Turnigy TGY 930 servo repair

[wedgehog]

Hi Guys, bit of an unusual one, anyone into R/C models? I am repairing a servo as in the title, it just doesn't work, dead. I took the pcb out and it is double sided with a 20 pin SMD chip on the top with various resistors and caps, all of which I have checked, and the underneath of the board has the four motor drive FET's ttwo N chan two P chan., a SMD cap and a component which has fallen apart! and I have no idea what it is.

It's about three mm squar, white on the top, it appears to have three solder points on two opposite sides, they are not pins, they are like pads on the side, when I removed what was left of it from the pcb with my hot air station I find that the three connections on the side go all the way across underneath so they are sort of wrap around pads, and in fact it has only three connections, and sure enough the pcb track is three strips under it.
I just cannot identify it, it has a white top with 20.00 written on it.

I have just been examining the component under a microscope, it looks like a licorice allsort, three layers, brown at the bottom then black then white. on top, Under the 20.00 it has T M3

Anyone any idea of what it may be, or at least what the package is, or better still has anyone got a circuit diagram for the servo please?

[MouldyOldGit]

20 MHz resonator, perhaps?

[wedgehog]

Hi Mouldy,
I thought that at first, or even an osc module, but why would there be anything like that in the power drive part of a servo? if indeed the circuit had one I would expect it to be on the top surface of the board with the control chip?
Bob

[wedgehog]

I examined the board under the microscope and traced the three connections and all three go through vias to the top and connect to the chip, So I thought Mouldy was right and I purchased a three pin ceramic 20 meg, but it doesent work, if I remove this component from another servo and fit it to this one it works and sure enough a 20 meg oscillator appears on pin 13, I suspect it may be a smd osc module, but I have spent hours searching and cannot find a three pin one like this.

I took a couple of pictures under the scope.

[fzabkar]

It appears to be a MuRata CSTCW_X series resonator:

https://www.enrgtech.co.uk/product/ceramic-resonators/ET16714093/CSTCW24M0X53-R0  (page 5)
https://www.murata.com/en-global/api/pdfdownloadapi?cate=&partno=CSTCW32M0X51-R0

https://www.digikey.com.au/en/products/detail/murata-electronics/CSTCW20M4X53-R0/2588739
https://au.mouser.com/c/?q=CSTCW20M

[wedgehog]

Hi and thank you for your reply.
I thought the same as you and I found these at Farnel but when fitted it doesn't work, yet when I remove a good one from another servo and fit it to this one it works fine. I have no ide what's going on :-(

[fzabkar]

Try adding a 1M resistor between input and output, if the circuit doesn't already have one. The CeraLock resonators incorporate two capacitors but require an external resistor. Maybe your resonator has an internal resistor???

[wedgehog]

Hi Fzabkar, I checked the connections the other day, two connections go through vias to the top of the board and then directly into the chip, the other one goes through a via to the top of the board then through a resistor to the chip, but there is another resistor going from that resistor to the chip as well. the pins in use by the resonator appear to be 11, 12,13 of the 20 pin chip, I am suspecting the chip is a PIC Microcontroler, but I'm not certain, it's just I have never seen a R/C servo chip with an external 20 meg osc before, I have seen many PIC chips with a 20 meg OSC. Of course the Chinese have ground the number off the chip, it's in a Turnigy 930 servo.

[fzabkar]

Just FYI, the centre terminal of the resonator is grounded.

[wedgehog]

Ah! interesting, thanks