Passat B6 key emits very weak

Hello again everybody,

I must say that I am a little ashamed by the fact that I only ask questions and don`t really have the expertise or time to answer other ones... My apologies for this...

I am having some problems with a VW Passat B6 key.. My godson`s key.. He has two of them, yet one emits very weak (the car only opens from near the driver`s door) and the other can open the door from 100+ meters.

I have an SDR and I can clearly see a huge difference between one key and the other. However, I`m not sure about what makes the key emit at such a weak level... I myself own an Skoda Octavia MK2 and my key transmission power is 10 times stronger than the B6`s good key TX. lol

I tried everything:
1. changed the battery
2. provided 3.1v from a bench psu
3. desoldered the oscillator (or is it something else?) and chip, clean everything, solder them back
4. replaced the buttons (he had problems with the locking button, clearly a mechanic failure - button was bent - I replaced all 3 buttons)

Unfortunately nothing helped.

I have only opened the "bad" key for replacing the buttons and (I hoped) to "fix" the bad range as I supposed it had water damage or something.. To my surprise the circuit did not had any trace of liquids, everything is nice and shiny.

I will attach some pictures here, should it be of interest. I suspect either the chip or the impedance matching circuit... but I have no values for those components and they are 0403 (or smaller) smd size.. overkill...

Thank you all!

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The good key (the one to the right) has quite some tx power while the bad key (the one to the left) has very very very little tx power... you can see in the waterfall...

I have also attached some photos of the circuit board, everything seems perfect...

As a side note - my Skoda key makes the entire graph red if near the antenna. Both keys

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The black SMDs are some axially wound inductors with black epoxy/rosin on top. The brown SMDs are definitely MLCCs... yet, what are the white SMDs?

I don't see "white" SMDs. There are brown and light grey MLCC, guess you mean the latter.

Although it could be a broken capacitor or inductor, it's more likely a defective transmitter IC.

Well... unfortunately the auto-save function of this forum didn`t worked this time.... I just lost about half an hour of research and writing... I tested it in the advanced reply mode and it does work...

1. Anyway, "white" - ok, MLCCaps, maybe with different dielectric? What should be the difference between the different color shades?
2. If the emitter/open-collector ant1/2 part of the chip is toasted, is it possible to amplify the signal with some general-purpose transistor? I don`t see how I could measure these teeny-tiny components and get their real values (even outside the circuitry)... The circuit in the datasheet is the recommended design, yet the VW engineers dd not followed it..

This is about what I had in mind.. Many many thanks for your interest!

Apparently there's an Atmel datasheet of the transmitter IC. Why didn't you post a link?

High permittivity MLCCs are mostly brown, low permittivity MLCC (RF caps) may be grey or brown.

My appologies... I could only get this link: https://pdf1.alldatasheet.com/datasheet-pdf/view/256253/ATMEL/ATAR862-4.html

The circuit in the datasheet is the recommended design, yet the VW engineers dd not followed it..

Click to expand...

The antenna matching circuit is very close to the datasheet, just has an additional low-pass.

Now I see that they used two inductors in parallel + two caps in parallel.. maybe I`ll try removing the lowpass, see if tx improves.

BTW: is there any way I could test the chip`s output directly, having the output filters / impedance matching circuit disconnected? ( I own no oscilloscope... )