[Roomba 765] No voltage detected

[victorc]

The Roomba turn on and charge the battery until full charge on the dock.

But if I put it outside the dock, it won’t turn on.

If I leave it some time then put back on the dock, I have to set the time again.
However, if I put the battery out then in, I can hear a click sound.

Even when charge is full, the Roomba says “Please charge Roomba”.
I also tried to put a new lithiom-ion battery and problem is still the same.

There is power on serial port and I can talk with roomba even outside the dock.

The bad news is that the data “Voltage” is at 0V outside dock and about 0.222V on his dock, whereas other paramaters like “Current”, “Battery power”, “Battery capacity”, “Battery temperature“ are correct. I guess there should be a way to get this fixed.

Could you please help me identifying which component should be changed?
Is there any schematics that I can use for?

Thanks in advance

[victorc]

On the dock the screen is not displaying any error and I can set time but outside dock nothing happens except that it still respond to Serial port communication.

Here are some HD pictures of motherboard : https://www.amazon.fr/photos/share/WsWv ... rqVsoX2VC3

Thanks

[victorc]

I have a LCR-T7 to test parts if needed.

[victorc]

Here are my measurements on battery then I did for L1 with AC and results differ

[RTC]

My first thought is that the D55 zener diode could be open and not limiting the voltage.
However I do not know its voltage rating, so I cannot be certain.

[victorc]

It seems to limit voltage because I get 5.1V regulated or may I misunderstand measurements.

[RTC]

You said you got 100V with AC. That seems excessive to me and indicating that D55 is doing nothing.
The 5.1V is just the battery and much less than the zener rating, so the condition of D55 has no influence on it.

Zener diodes clamp a voltage at their rating and hold it there regardless of power source.
For it to do what you are seeing, either D55 is a 100V or more diode, or it is blown open failed.
Note: zener diodes CANNOT be tested using a typical multimeter!

A 100V zener diode makes no sense to me in this circuit.

[victorc]

Right, I just unsoldered and tested the component.
Here as the results :

[RTC]

OK, you have a smart tester that knows zeners!

However if its zener voltage is 0.214V as shown, then regardless of if powered by battery or AC it should be clamping to 0.214V --- not 5.1V or 100V, which are both far above 0.214V.

I think your meter is being confused, as zeners below about 1V are rare to nonexistent!
1V to 2V zeners are the lowest voltage ratings I have ever seen, I have worked on electronics since 1967.

The lowest voltage zener listed by Mouser is 1V.

[victorc]

In fact if I put the Anode and Cathode on the meter I get the first picture (damaged component), but if I put Anode in Cathode slot and so on, I get a Zener at 214mV, which may be the voltage drop in diode.
I am now just waiting to receive a new one (which could take time due to Corona Virus).
I will keep you in touch. Thanks for your help.

[RTC]

BTW, 0.214V sounds like a forward biased germanium diode, not any normal reverse biased zener diode.

Note: the mode of operation of a zener diode is always reverse biased and should only be tested that way (never forward biased). The zener breakdown effect (which occurs sharply at their zener voltage) only happens when the diode is reverse biased.

Low voltage zeners are built of just the zener diode and thus behave like an ordinary diode when forward biased.
High voltage zeners are built with the zener diode and a reversed ordinary diode, so one or the other diode will always be reverse biased. This protects the zener from overcurrent forward current damage if accidentally installed backwards.

[victorc]

I received the new Zener diode 1N5818 today.
If I am trying to test with T7 the Zener diode, I get the same result. However, once diode replaced on the motherboard, the robot now works like a charm.
Thanks all for your help. Appreciated.

[RTC]

I received the new Zener diode 1N5818 today.
If I am trying to test with T7 the Zener diode, I get the same result. However, once diode replaced on the motherboard, the robot now works like a charm.
Thanks all for your help. Appreciated.

No surprise.

I looked up the 1N5818. It's not a zener, it is a Schottky Barrier diode. Entirely different animal. These have ultralow forward bias voltage (less than 0.1V), super fast switching speeds (well under a nanosecond), and use a metal-semiconductor junction not a PN semiconductor junction. Not sure why such a diode would be used in this circuit. This one has a 30V reverse breakdown voltage.

[RTC]

Yep, I finally looked it up and the symbol for D55 with the "S shaped" cathode indicates it is a Schottky barrier diode.

The symbol for a zener diode only has angled ends on the cathode.

[victorc]

Repair has not worked too much time, same problem again.
Maybe the diode has died again.
There should be another component that send too much voltage.
Seems like the problem is in the charging circuit.