Repairing a Fujitsu Halcyon Split Unit AC with “Manual Auto Switch error 35”

I have a Fujitsu Halcyon Split Unit AC that is about ten years old. Last week, it stopped turning on or responding to the remote control. Mysterious error lights were blinking on the front panel. Here’s how I fixed it:

First, I noticed that there were some blinking LEDs on the front panel, and sure enough, referring to the installation manual, these were diagnostic LEDs. I counted three pulses on the leftmost green LED, and five pulses on the middlemost LED. This indicated that the AC unit was experiencing error code 35. I looked up error code 35 in the installation manual, and it said that this was called the “Manual Auto Switch error.”

The user’s manual suggested two possible culprits for the error:

1) There is a small button on the bottom-right corner of the indoor half of the AC unit. If this button is held down for too long, the error code is triggered. Therefore, the most likely culprit was that this switch had gone bad. In ten years of owning this AC, I have never pressed the physical button once, always preferring to use the wireless remote; nevertheless, checking the switch made the most sense to me.

2) The main motherboard controlling the AC unit had malfunctioned and would need to be replaced. This would be a more difficult and costly repair, so I hoped to avoid it.

Feeling out of my depth, I called the HVAC company and asked them to have a technician look at the machine. His suggestion? Replace the assembly containing the switch board, replace the main motherboard, and replace an auxiliary motherboard. Estimated repair cost? Close to $1500. This was not the news I’d hoped for.

Whenever I repair broken electronics, if I can help it, I prefer to repair the circuit boards themselves, rather than guessing and replacing entire assemblies blindly. This component-level repair approach is usually more difficult and more time-consuming than board swapping, but it is less wasteful and when it is successful, it can sometimes result in significant cost savings.

Once the repairman had left (leaving the AC unit in pieces,) I took a look inside the machine to see if I could determine for myself what had gone wrong. At the very least, I hoped to test the functionality of the switch with a multimeter, which the technician hadn’t done for some reason.

After disconnecting power to the AC unit at the circuit breaker, I pulled the switch board out of the the AC unit to examine it, which the repairman had not done. It was immediately obvious to me that this switch board was not in good shape: a capacitor had leaked, corroding several traces and leaving deposits of electrolyte all over the PCB!

I gave the main motherboard visual inspection too, but all of its Nichicon-branded capacitors looked like they were in good shape. Not seeing any obvious visual issues with the main motherboard, I concluded that the corroded switch PCB was most likely the source of my problems. Since this AC unit was already broken, out of warranty, and in pieces, I didn’t have much to lose. I gave the switch PCB another look, and it seemed like a simple fix, so I decided to try repairing it myself.

A note for others attempting a similar repair: if you encounter a similarly damaged circuit board but aren’t handy with a soldering iron, that’s okay! You can just replace the switch board assembly, and skip replacing the main motherboard as suggested by Fujitsu. This will still save you a lot of money.

A view of the malfunctioning and corroded switch PCB; C204 is the electrolytic capacitor that leaked.

The switch PCB was single-sided and had relatively few components: just a few LEDs, a small button, an IR reciever, a couple diodes, and of course, the exploded capacitor. My repair procedure was as follows:

• First, I used cotton swabs soaked in isopropyl alcohol to rinse off all of the spilled electrolyte from the leaky capacitor.

• Next, I carefully scraped away all the chunks of soldermask which had bubbled up from electrolyte corrosion. The copper traces underneath this damaged soldermask were not in great shape.: the copper had oxidized and was partially eaten away.

• After scraping and rinsing away all of the damaged sections of soldermask, I used a soldering iron to coat the exposed traces in a thick layer of fresh solder. In order to get solder to stick to the heavily oxidized copper traces, I used copious amounts of flux. My reasoning for this step is that coating the exposed copper traces in solder should help prevent them from oxidizing further. Adding a layer of solder should also strengthen the traces and allow for more current to pass through them.

• I removed the damaged capacitor and replaced it with a new one of the same value: 47uf, 10V, 105ºC temperature-rated.

• I reflowed every solder joint on the PCB as many had grown dull, thanks to the electrolyte to which they’d been exposed.

• Lastly, I used a multimeter to confirm that there was continuity on every trace throughout the PCB. I added some bodges where the traces had broken using the metal leg scraps that I’d clipped from the replacement capacitor.

The PCB after I’d scraped away the corrosion and damaged soldermask.

Here’s a view of the PCB after I repaired the traces repaired with a coating of fresh solder. It’s not too pretty, but it works. In the interest of time, I opted against painting on fresh UV soldermask. Instead, I just stuck a few pieces of Kapton tape over the back of the PCB when I was done.

The two photos above show the top of the switch PCB, before and after replacing the capacitor.

Cleaning up the PCB took me only about an hour of work, and compared to similar repairs I’ve done on vintage Macintosh computers with electrolyte-corroded SMD circuit boards, this wasn’t such a tricky job.

Unfortunately, the replacement capacitor I’d selected was not low-profile like the original one, so the PCB would not neatly snap back inside its plastic carrier. I could have gotten around this issue by either installing a different replacement capacitor, or by relocating the capacitor I’d installed (perhaps by bending it flat against the PCB.) I choose to cover the back of the PCB in a layer of Kapton tape and cram it back inside the machine the way it was. My reasoning was that the PCB still fit well enough inside the plastic carrier that everything important to me would still work. The only consequence is that I lost the ability to use the tiny button. We haven’t used this button once in ten years of owning the AC unit, so losing its functionality was a non-issue provided that the remote control would be fixed. The layer of Kapton tape was intended to provide additional insulation in case the back of the PCB were to rub up against something it didn’t before. I believe the tape was probably unnecessary, but it made me feel better.

Now for the moment of truth: I reinstalled the fixed-up switch PCB into the AC unit, and threw the circuit breaker. The AC sprung to life! Because the outer plastic housing/shroud was removed, the AC now threw a new error code. More importantly though, the remote control worked again, and I could once again control my AC with it. Once I reinstalled the plastic housing, the error codes all went away. This is because there is a microswitch that checks if the housing is installed and complains if it isn’t.

Now my repair was complete! Once the machine was reassembled, all the error codes went away, and the AC was once again working perfectly! Total cost of my repair: 13 cents for one capacitor. Not bad compared to the $1500 repair estimate I received!