TT XXL - Unusual problem with charging

[SlurmMcKenzie]

Hello, i have here a broken TomTom XXL System with a charging problem.

(TomTom XXL, Canada 310, IQ routes Edition)

When i got the device, it wouldn't work. But the green light was on, when i pluged it in.
But it wouldn't start.
So i replaced the battery.

And that helped a little. the battery was precharged and as soon as i put it in, the green led was on.
the device even booted.
but only while on battery.
as soon as i plug in the charger (original or replacement or even pc-usb) the device is off, instantly.
however the green light is still on.

so i guess the problem is with the charging circuit.
runs fine on battery, but doesn't charge the device, and even "short circuits" it ?
because it's off instantly when pluged in.
guess that's what killed the original battery too. didn't charge until it ran out of power and broke.

i inspected the circuit board, but there is no visible damage.
i read on the forum that there are problems with the zener diodes. but the one marked in the picture (point 1) seems ok.
there is a voltage of 4.6V.
i measured the voltage going to the battery. (picture, point 2) (without the batterie installed)
and it is just about 1.7V
so the charging circuit doesn't have enough voltage (battery is 3.7V)

but i don't know what part is damaged, or even where to look.

any suggestions ?

 

[canderson]

That 4.6V should probably be 5.1V, but at least you're in the ball park. What usually happens is that the zener diode goes by itself, or the charging chip goes along with it. I wonder if in your case the charging chip has died by itself? 1.7V certainly isn't enough to charge the battery. The chip in question is a nasty little bit of SMT work unless you're really good with an iron. Sure nothing else looks toasted there?

 

[SlurmMcKenzie]

went over it with a magnifing glass, everything looks good.
which one of the chips is the charging chip ?

 

[canderson]

Don't have board with good pic. Look for LT3455.

 

[SlurmMcKenzie]

Don't have board with good pic. Look for LT3455.

i found the LT3455, but it's not easy to access and even harder to solder.
i changed the zenner with a 5.1V but there is still just 4.6V, so that is not the problem.
at this point i give up. it's not worth any more time.
i'll dispose the unit and forget about it.
however, many thanks for your help.

 

[canderson]

There's a trick to soldering fine pitch SMT parts. The trick is getting them off the board cleanly (e.g., without pulling up any of the traces). If you can accomplish that, then flowing solder fully over the pins and picking up the excess with Solder-Wik or similar really does work. I once soldered a VERY large fine pitch SMD processor (on the order of 200 pins) to a prototype board that way, and after picking just a couple of solder bridges off, it worked like a champ. Buddy in Japan showed me that trick.

 

[SlurmMcKenzie]

it's pretty easy to desolder the chip using hot air, but i don't have that kind of tools.
however, the chip is sitting under that "cage", i would have to remove that first.
then i would have to order a new chip, and even then, it's not guaranteed to work.
if the chip is fine and one of the many many smd capacitors is short circuited, it is a hell of finding out which one.
this tom tom is trash.

 

[canderson]

Recommendation - pull the display and sell it separately. Someone is always looking for one.

 

[Andy_P]

Or even the whole thing... someone may have one with a cracked case or broken power switch (I mean the visible bit on the case that presses on the actual switch on the PCB)