Roomba 775 (aka 770) issue : turns in circles and no sound

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Roomba 775 (aka 770) issue : turns in circles and no sound

Postby Nano69 » January 29th, 2018, 9:36 am

Recently bought a Roomba 775 (which in fact seems to be a 770 board) for a cheap price, but out of order. I’m trying to repair it…

Roomba switched off, I press one time the clean button and it turns on normally (active keyboard, screen OK, but no sound).

When I press the clean button one more time, it starts (bin motor ok, brushes OK, right wheel OK, side brush OK, but left wheel doesn’t work) and turns around itself. The left wheel does not work.

Tried to replace the left wheel, and the bumper sensors, but it didn’t change anything.

A few voltage measures on the main board pointed out that after starting up the Roomba (the first time clean button is pressed) :

U25 (TPA 751), 0,9 V between pin 7 (GND) and pin 6 (V+) => this is audio amplifier, it cannot work if not powered => no sound, ok… :D
U10 (LM358), 0V between pin 4 (GND) and pin 8 (V+), :shock:
U2 (LM393) 0V between pin 4 (GND) and pin 8 (V+), :shock:
U1 (LM324), 6,15V between pin 11 (GND) and pin 4 (V+)

It seems like a part of the board is not powered….
Any idea ?

Many thanks for your help,


Arnaud
Nano69
 
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Re: Roomba 775 (aka 770) issue : turns in circles and no sou

Postby Nano69 » March 23rd, 2018, 10:12 am

Hello,


Still trying to troubleshoot this main board...

Does anyone have this board schematics or any maintenance/troubleshooting guide ?


Many thanks,


Arnaud
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Re: Roomba 775 (aka 770) issue : turns in circles and no sou

Postby a1robotrepair » March 23rd, 2018, 10:30 am

http://www.robotreviews.com/chat/viewtopic.php?f=4&t=19533
Go to this thread, under 700/800 look at right wheel H-Bridge thread, it has the info you seek.
There was a time early in the 770s line that many failed for the left wheel.

A few I repaired were bad solder joints on one of the gate circuits of the duo FETs.
Can't remember which one but pushing down with a little force on them, while in bit mode, caused them to run.
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Re: Roomba 775 (aka 770) issue : turns in circles and no sou

Postby Nano69 » March 30th, 2018, 10:05 am

@a1robotrepair : Many thanks for your help. I'll check soldering of duo FETs ...

I also have another problem : U25 (TPA 751) is not powered (no voltage on pin 6), and i don't get any sound.
Don't know if it could be linked with the wheel problem, but i'd like to solve this.

Does anyone know how this chip is powered ? It needs 5V, but where does this voltage come from ?


Thank you,


Arnaud
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Joined: January 29th, 2018, 7:07 am

Re: Roomba 775 (aka 770) issue : turns in circles and no sou

Postby a1robotrepair » March 30th, 2018, 7:17 pm

It is very possible you have lost the 5Vreg to these circuits. It is supplied throughout the board in a substrate layer. Good luck finding the bad point, looks like another jumper. Find the like points in like circuits, left wheel, right wheel circuits, and check a jumper wire from there.
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Re: Roomba 775 (aka 770) issue : turns in circles and no sou

Postby Nano69 » April 9th, 2018, 3:15 pm

Hi

Wired a jumper from U25 pin 6 to R175, and sound came back :D

CI.jpg


But this sound problem had nothing common with the right wheel definitly refusing to work ... :evil:
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Re: Roomba 775 (aka 770) issue : turns in circles and no sou

Postby Nano69 » April 9th, 2018, 3:24 pm

Sorry, left wheel....
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Re: Roomba 775 (aka 770) issue : turns in circles and no sou

Postby sabrina » July 2nd, 2018, 10:14 pm

A similar thing happened to 570... Roomba as it was going in circles and one wheel was not working. The corresponding H bridge was the apparent cause, as changing the transistors in that bridge healed the Roomba. The failure was probably caused in this case by some spill that also coloured that part of the board. We bought the Roomba after it was "cooked". Luckily, now it works, but it needed a lot of work on that MB.
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