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Old 09-26-2020, 02:15 AM   #1
Stigsolbjerg
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Default Gardena R38li lawnrobot mainboard power issue

Well i have this Gardena R38li lawn robot, that is giving me a headache. it had a blown somthing and a bit of corrosion on it.
attempt no 1. when i put the battery cable in, it shuts off the 18 volt battery.

attempt no 2. i tried soldering a new cap in where the blown thing was, battery still shuts off.

attempt no 3. desoldered the cap from attempt no 2, used and external powersupply and it instantly draws 6 amp at 12 volt (max on my gauge).

but i never can measure a short from + to - .

please help me, get this thing alive again

Last edited by Stigsolbjerg; 09-26-2020 at 02:18 AM..
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Old 09-26-2020, 02:17 AM   #2
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Default Re: Gardena R38li lawnrobot mainboard power issue

Here is some pics off the board
Attached Images
File Type: jpg mainboard.jpg (732.3 KB, 20 views)
File Type: jpg burnt.jpg (686.0 KB, 18 views)
File Type: jpg powersupply.jpg (653.5 KB, 17 views)
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Old 09-26-2020, 03:20 PM   #3
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Default Re: Gardena R38li lawnrobot mainboard power issue

how do you figure that actually a cap was there first place? Trace both pads and see where they go.

Last edited by CapLeaker; 09-26-2020 at 03:22 PM..
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Old 09-26-2020, 11:42 PM   #4
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Default Re: Gardena R38li lawnrobot mainboard power issue

Hi Capleaker, that was also one of my problems. I thougt it was the most obvies, since there is this giant forum named after it. But i asked in a robot forum, for close up pics of the board.
Do you have better idea than a cap?
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Old 09-27-2020, 10:00 AM   #5
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Default Re: Gardena R38li lawnrobot mainboard power issue

could have been a diode, a resistor, a jumper (0 Ohm resistor) or an inductor... we need to know where these traces go.
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Old 09-27-2020, 10:58 AM   #6
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Default Re: Gardena R38li lawnrobot mainboard power issue

Quote:
Originally Posted by CapLeaker View Post
could have been a diode, a resistor, a jumper (0 Ohm resistor) or an inductor... we need to know where these traces go.
An obvious approach is to see if it is a separate subcircuit vs. one of the many (likely identical) FET circuits in that area. In the latter case, you can examine the non-toasted FET circuits to identify the component and value.
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Old 09-28-2020, 12:05 PM   #7
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Default Re: Gardena R38li lawnrobot mainboard power issue

HI now i am measuring on the board.

no. 1. in the battery connection between + and - i get 6 kohm and rising fast to 1000 kohm, maybe starting from zero ohm.

no 2. on the big electrolytic cap (next to the blown something) there is only 2,4 ohm

no 3. on the big electrolytic cap same size as no. 2 there is 3,66 kohm

no 4. from the battery connection there is 2,5 to 3 ohms on both sides of the blown thing.

no 5. from the blown thing to the electrolyte there is 2,7 to 4,5 ohm.

these low values are going to zero when swithing to diode mode.

no 6. i cant see where the traces are going, it goes under the electrolyte.
Should i remowe it and look, What value should it be reading when i
desolder it from the board?

P.s it is 4 layer pcb with lacuer on.
Attached Images
File Type: jpg mainboard2.jpg (755.1 KB, 7 views)

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Old 09-28-2020, 05:19 PM   #8
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Default Re: Gardena R38li lawnrobot mainboard power issue

sounds like something is shorted at your step number 2. Make pictures each step as you go along, so that you know where things went and in which direction they were sitting on the board, when you take parts off the board. That electrolytic cap has a value of 220uF at 50V. My guess is, that when you take the capacitor off, the cap tests good, but you got a short across the 2 pads where the cap was sitting on. Check and compare the 9 FETs below that cap. Maybe one or more are shorted.
Can you clean that area where the burnt mystery device was up with rubbing alcohol and make another picture of it to see where the pads go?

Last edited by CapLeaker; 09-28-2020 at 05:58 PM..
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Old 09-29-2020, 02:06 PM   #9
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Default Re: Gardena R38li lawnrobot mainboard power issue

you were right, took the cap of today, and the short is still there. I sadly tore one of the traces of the board and damaged it a bit, that cap was very hevily solded under its socket. but i think it can be repaired.(see attached pic).
you said there could be a short in the nine fets. How do i check that.
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Old 09-29-2020, 02:16 PM   #10
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Default Re: Gardena R38li lawnrobot mainboard power issue

hopefully a better pic here
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File Type: jpg blown2.jpg (510.8 KB, 10 views)
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Old 09-29-2020, 02:21 PM   #11
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Default Re: Gardena R38li lawnrobot mainboard power issue

Based on internet pictures of the board, it kooks like the "blown something" is a mlcc (ceramic cap), the cap that was missing is a 220µf/35v. Like suggested there is likely one or more shorted mosfets.
There is liklely a diode for polarity protection, or the POWER SWITCH is not connected, that is why there is no short measured across the BAT connection.

Last edited by R_J; 09-29-2020 at 02:34 PM..
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Old 10-01-2020, 09:38 AM   #12
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Default Re: Gardena R38li lawnrobot mainboard power issue

hi Capleaker and R_J. you got me thinking could it be the powerswitch, hooked it and and the problems still there.
so i started looking for shorts on the mosfets. but i dont have a clue on how to look on mosfets, so here is what i found. all of 9 mosfets are shortet like the one marked whit green, but only nr 8 are shorted 2 more places marked blue and red on the pic. is this right done?
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Old 10-01-2020, 05:23 PM   #13
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Default Re: Gardena R38li lawnrobot mainboard power issue

try cleaning the coating off one of the mosfets to see if you can read the number (don't use sandpaper) try cleaning it with alcohol.

Last edited by R_J; 10-01-2020 at 05:25 PM..
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Old 10-02-2020, 09:30 AM   #14
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Default Re: Gardena R38li lawnrobot mainboard power issue

ha its a 1F33AB FDD 8424H. im suspecting i need a new one, does enyone have any ideas where to find one (laptops-printers-tv´s) or do i order in china and wait for 3 weeks.
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Old 10-02-2020, 10:03 AM   #15
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Default Re: Gardena R38li lawnrobot mainboard power issue

The mosfet number to use is FDD8424H, most suppliers like digikey, mouser etc. have it. Now that you have the number, here is the datasheet. You might be able to remove the one bad mosfet and check that the short is no longer there.
The short you measured (yellow) is S1 to S2, this might be normal and the source pins are connected together, Check for shorts between D1/D2 and S1/S2
Attached Files
File Type: pdf FDD8424H-92711.pdf (422.9 KB, 2 views)

Last edited by R_J; 10-02-2020 at 10:07 AM..
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Old 10-04-2020, 10:43 AM   #16
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Default Re: Gardena R38li lawnrobot mainboard power issue

Hi R_J im sitting here and looking to buy new mosfets, you linked a data sheet to a 20 amp one, but at digikey they only have a 9-6,5 amp one. and at r.s.online they have both, but calling the 20 amp fdd8424H-f085a.

so now im totally lost, is the 9-6,5 amp ok? i can buy minimum of 5 of that.
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Old 10-04-2020, 01:26 PM   #17
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Default Re: Gardena R38li lawnrobot mainboard power issue

did you actually find a blown FET on the board, before you go out and order something?
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Old 10-04-2020, 01:49 PM   #18
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Default Re: Gardena R38li lawnrobot mainboard power issue

No sorry, i was only Looking for cheap plaeses to buy now. I am still trying to figure out how to desolder the FET from the PCB, i do NOT have a fancy hot air soldergun/blower thing.
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Old 10-04-2020, 08:01 PM   #19
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Default Re: Gardena R38li lawnrobot mainboard power issue

Once you find the shorted part, lets say it is one of them fets, you can clip the legs with the side cutter, drill a big hole in the top of the fet until you get metal (dont drill through) then take a large wattage soldering iron / gun and unsolder the rest.
First things first... you NEED to find the broken part first. Do you have or can you borrow a CC / CV lab power supply?
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Old 10-17-2020, 02:59 AM   #20
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Default Re: Gardena R38li lawnrobot mainboard power issue

hi happy news from Denmark.

its working
.
i did order the 5 mosfets from the danish supplier, the 6,5-9 amps one.
my soldering iron had no chance at the mosfet, so i used the Dremmel and grinded the most off it away. Then i soldered in a new one and the rest of the componets. hooked it up, flipped the on/off switch and the display lights up.

Damm good feeling.

Thank u so much Capleaker, Curious.George and R_J for helping me
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