
Marcel

 

[peapvp]

Ugh...not going to give up just yet, but the thought of tossing it into Lake Norman as point-of-interest for young carp seems quite appealing right now .

Marcel,
Could you please desolder this part now and post a pictures of it from both sides? This way I can get better understanding of what actually happened.
Since this part is directly next to the large caps it is most likely a inrush current limiter
But I need to verify this

 

[kloppk]

Here's a picture of one.

 

[peapvp]

I would like to see the defective part

 

[peapvp]

Kurt, the one you got is Cermaic Cap, no question. Is there anywhere in your circuit a inrush limiter that your aware off?
It is hard to believe that they have no inrush current limiting with those 12 large caps. The inrush current is pretty high when they are empty and the system first fires up

 

[peapvp]

Peter,
should I be checking the alternator first before going any further with the inverter? Could the problem have originated there? Certainly just prior to the smoke pouring out I heard something which could best be described as some sort of mechanical rubbing sound.

Thx,
Marcel

Marcel,

i still would like to see a picture of both sides of your failed part to see how it failed besides the obvious that it got hot

Peter

 

[peapvp]

Marcel,

once you remove the part could you please check for continuity between both pins?

Ceramic Capacitors can fail. They start decreasing their DC Resistance from near infinite Ohm down to full short.
But this usually takes quite a while for this happen.
A system with a failing ceramic cap usually becomes unstable first. It will keep operating but with noticeable differences. In your case it would have been drifting voltages or voltage fluctuations which increase overtime before the ceramic cap completed fails