Repairing Mitsubishi Mini-Split Inverter Board

RedOak

ratio said:

It appears to be part of the main board? At least, it's inside the dashed line. It's some kind of transistor H-bridge module to drive the compressor with positive & negative pulses if the notations on the drawling can be believed. I can't guess if it's an off-the-shelf part or a Mitsubishi custom job. Looking into IC932 right below it might give you some insights, it's basically the same thing but for the outdoor fan. I would expect similarities between them.

IIRC most/all Mitsubishi mini splits have some LEDs on board. Are any of them lit up? They might even be flashing an error code.

As I'm sure you're aware of, the schematic "block diagram" depicted in this thread - which is exactly the same as the one I have - is far from comprehensive. There are many components on the Mitsu E12 P65 451 Inverter Board that are not shown in the schematic.

Yes, judging by what's shown in the drawing, IC932 could very well be exactly the same IC as the IPM. The components labeled "IPM" and "IC932" in said drawing are both shown with an NPN bipolar transistor symbol on them, they both have two inputs and three outputs. In addition, both are driving motors, so these (2) ICs seem to be serving very important roles.

As I recall, there is one solitary red LED near the center of the inverter board. During my hurried testing, it never lit up at all. It's really too bad that it's configuration isn't included in the drawing, either.

We have one more HVAC tech that's supposed to come out "next week," so I'll leave the MUZ-FE18NA outdoor condenser unit fully assembled until this last guy has had a look at it. The difference between this guy and the others, however, is that he's supposedly coming out with repair work in mind. We'll have to wait and see.

mattmia2

Are tab5 and tab6 connectors? If you can disconnect the motor control there then you can see if the fuse still blows (or put a lamp in series with the fuse to limit the current so you don't keep blowing fuses) and see if it is still shorted at that point. If there is a power supply on this board that isn't shown, either that or the thermistors having welded together would be my guess.

MaxMercy

RedOak said:

Do you think that a replacement for the IPM could be found anywhere?

I seriously doubt it. There will be numbers on the device - it could be the OEM's identification or it could be Mitsu's house numbers if it's a custom module.

Back when I used to work on plasma TVs, a lot of the panels were driven by IPMs (others used all discrete components). I was able to get a lot of them out of China through Alibaba, but just as many were salvage "pulls" and were defective on arrival, and even previously reliable suppliers would start sending pulls..

Virtually 100 percent of the failures I saw were in the output sections of the IPM and were easily found by measuring the output lines to ground. On some of the ones that were potted in a jelly material, I would be able to remove the jelly and cut the lines to the output devices. I would then mount external IGBTs to the heatsink and connect by wires. Back when I was doing several a week, it got easy and saved a lot of money and the external devices actually ran cooler than the ones printed on the deck.

Now, the problem with a power surge is that it's far more likely to be an input failure (like a rectifier or MOV) or a small signal failure (like the microprocessor) than an output device. Without a schematic it gets iffy to run down the problem.

The schematic doesn't show it, but there might be MOVs on the board whose job is to divert excessive and abnormal AC voltage to ground. These look like capacitors and will either be across the AC input (after the fuse) or from the AC lines to ground. If any of these were damaged, they'll read like resistors and must be cut out (at least) or replaced. I would check for any leaky MOVs and move on to any rectifiers I see. Sometimes you get lucky and sometimes you don't.

ratio

I don't think that IC392 and the IPM are the same part, as the current rating is vastly different. I think the same engineer specced them or specced the IC & designed the module; so I expect them to have similar operational modes, similar headroom, etc. Like I said, insights.

RedOak

mattmia2 said:

Are tab5 and tab6 connectors? If you can disconnect the motor control there then you can see if the fuse still blows (or put a lamp in series with the fuse to limit the current so you don't keep blowing fuses) and see if it is still shorted at that point. If there is a power supply on this board that isn't shown, either that or the thermistors having welded together would be my guess.

I don't have an answer for this question, but I will definitely keep this in mind when I get a chance to take the MUZ-FE18NA condenser unit covers off again.

RedOak

I "borrowed" this image from the one originally placed in this thread - by member 109A_5 [Thank You] - to make things a bit easier.

In the upper-left corner of this drawing, the labels "POWER SUPPLY" and "CIRCUIT BREAKER" are employed. The HVAC folks have seen a lot more of these drawings than I have, so I want to be sure that the fellow who created this drawing is referring to the breaker panel that's supplying the outdoor disconnect, right? I'm simply not used to seeing the label "Power Supply" used in this way.

MaxMercy said:

QUOTE from @MaxMercy:

The schematic doesn't show it, but there might be MOVs on the board whose job is to divert excessive and abnormal AC voltage to ground. These look like capacitors and will either be across the AC input (after the fuse) or from the AC lines to ground. If any of these were damaged, they'll read like resistors and must be cut out (at least) or replaced. I would check for any leaky MOVs and move on to any rectifiers I see. Sometimes you get lucky and sometimes you don't.

I seem to recall mentioning this before, but the lightning struck MOVs that I've seen looked like black relays before they shunted too much current and [literally] burst open. My eyes are old, but I don't see any proper symbols for a varistor in this drawing. I suppose that means that I'll have to track them down visually.

There are definitely no signs of excessive heat anywhere on the board, so, if I find any MOVs on the board, I have to try to find the ones that are testing at a low resistance value and look into replacing them.

IMAGE OF "BLUE MOV" BELOW...

mattmia2

The dashed lines are usually field wiring so the breakers are the overcurrent protection on the feeder. you also have to remember this is sold internationally and british, european, us, australian, and numerous asian countries all have different power systems and electrical jargon.

RedOak

mattmia2 said:

The dashed lines are usually field wiring so the breakers are the overcurrent protection on the feeder. you also have to remember this is sold internationally and british, european, us, australian, and numerous asian countries all have different power systems and electrical jargon.

Considering that I spent most of my active duty time overseas, I should've thought about that. The bit about these things being sold world-wide is a very good point.

bio_guy

I don’t know if this is helpful or not. I have two MXZ-2b20NA (and one MXZ-3b24NA). One of the former went belly-up. I downloaded service manuals and trouble-shooting trees. Indications were that it was the power board. (I’d have to go back over my narrative to see exactly what I did, blown fuse, test points….) I ordered a new power board, ouch. That is the one with three big capacitor cans and a vertically-mounted encapsulated sub board on one side and two huge heat-sinked chips on the other side (E12 C15 44G).

A year or two later, the other one failed with similar diagnosis. At that point, it was early in the COVID pandemic and it was heating season and I still have a ducted gas furnace in the house. I packed up the board and wrote all the info that I had about the electronics to a DVD along with my narrative and took it to a nearby electronics repair shop. It seemed like there was a little bit of everything lined up for repair, stereo systems, guitar amplifiers, I don’t remember it all. I also came to know that he fixes big, industrial VFDs for industrial customers. I dunno if he used the info on the DVD. Communications is not his strong point and I don’t think that I ever saw him. I always talked with a woman in the front end.

It took a while. Shipping issues were blamed, and I believe it. He replaced the two huge heat-sinked chips. It is still working fine. Cost was half of a what a new board cost me. At some point after dropping off the board, I was reading reviews about the place. I had noticed lots of conservative political slogans posted. One reviewer wrote that he made a (negative) comment of some sort about the politics and the repair guy launched himself over the counter ready for fisticuffs. At that point, I was wondering if I made the right choice. Probably just as well he stays at the bench and the woman that I am guessing is his wife handles the customers.

RedOak

Thanks for posting, bio guy. Are you located in the US? If you are, which part of the country? The guy you mentioned sounds like a lot of guys who pursued starting an electronics repair business. They usually end up biting off a lot more than they can chew, and it ends up being a very stressful way to make a living.

As far as the politics goes, it's a real shame that regular folks get so involved with these politicians. Let's face it, our wonderful so-called "news media" specializes in inciting anger. Hell, it's not much different than listening to Howard Stern and ending up upset about something that doesn't matter much anyway. I mean, that's what the guy does, so why subject yourself to it?

In the end, the puppet career politicians could care less about working people, so why should working people care about them? I believe that Jack Gargan was really onto something, back in the 90s, when he started the THRO movement. Mr. Gargan believed that we should all use our vote to Throw the Hypocritical Rascals Out! In other words, he believed that we should all start our own fool-proof term limits by consistently voting against the incumbents. That way, there will be no such thing as a "career politician." Perhaps, once the possibility of building a career based on taking from the American public has been eliminated, we'll end up with true leaders who have no problem making realistic decisions.

bio_guy

I am in the New Orleans area though I moved 5 times in 15 years before moving here. It seems like the repair guy has plenty of work so he can probably charge enough to make it worth his time. I know that I was happy to have the board back for half of what a new one would cost. I am finding a relatively new friendship pretty interesting. For over two years now, I've been working near a guy that is very, very smart and a highly trained electronics expert. Basically, his smarts and his work over the years in and then out of the military made him able to fix anything that makes or detects electromagnetic radiation. Then he branched out even more as time went on. He could (and used to) be making lots more money working on work boats, but has enough and loves his current work because there is something new every day.

I don't really want to get into politics here, but I will simply say that the biggest problem I see are uncreative, selfish jerks in office that stir up fears of closet monsters because they can't think of anything new (that their opponents have not already brought up) that is constructive. In an increasingly complex world, closet monsters are easier to understand than the real problems. The electorate is to lazy to take the time to think through issues. We don't deserve democracy because we are not willing to work for it, and it is a lot of work.

Erin Holohan Haskell

Please follow site rules and refrain from political discussions. Thanks.

RedOak

Wow, what a coincidence! We were [literally] blow out of NOLA by storm Katrina back in '05!

Like your new friend, I was military trained to calibrate, align and/or component-level repair electronic test equipment (PME). I worked on the bench for years until I moved over to the engineering side of the house, but the immeasurable difference between working on military PME and working on an HVAC system, like the Mitsubishi MUZ-FE18NA condenser unit discussed in this thread, is that the military wouldn't contract for PME unless it came with technical support in the way of detailed schematic drawings and a theory of operation at a minimum. The MUZ-FE18NA being discussed herein comes with a basic power supply diagram - and not much else.

Regardless, I haven't given up on reviving this unit. It's simply in too good a shape from me to believe that it was severely overloaded throughout its circuitry. In our case, I believe that the protection circuitry did its job, but it was used strictly as an AC unit and the weather is much cooler here now, so further troubleshooting has fallen way down the priority list. I appreciate your post.

bio_guy

Newish friend was Navy. At one time, he explained that they might do things a little differently from the other branches as in fixing stuff right then and there rather than getting boards sent and swapping them out. He is also a pretty skilled machinist, daddy-taught and self-taught. That makes him a pretty handy guy to have around. I explain to people that he can make or fix just about anything. The only question is, is it worth it. In addition to doing boat work around here, he also worked for several radio and TV stations after he left the Navy.

Don't give up on finding information. The info that Mitsubishi posts at mylinkdrive changes. Have you seen the engineering info for some of their units? I found the reading pretty interesting. In trouble shooting info about the LEVs, they give a voltage range and have you check the coils.I later found some information about the frequency control of the valves I was thinking that I might put a 'scope on the LEVs to see if they were working properly. I never got that far.

Here is the TOC from a document entitled 22_2013_P-and_M-Series_Engineering_Application_Notes

P-SERIES AND M-SERIES
APPLICATION NOTES
P-SERIES A CONTROL WIRING DIAGRAM. ...................................................................................................................AN-1
MS-A WIRING DIAGRAM.................................................................................................................................................AN-3
MSY-MSZ WIRING DIAGRAM. .........................................................................................................................................AN-5
MXZ-2A20NA WIRING DIAGRAM....................................................................................................................................AN-7
MXZ-3A30NA WIRING. ...................................................................................................................................................AN-10
LOW TEMPERATURE HEATING CHART......................................................................................................................AN-13
P-SERIES TWINNING WIRING DIAGRAM....................................................................................................................AN-15
MXZ-4A36NA WIRING DIAGRAM..................................................................................................................................AN-17
SI-1730-230 WIRING DIAGRAM....................................................................................................................................AN-20
SI-3100-115 WIRING DIAGRAM. ....................................................................................................................................AN-22
SI-3100/230 WIRING DIAGRAM. ....................................................................................................................................AN-24
DRAIN LINE APPLICATIONS.........................................................................................................................................AN-25
RATED CAPACITY VS MAXIMUM CAPACITY. ..............................................................................................................AN-28
M-SERIES DRY MODE DESCRIPTION.........................................................................................................................AN-30
M&P-SERIES AIR OUTLET COVERAGE RANGE.........................................................................................................AN-33
M&P-SERIES REMOTE ACCESS..................................................................................................................................AN-36
M-SERIES COOLING CHARTS. .....................................................................................................................................AN-40
M-SERIES COOLING CAPACITY CORRECTION FACTORS. .......................................................................................AN-53
M-SERIES COOLING PERFORMANCE DATA..............................................................................................................AN-55
M-SERIES OUTDOOR UNIT CLEARANCES. ................................................................................................................AN-58
M&P SERIES HIGH ALITITUDE APPLICATIONS..........................................................................................................AN-67
REGARDING REFRIGERANT ACCESSORIES.............................................................................................................AN-70
P-SERIES INDOOR FAN CYCLE OFF. ..........................................................................................................................AN-72
SUPPLEMENTAL HEAT CONTROL...............................................................................................................................AN-74
NO SIMULTANEOUS WITH MXZ...................................................................................................................................AN-81

RedOak

@bio_guy - Did you find those Engineering Application Notes on a special Mitsubishi website? Would you kindly elaborate about where you found those notes? I don't recall what "mylinkdrive" means...

109A_5

https://mylinkdrive.com/USA/M_Series/R410A_Systems-2/Outdoor_Equipment/R410A_Outdoor/FE_Models-1/MUZ_FE18NA?product

https://mylinkdrive.com/USA/

https://mylinkdrive.com/USA/App_Notes

RedOak

Thank you for the post, @109A_5 !

Porcupinepuffer

HVAC tech with intermediate knowledge in electronics chiming in.

Had a similar situation with a unit. Mine is a model MUY-D30NA- [1]

Have almost the same schematic situation as you with the exception that I have an black (NR64) MOV added in between the F64 fuse and L2… On mine, if that MOV doesn't have a reading in the megaohms with a low resistance/short, it will easily blow the slow blow 2a fuse between L1/L2. For the two PTC64/P65 resistors in parallel. These are positive temperature coefficient resistors. Total resistance of the two of them combined is 16-17 ohms in the circuit. If you pop off the black cover, they simply look like a watch battery pressed into place. It's nothing complicated or mysterious. This circuit is a switch mode power supply. The first thing it does is let power safely charge the capacitor bank through the 2a fuse via the PTC64/65. When all is good and done, then it moves on to closing the X64 relay, which bypasses the fuse and PTC64/65 to allow full operation. This is all done to limit any crazy in-rush current that can damage rectifiers, diodes, etc… In my case, a snake caused a short between the back of the power board that damaged the components and the 2a fuse blows the second power is applied.

Replacement of the power board fixed my issue. The noise filter board was still good with a new slow blow fuse.

RedOak

Hi Porcupinepuffer ~

First, cheers for adding your thoughts to the thread - I appreciate your input.

As I didn't have a solid approach to the problem - that and the weather changed - this issue was put off until the warm weather months hit us again. As it got warm and humid quite suddenly, it was decided that we would begrudgingly lay down the extravagant amount of cash being asked for the Inverter PCB. Unfortunately, even that didn't pan out because we could no longer find a replacement board for under the $600 mark, so, considering that there was no guarantee that there was no other damage to the internals of our Mitsu condenser unit, we went to Plan B and purchased a pre-charged 24K BTU ductless system and installed it ourselves.

My only experience with AC systems was with big central air systems back home - in central FL - so the newer R134A system presented a few challenges. First, we were advised that the reliability of an R134A system would be compromised if the LL and the SL set was brazed together. This being the case we purchased a 25' loop of both lines and set about bending each line to fit the run between the evaporator head unit and the condenser unit. Considering that our weather called for a custom stand - due to snow drifting - I welded one together and anchored it to a concrete pad. As the condenser unit was now fairly high above ground level, putting together the line runs and the decorative line covers required scaffolding … but I digress.

The point is that working with continuous (single-piece) runs of copper was challenging: especially when one considers that the lengths of the lines has to be spot-on to ensure the best seal at each end. After taking my time to carefully cut, ream and flare each end of the LL and the SL, I employed Nylog to the flares to improve the connection. Once the line set was Nitrogen tested for leaks [read: we used Rectorseal Better Bubble liquid], we moved on to pulling down a vacuum. Much to my surprise, we were able to pull the line set down to less than 200 microns in under 5 minutes. After letting the vacuum rest for over 20 minutes, the vacuum gauge settled at 340 microns and was holding. For the sake of brevity, I'm leaving out certain details, but I trust it will suffice to write that the valves were then opened and the R134A refrigerant was released.

The system has been working for nine days, as I write this, and we're very happy with its performance. In short, it's very nice to have a cool/dry indoor environment again. Having written that, we now have an 18K Mistu system that we have to do something with. If I can find an Inverter PCB for it - at a price that we can possibly afford - perhaps I'll look into installing the Misu system at the other end of the house.

Thanks again ~

Red