I think the Frigoboat Merlin II mini smart speed board has a design flaw

[Bill O]

Originally Posted by s/v Jedi

From memory I think the RPM is chosen by the value of a resistor installed between two pins. You can simply take a rotary switch and some resistors and make every supported speed setting available with the switch.

Yes, you could make a board w/the proper resistors for preset speeds/possibly add a rotary switch or could purchase a manual potentiometer type variable speed board from Coastal (~$30).

The digital thermostat (Inkbird) is superior to the mechanical one and is inexpensive. Still can have the manual speed control w/the digital thermostat.

[Bill O]

Originally Posted by Katka

Hi, do you know how did they fix the issue. Our Merlin was purchased in April 2022. It does not work correctly. According to Coastal it is very rare to have problem with the Merlin. We need to send it to them and they will test it. But to my understanding the new Merlin’s are still not available and they are still on pre order status.

It is a known issue that we addressed last year after you purchased your unit.
Not sure what type of programing they did, but the updated boards that I have are working well/no problems.
Have Coastal (Maureen) send you a return box and ship the Mer. back to them. They were very responsive once they have the board.
In the mean time, your system should work fine w/o the Mer. board at 3000 rpm.

[rslifkin]

There's also the AEO version of the Danfoss/Secop control module that does a similar variable speed thing (with slightly different logic), but has everything built in rather than using an external board.

[PNWSalmon]

Hello, I’m late to this thread, but could someone explain what the problem looks like with the Merlin boards? In other words, how do we know if ours is working properly?

The fridge is constantly having a hard time getting down to the programmed temps. We are in the tropics, so it’s quite warm right now, but we’ve checked all of the basics, like the voltage and the keel plate being clean (keel cooled version) and we added a fan to the compressor, none of it seems to make a difference.

I’m about to “top off” the gas, but I don’t want to go down that rabbit hole if I don’t need to, so I’m trying to understand if the Merlin board could be contributing to our cooling issue.

This is a danfoss k 35, compressor and evaporator all new from 2019. With a Merlin II board that I swapped over from these old unit. I think I added the Merlin II back in 2015.

[Bill O]

Originally Posted by PNWSalmon

Hello, I’m late to this thread, but could someone explain what the problem looks like with the Merlin boards? In other words, how do we know if ours is working properly?

The fridge is constantly having a hard time getting down to the programmed temps. We are in the tropics, so it’s quite warm right now, but we’ve checked all of the basics, like the voltage and the keel plate being clean (keel cooled version) and we added a fan to the compressor, none of it seems to make a difference.

I’m about to “top off” the gas, but I don’t want to go down that rabbit hole if I don’t need to, so I’m trying to understand if the Merlin board could be contributing to our cooling issue.

This is a danfoss k 35, compressor and evaporator all new from 2019. With a Merlin II board that I swapped over from these old unit. I think I added the Merlin II back in 2015.

For a good description of the problem w/the MerII not functioning correctly, see post #1 of this thread.

Hard to tell from you description of your cooling issue, but if the MerII is functioning properly and can't get to temps, maybe the box insulation is the problem.

Can you hear the "gurgling" in the plate when running and is your plate frosting over on the whole plate? If both of these are a yes, then you refrigerant amount is fine.

What is the water temp?

[PNWSalmon]

Originally Posted by Bill O

For a good description of the problem w/the MerII not functioning correctly, see post #1 of this thread.

Hard to tell from you description of your cooling issue, but if the MerII is functioning properly and can't get to temps, maybe the box insulation is the problem.

Can you hear the "gurgling" in the plate when running and is your plate frosting over on the whole plate. If both of these are a yes then you refrigerant amount is fine.

What is the water temp?

Thank you for your reply.

Yes, post number one is clear, but the thread had morphed since then, so I lost the plot a bit, to be honest.

Indeed, I also left out a crucial detail. My Merlin seems to be saying one thing and the amps another.

The Merlin II blinks 6 times, green, indicating full compressor power, but when I throw the amp meter on the positive, I’m only measuring about 2.3 amps DC, on this 12v system (multimeter says 12.6 volts dc).

So, this amperage seems to show that the lowest compressor speed is actually running. Is the Merlin lying to me?

Going through the standard flow chart of troubleshooting, the fridge starts up normal, I get gurgling, and we get nice even frosting, and we can even make ice in the evaporator trays. But about 45 F is the lowest the fridge will go, using the digital thermostat and confirmed with a handheld thermometer. We would like it to be closer to 41.

The water is an ungodly 90 degrees F. I just had the fridge plate cleaned three days ago. No difference there. And I’ve put a fan on unit, the external temperate on it is down a 27 degrees, yet no difference in cooling.

According to the handy flow chart of troubleshooting, this seems to indicate the compressor not running fast enough. And I guess, what’s more, the amps seem to confirm this.

I always have to slow myself down and get back to first principles on these units. I just want to shoot some gas in there. Lol!

Anyway, back to the Merlin and why is it blinking 6!?!?

[Bill O]

Ok much better description of what is going on.

The 6 green flashes is telling me you are running the compressor at 3500 rpm (max) trying to keep up w/the heat getting into the box.

Agree, something isn't adding up correctly. The 2.3 amps is my approx amp draw at 2000 rpm (lowest) in 83F water read with an inline meter. You are getting good gurgling and full plate frosting, so "normal" and would not add any refrigerant.

Turn off the fridge and pull the MerII, then directly connect the thermostat leads to the C and T terminals. This will make the compressor run at 2000. What is your amp draw?

[PNWSalmon]

Originally Posted by Bill O

Ok much better description of what is going on.

The 6 green flashes is telling me you are running the compressor at 3500 rpm (max) trying to keep up w/the heat getting into the box.

Agree, something isn't adding up correctly. The 2.3 amps is my approx amp draw at 2000 rpm (lowest) in 83F water read with an inline meter. You are getting good gurgling and full plate frosting, so "normal" and would not add any refrigerant.

Turn off the fridge and pull the MerII, then directly connect the thermostat leads to the C and T terminals. This will make the compressor run at 2000. What is your amp draw?

I’m on it. I’m going to pull the Merlin and run the test with the manual speed board and/or jump the terminals, will measure and report back.

Thank you for your patience and responding back. One nice thing about the off season cruising here in the topics is getting it the bottom of some these pesky gremlins. This would be a good one to solve.

[Bill O]

Where are you, in the Sea of Cortez?

Awesome you have the manual speed board (I have one on board also). Let me know how this works for you.

If this doesn't work, I'm afraid your insulation may be the problem. We run our 6 cubic ft fridge at 35F at the low rpm and use ~35ah w/the water temp at 82F (BD50 w/keel cooler). With a water temp at 90F, we run up to 40ah

[PNWSalmon]

Originally Posted by Bill O

Where are you, in the Sea of Cortez?

Awesome you have the manual speed board (I have one on board also). Let me know how this works for you.

If this doesn't work, I'm afraid your insulation may be the problem. We run our 6 cubic ft fridge at 35F at the low rpm and use ~35ah w/the water temp at 82F (BD50 w/keel cooler). With a water temp at 90F, we run up to 40ah

Thanks Bill, you really helped us out today. So much appreciated.

I swapped the Merlin out for the speed board and we were still having issues with low amps. So, in the end our Merlin seems ok!

But this was a good rabbit hole to fall down today.

It turns out a really thick piece of ice had formed and then wedged between the evaporator and box top which somehow didn’t get dislodged during the last defrost. This was right where the copper tube enters the evaporator.

This was apparently enough to keep the compressor from reaching full amps. So odd, refrigeration is really interesting and complicated. As soon as I dislodged this we were up to full amps.

As it’s gotten hot these systems are touchy to the smallest issues. I think it comes down to not enough insulation in the end. All I can do for now is to add more shade to that side of the boat for the rest of the summer, and more thorough defrosting.

We are in banderas bay this summer. We went up to Santa Rosalia and over to Guaymas last year. And we thought we would spare ourselves that heat this year.

[Bill O]

Will say you did some excellent sleuthing to recognize and fix the issue.

Really glad you fought the feeling to add more refrigerant. Typically these cap systems work fairly well, but can be finicky with the amount of refrigerant in them. If you add refrigerant to the "proper" amount in cooler waters, then go to very warm waters, your system maybe slightly over charged. This will cause the compressor to work harder (more amps) and possibly frosting on your tube as it exits the box.

Any way, good job.

[George Elliott]

I'm sorry to log into such an old thread, but having worked on marine refrigeration since I built my own engine drive/bd2,5 system in 1991 using Richard kollmans diy book. The BD 2.5 died and I replaced it with a BD 50 and a hard start module, as it was relatively well understood that BD 50s can be reluctant starters. Ok, sorry to be so wordy, but I've just rebuilt my whole box, freezer/fridge spill down system. While I still have the hard start module I used a variable speed module and built it with the 3 speed specific resistors and a rotary switch. I'm experiencing restartt issues when switching speeds, unless when going to a higher speed it's been running for a very long time. To ensure a successful restart I'm finding I need to leave the system off for about 2 hrs, if you leave it on, it will continuously try to restart, I believe, unsuccessfully. This is a fairly old bd50 and this is the 1st time I've used the no210 thru 240 controller and the conclusion I arriving at is the bd50 is still hard to start. This may be aggravated by the fact I've quite a long hot gas line to my condensor, which is a fresh water tank. So to tank and return is close to 20' of 3/16 and 1/4". On the plus side while it's a lot of material to get moving I've a sea water temperature condensor. Not to be confused with the galley sink thru hull/condensor which I'm not sure is particularly effective in 80+° f water.ii consistently get hi pressure readings of 85-95 psi with a danfoss expansion valve with a 00 orifice. Other than the restart it's great. I'm responding to the Merlin posts. Thanks for putting up with a long post. Cold beer is a blessing. George, sv Thalia, la paz, Mexico

[George Elliott]

Sorry I should mention that I've virtually no experience with danfoss module #300/320 with aeo, but it sounds like the module is deciding compressor speed. It may allow the unit to restart at minimum speed and ramp up at a variable rate, possibly clearing BD 50 hard start. The good news is at any given speed my compressor is drawing a little less than 1/2 of danfoss suggested current consumption