Sears Kenmore AC model 73089 (Plasmaire)

[bobbeyer]

An earlier discussion topic prompted me to post this.

I have two Sears air conditioners of this model (73089); one is in my son's bedroom and the other in my daughter's. One is about a year newer than the other. My best recollection is that they are about 5 to 7 years old. We have them both installed in a through-the-wall fashion and not in windows. They have run quietly and reliably the entire time we have owned them and have never had any problems with them. All we have ever needed to do is clean the filter periodically.

This week, my wife turned them both of them on one night when it was warm but not overly hot. She decided to run them both in "power saver" mode. Something she has only done once or twice before the entire time we've had them. Some time during the middle of the night, both units burned out their AC control boards. There were no thunder storms anywhere in our region so it was probably not lightning related. No other electrical or electronic device in our house had any disruption or malfunction. My wife heard one of them shut off and woke up briefly about 1/2 hour later to discover the other was also dead. My suspicion is that their demise was related to one of two things. Either there was a power line event that killed them both (but did not affect numerous other consumer electronics left on downstairs including three PC's, three cable TV boxes, two TiVos, a microwave, etc.) or operating in power saver mode some how put an extra strain on the AC control board. I have replaced both boards and both units are operating normally once again.

I did some research and discovered that these units were made by LG (formerly Goldstar) for Sears. This is what the 580. prefix denotes. One thing I can tell you is that the schematic shown in the earlier discussion is not for the control board for this AC unit (however, the correct board is shown in some pictures near the top of the thread). I haven't found a schematic for this board. However, I did find one LG service manual that has a diagram of the board with the component layout on it (LW1500PRY3, page 26, upper right corner diagram). The earlier diagram shows a similar board that is popular in many other LG units. The board shown in that schematic uses a linear power supply design. The board for these units (6871A20271A) uses a switched-mode power supply design. By the way, the replacement part is now 6871A20271B, and I suspect that this newer B model has some improvements which could have addressed design flaws in the older A model.

The air conditioner being discussed here has an AC control board which consists mainly of a switched-mode power supply, 4 relays, and 5 connectors to other parts of the air conditioner. The switched-mode power supply in these units uses the TNY266P "Enhanced, Energy Efficient, Low Power Off-line Switcher" integrated circuit made by Power Integrations, Inc. This device can supply about 10W in an "open frame" design (meaning adequate free air ventilation for cooling e.g. a DVD player) but only 6W when used in an "adapter" which the manufacturer defines as "Typical continuous power in a non-ventilated enclosed adapter…” The temperature is in an enclosed control box so 6W should be considered the useful rating. On pages 11 and 12 of the spec sheet for the TNY266P, the manufacturer states that the TNY266P should be connected to large copper trace areas on the circuit board because these act as a heat sink for the device. This circuit board for the Sears (LG) air conditioner has very little copper foil under this device and therefore offers almost no heat sink function. It is possible that since this is running in an enclosed environment, with inadequate heat sinking (poor circuit board design), the TNY622P is running hot. Although I can't imagine why, it's possible that energy saver mode puts some additional strain on the AC control board that eventually overheats the TNY622P and kills it.

I put one of my dead boards on the bench. The +12V DC output was only 1V and there was 0V where there should have been +5V DC. There was 120V AC input to the bridge rectifier and 160V DC on the output side of the bridge rectifier. This means that the input to the TNY266P was operating correctly. Switched-mode supplies are complicated and it is possible that one of the supporting components is what died instead, but to trouble shoot any further would have required de-soldering components from the board and I had already ordered the replacement boards so the point was moot for me. Besides, testing a live power supply outside of the case is a tricky and somewhat hazardous enterprise. Especially since I still had the motor start capacitor connected. I decided to resign myself to the satisfaction of knowing it was a dead switched-mode supply, somewhere downstream of the AC mains bridge rectifier (four 1N4007 diodes), be happy I hadn't made any sparks or gotten a nasty (possibly lethal) shock and wait for UPS to show up with the new boards the next day.

We intend to forgo using the energy saver mode from now on. Although I cannot definitively say that was related to the seemingly synchronized demise of my two units, lacking further evidence, it seems as likely as my next best guess which is a power line surge.

[RegUS_PatOff]

It may just be that the electrolytic caps are bad ...