Repair of Amana / Goodman Hotel Air Conditioning Control board
My friend is a Refrigeration mechanic fixing ice machines, freezers, and air conditioning. He calls me when he has trouble with electronic controls for this equipment.
I was asked to respond quickly to a hotel room air conditioning system which had a “fire”. A fire in a hotel is a serious problem and the manager wanted quick action. Luckily there was no fire, just some smoke in a guest room. I met him at the hotel and saw that there was some residual smoke at a through-the-wall heat pump. For those not familiar a heat pump is an air conditioner that provides cooling in summer and through reversing the refrigerant provides heating.
We removed the panel and found the board shown partially in the picture. Notice the burnt and smoke damaged components.
I removed the board and brought it to my shop. Because the burnt component was exploded and burnt beyond identification we went to his shop and picked up a spare board for comparison. Second picture shows the spare board.
The damaged component is wired directly across the 24vac input. Note the push-on connectors. This is an externally mounted transformer stepping down the mains 120vac (in US) to the 24vac which powers the control board. Close inspection revealed that the failed component is D400 on the board. This was somewhat confusing as other components D 401-404 are clearly surface mounted diodes in a bridge configuration.
Using a multimeter I measured across the good component. It measured high resistance in both directions. So it was not a diode. It looked somewhat like a ceramic capacitor. But what could be its purpose? Possibly to suppress noise coming in from the 24vac transformer. But to confuse matters further I found it was marked TVR 07470 which didn’t seem like a capacitor.
I consulted a few databases but found no matches. Finally I thought what else could be wired directly across a transformer secondary and fail in such a way?
The analysis goes like this. This control board is in an environment where voltage spikes might be present in equipment just from compressor motors, fan motors, relays, contactors, etc all of which can induce voltage spikes. At the transformer secondary was a good place to put a component to absorb these surges. I finally realized the component must be a MOV, metal oxide varistor.
On the 24vac side I knew that the allowable RMS voltage must be around 30-35vac. I looked through the catalog of a surplus parts supplier here and found some MOVs with similar ratings but nothing exact. Search on those parts I finally came up with a catalog sheet for the exact MOV and of course its specs. But no local parts supplier had such a MOV in small quantities.
Searching on eBay found a suitable replacement; a TVR 10470 which I knew from the catalog had the same 30vac spec, same clamping voltage but a slightly higher energy (joule) rating indicating greater energy absorbing qualities. At 90 cents (US) each we bought 10. My friend wanted to repair the failed board so as to have a spare. From the distributor the replacement board costs $140 so repairing one saves considerable money.
Burnt MOV was removed and the board cleaned. New MOV installed. Repaired board was tested. It performed properly.
Analysis of Failure
Voltage spikes occur constantly in motor driven equipment and heavy duty relays (contactors). The MOV eventually loses its ability to absorb spikes and heats up. It finally self destructs. In this case it was probably under specified as to energy (joule) rating.
This article was prepared for you by George Persico from USA. He is 66 years old and has more than 40 years’ experience in TV repair both Tube and Flat Screen. He also currently repairs small appliances, battery operated tools and computers.
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