# Repair old inverter board

#### PSteward

Joined Nov 7, 2020
3
Hello,
I recently started to attempt to figure out what is wrong with an old fridge that I need to get working (Norcold DE-251E) and it appears that the DC circuit side of the inverter failed, this board outputs power to the 23vac compressor.
Typically this board has two main functions, take 120vac and output it to 23vac to compressor and the other is to take 12vdc and output to 23 vac. The transformer is 7 legged, 2 for 120vac input, 3 for 12-0-12vdc input, and 2 for 23vac output.
The board also no manages the evaporator thermistor, condenser thermistor, and the temperature control sensor.
Currently I have the board / transformer off for testing purposes and have observed that 120vac passes properly but 12vdc is not.
I observed that the relays are properly functioning but at the output of the alternating 12vdc there is only +12vdc on the center leg, which implies the failure is with the oscillating circuit.

My electronic knowledge is on and off as I did study circuit design but it's become more of a hobby than my career so while I could figure out an alternative circuit to produce (research and copy) a true sine (does it really need to be a true sine??) at 12-0-12 from a 12vdc input, preferably I repair the original circuit.

Any ideas??

Thanks,

--Phil

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#### Dodgydave

Joined Jun 22, 2012
11,234
Are you getting AC voltage from the Transformer 23V and 12-0-12v ?

#### PSteward

Joined Nov 7, 2020
3
Are you getting AC voltage from the Transformer 23V and 12-0-12v ?
The oscilloscope attachment (pic) is the output of the transformer when the circuit is fed 120v. The transformer shown on the board is not the main transformer, instead it drops the 120v down to ~12v to run functions on the base circuit (like the relay for example), the 120v feeds through that relay as well and direct to the main transformer which brings it down to ~23 vac.
On the 12-0-12v side there is no output to the transformer so the voltage out of it (should be ~23vac) but is 0.

#### PSteward

Joined Nov 7, 2020
3
Here are some notes on the main board and a picture of the main transformer.

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#### debe

Joined Sep 21, 2010
1,389
Looking at the boards & the compressor on the circuit diagram, they are simmilar to an Engel fridge. What are the part numbers on the 2 switching devices i have marked ?, these actualy generate the 23V AC from the transformer to feed the compressor. The 12V circuit puts out a square wave, the compressor runs on either sine or square wave.

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#### debe

Joined Sep 21, 2010
1,389
This a few pictures of the wave forms on the switching MOSFETs & the boards of an Engel equivelent fridge, ( the freq used here in Australia is 50 HZ )

#### debe

Joined Sep 21, 2010
1,389
This is the basic circuit of the 12V inverter output to drive the compressor. The Transformer is used for 12V as well as Mains voltage.