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Old 19-02-2015, 09:52   #1
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Refrigerator 12v/110v power supply

O n 15/2 2015 I posted I have problem locating a replacement, or a fix for my NORCOLD SCQT-6407 refrigerator power supply. I am thinking of a DIY 110v power supply unit. The compressor needs 24v AC +-1v supply, so I am thinking a universal power supply should do the trick. However I will need a thermostat, and do any of you HVAC doctors have an idea of what's simple and best to do??
Please help.
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Old 20-02-2015, 08:30   #2
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re: Refrigerator 12v/110v power supply

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Originally Posted by J. Nikitaridis View Post
O n 15/2 2015 I posted I have problem locating a replacement, or a fix for my NORCOLD SCQT-6407 refrigerator power supply. I am thinking of a DIY 110v power supply unit. The compressor needs 24v AC +-1v supply, so I am thinking a universal power supply should do the trick. However I will need a thermostat, and do any of you HVAC doctors have an idea of what's simple and best to do??
Please help.
Isn't there already a thermostat in the refrigerator?

If I understand your post, in the end the refrigerator will only work on 120 volts AC. Won't that be a problem?
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Old 20-02-2015, 12:47   #3
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re: Refrigerator 12v/110v power supply

Hey Ron,

Here's the deal with the thermostat issue. The original SCQT-6407 power supply is nowhere available and I have a SCQT-4407 power supply which is the replacement unit that replaces the 6407. Now that unit only works on DC 12V and not on AC as I previously stated. So now the thermostat that's in the sailboat, has a four wire lead on it, and the 4407 that I want to use, has 5 wire lead. The wires are all the same color with an added orange wire in it. So the plugs on the end of the lead will not now plug into each other. I will need to source a 5 wire thermo, or make up a thermo and a power supply. NORCOLD is very disappointing, and I don't want to replace the entire unit with "an updated model", when they can not support parts for their predecessor models. Engel makes a SCQT-4408FL which is identical but far better and more reliable unit, which I will opt for later to use. For now I just want to do a patch on this junc thing that's plaguing my boat. (I hope you are not with Norcold!)
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Old 20-02-2015, 13:04   #4
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re: Refrigerator 12v/110v power supply

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Originally Posted by J. Nikitaridis View Post
Hey Ron,

Here's the deal with the thermostat issue. The original SCQT-6407 power supply is nowhere available and I have a SCQT-4407 power supply which is the replacement unit that replaces the 6407. Now that unit only works on DC 12V and not on AC as I previously stated. So now the thermostat that's in the sailboat, has a four wire lead on it, and the 4407 that I want to use, has 5 wire lead. The wires are all the same color with an added orange wire in it. So the plugs on the end of the lead will not now plug into each other. I will need to source a 5 wire thermo, or make up a thermo and a power supply. NORCOLD is very disappointing, and I don't want to replace the entire unit with "an updated model", when they can not support parts for their predecessor models. Engel makes a SCQT-4408FL which is identical but far better and more reliable unit, which I will opt for later to use. For now I just want to do a patch on this junc thing that's plaguing my boat. (I hope you are not with Norcold!)
I am not with Norcold but I suspect they at least read boating forums. Norcold doesn't have a great reputation.

My understanding was that you were going to build a power supply to provide 24 volts AC to the unit. I would think that somewhere in the existing thermostat is a switch that closes when the temperature gets above the set point and opens when the temperature gets below the set point. My thinking is, you find which wires go to the switch and use them to control your power supply. You might need a relay to handle the current.

Of course there could be more to the thermostat than what I said, I've never seen one or had it apart. I do know how a basic thermostat works though.

You could save yourself a lot of work and hassle by just replacing the refrigerator with a better one. I have a Nova Kool and have never had a problem with it.
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Old 20-02-2015, 13:10   #5
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re: Refrigerator 12v/110v power supply

Yes. I will look into a NOVA COOL. I was thinking that I could build my own power supply better then the junc that Norcold has on the market. Whatever I do, I vouch that this boat does not see a norcold product again.
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Old 20-02-2015, 19:01   #6
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re: Refrigerator 12v/110v power supply

I have an old Nova Cool DE-561 which is a combination fridge/freezer. It was originally 110ac/12vdc unit. 7 or 8 years ago it starting giving me problems. After some research I was informed that it was better to convert to a 12vdc compressor as the newer equipment was more efficient. I removed the entire unit and brought it to a marine refrigeration man in North Vancouver. He replaced the old refrigeration parts altogether ( the box was in perfect condition). It is in constant use as I live aboard. Draws at max 4.5 amps (1.5 amps less than the old unit). It is quiet and efficient and I don't consider dual voltage. good decision.
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Old 21-02-2015, 05:24   #7
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re: Refrigerator 12v/110v power supply

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I have an old Nova Cool DE-561 which is a combination fridge/freezer. It was originally 110ac/12vdc unit. 7 or 8 years ago it starting giving me problems. After some research I was informed that it was better to convert to a 12vdc compressor as the newer equipment was more efficient. I removed the entire unit and brought it to a marine refrigeration man in North Vancouver. He replaced the old refrigeration parts altogether ( the box was in perfect condition). It is in constant use as I live aboard. Draws at max 4.5 amps (1.5 amps less than the old unit). It is quiet and efficient and I don't consider dual voltage. good decision.

Both of our NovaKool fridges/freezers are dual voltage. Both have DC compressors. Both run on DC all the time; their AC "power supply" simply converts AC when present to DC. No problems with either unit.

-Chris
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Old 21-02-2015, 10:58   #8
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re: Refrigerator 12v/110v power supply

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Both of our NovaKool fridges/freezers are dual voltage. Both have DC compressors. Both run on DC all the time; their AC "power supply" simply converts AC when present to DC. No problems with either unit.

-Chris
The NovaKool sure look like a good unit. I also like Engel. I have an Engel 40LT portable unit and this has been banged around like nobody's business. I bought it back in 1981 in South Africa, where I used it on my marlin fishing boat, and I mean I used it. It has been exposed to transporting by 4x4 on dirt roads, rough terrain, then strapped on my boat, I would be beach launching at Sodwana Bay in Northern Zululand, and more often then not, I would have to punch the boat through huge dumper waves to get out from the beach launch. The result of that is that the waves pass right over the small boats, and everything on the boat is saturated. This fridge walked through that gauntlet, and I gained so much respect for it, that I was not going to leave it behind. I flew it out here with me in 2011, using it on my sailboat now on 12v, because mains is 220V only. Bottom line, it is going to be hard for me not to go with Engel, they make the SCQT-4408FL for just $799.
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Old 23-02-2015, 13:38   #9
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re: Refrigerator 12v/110v power supply

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I am not with Norcold but I suspect they at least read boating forums. Norcold doesn't have a great reputation.

My understanding was that you were going to build a power supply to provide 24 volts AC to the unit. I would think that somewhere in the existing thermostat is a switch that closes when the temperature gets above the set point and opens when the temperature gets below the set point. My thinking is, you find which wires go to the switch and use them to control your power supply. You might need a relay to handle the current.

Of course there could be more to the thermostat than what I said, I've never seen one or had it apart. I do know how a basic thermostat works though.

You could save yourself a lot of work and hassle by just replacing the refrigerator with a better one. I have a Nova Kool and have never had a problem with it.
Ron,
Do you or anyone else know whether the 4407 model has 24vac or 24vdc onto the swing motor? Looking at the manual troubleshoot test procedure on the predecessor model, 6407, there they show 24ac on the swing motor, which would mean that Norcold converts the 12vdc input supply to 24vac for the swing motor. My question is and I suspect, that this is not changed in the updated models that followed on after that, and so the 4407 is probably the same?
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Old 23-02-2015, 16:00   #10
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Re: Refrigerator 12v/110v power supply

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Ron,
Do you or anyone else know whether the 4407 model has 24vac or 24vdc onto the swing motor? Looking at the manual troubleshoot test procedure on the predecessor model, 6407, there they show 24ac on the swing motor, which would mean that Norcold converts the 12vdc input supply to 24vac for the swing motor. My question is and I suspect, that this is not changed in the updated models that followed on after that, and so the 4407 is probably the same?
I'm not a marine refrigeration expert, just an old electronic technician with a working knowledge of other trades. And I've never owned a Norcold product. I'm just passing on what I do know.
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Old 23-02-2015, 17:59   #11
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Re: Refrigerator 12v/110v power supply

According to the service manual (http://www.thetford.com/LinkClick.as...d=501&mid=1326) the swing motor runs on 24VAC.

The "thermostat" is just a potentiometer and a switch with a series LED to indicate "ON". The temperature measuring device is a temperature dependent resistor (sometimes called a thermistor). The controller compares the thermistor resitance to the potentiometer in the thermostat. Then it turns the compressor on and off accordingly.

If the power supply is dead most likely the problem is one of the fuses is blown. This happens if the 12VDC is connected backwards. There may be 2-3 fuses depending on the vintage of the oscillator assembly.

Another possibility is that one or both of the output transistors on the oscillator assembly have failed. If you have the oscillator assembly and can find the transistors it is easy enough to test them with an ohm meter. All these transistors do is switch on and off to create AC on the primary of the transformer. If you can read the type of the transistor on the case it should be possible to find replacements. There isn't much else within the unit to go wrong.
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Old 24-02-2015, 10:47   #12
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Re: Refrigerator 12v/110v power supply

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According to the service manual (http://www.thetford.com/LinkClick.as...d=501&mid=1326) the swing motor runs on 24VAC.

The "thermostat" is just a potentiometer and a switch with a series LED to indicate "ON". The temperature measuring device is a temperature dependent resistor (sometimes called a thermistor). The controller compares the thermistor resitance to the potentiometer in the thermostat. Then it turns the compressor on and off accordingly.

If the power supply is dead most likely the problem is one of the fuses is blown. This happens if the 12VDC is connected backwards. There may be 2-3 fuses depending on the vintage of the oscillator assembly.

Another possibility is that one or both of the output transistors on the oscillator assembly have failed. If you have the oscillator assembly and can find the transistors it is easy enough to test them with an ohm meter. All these transistors do is switch on and off to create AC on the primary of the transformer. If you can read the type of the transistor on the case it should be possible to find replacements. There isn't much else within the unit to go wrong.
Thank you much for this useful information. I did believe there is a way to repair the oscillator board, I have it removed, and even sent it in to electronic shops to repair it for me, and they gave up on it, saying that they were not able to get values on the components as they are sealed in a form of solid state components, kinda filled with epoxy. Given your information however this is now narrowed down somewhat. I should be able to test on the connections on the board for failed triacs or transistors, I have checked the fuses they are good. I am trying to temporally repair the unit until I can replace it with an Engel SCQT-4408 unit.
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Old 24-02-2015, 11:15   #13
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Re: Refrigerator 12v/110v power supply

If you can rig a 110-24 AC step down transformer it will run the swing compressor. You won't have temperature control but that's ok. Just let it run 24/7 until you get a new one.
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