Model 1U71SABFRA
Not amateur radio related but might serve as a reference for someone, and a future reference for me as I have 4 of these.
The indoor unit started giving an error E7 flashing on the display and no cooling or heating – the outdoor fan was not starting at all.
I managed to find the service manual online, the PCB layout was not a 100% match but very similar, looks like some through-hole parts are SMD on my version, not to worry.
My usual disclaimer and WARNING – this time more severe than usual, usually I warn you that a mistake may damage your radio or device more, a mistake on this WILL KILL YOU ! If you are not confident with working on mains connected rectified DC (+330V at mains current), and do not have means to isolate the mains, your equipment and yourself and any other instrument you may use, DO NOT ATTEMPT this repair, engage a professional or someone who is capable of working on this. I will not be held responsible for any damage you cause to the unit, your equipment or your life, no one, including your next of kin or insurer will not hold me responsible for any damage you cause, including killing yourself by electrocution. Proceed at your own risk, or just read this article for educational purposes.
The unit uses 2 boards, a mainboard that seems to do all the control, and another board that drives the fan motor with PWM – I’m assuming this is a VFD controller – Variable Frequency Drive.
I did some quick checks before removing the boards from the outdoor unit, the service manual advises doing various checks, with most being a flow-chart, with most flows ending in “board fault – replace”, one check is if 330V DC is present on the 2 large smoothing capacitors, which there was, it was windy and cold out so I just removed the boards for inspection on the bench, hopefully, it can power up without many ancillary items. I was wrong, on the bench, rectified mains wasn’t available! looking at the layout, it seems the bridge rectifier is on the VFD board, with the mains filter caps on the mainboard, however, there was no power still, looking again it seems that an off-board PFC choke is connected to the VFD board, which remained in the unit on the roof, a quick bridge with crop-clips lead took care of that, hopefully, the PFC circuit won’t explode! – Note – Apart from using an isolated mains transformer, I current limit with a 100W light bulb in series when doing these checks, I learnt the hard way with MOSFET’s running off with fire trails on the bench!
After powering up I checked the main DC low voltages, 5V and 15V, they were not present, connected an (ISOLATED!) scope to the main switcher FET gate, and there was some pulsing, the SMPS was trying to start but was dribbling off, checked the datasheet for the NCP1200 SMPS controller, and it should have HV DC on pin 8 and a smoothing capacitor on pin 6, pin 8 was at around 290V DC and pin 6 at around 2V DC and “twitching”, hmm, let’s check the cap !
This is the part where I remember that I keep telling everyone, DO NOT test electrolytic capacitors with your multimeter or capacitor meter, 99% of the time they will test good, I keep telling this to a ham friend in particular ! mehhhh — Read about ESR https://en.wikipedia.org/wiki/Equivalent_series_resistance
Now lets test it as we should do an electrolytic ! Lets look at its ESR at 100KHz
Replaced the capacitor and the SMPS kicked back into life, +5V and +15V now present, some relays even started to click on the board. I gave a good check to most other susceptible components, but all looked fine, there was some corrosion on the VFD board which I cleaned up and I installed the board back in the unit, I am now writing this article in a nice and warm room 🙂
