Thursday, 22 October 2020

BMW Diversity antenna amplifier.

On old school pal, a once estate agent and daytime telly presenter, now a famous comedian (so we'll let him off for the first couple of misdemeanors) dropped me a message on Facebook....

"I've followed some of your repairs with astonishment and very little understanding.

Do you think you could look at something for me?"

Yeah .. why not ..

It's a diversity amplifier from a BMW E61 Tourer. 

They're apparently a really common fault on the E61 tourer, as they're situated in the tailgate, and suffer with water ingress, and Nick confirms this one was full of water when removed, and he'd left it to dry out on a radiator for a few days...

I expect it to be full of corrosion ... 

The plastic case just unclips, and reveals the PCB. 

Which is surprisingly clean and lacking in corrosion... 

So. The plan is to first work out what on earth this thing is supposed to be doing. The faults are the remote locking wasn't working, and the radio would reset to Radio 2 with the bass and treble set to maximum whenever the ignition key was removed... hmmm.

So, a spot of reverse engineering is required. 

The antennas (there are two) seem to feed into the red and black sockets on the top edge of the PCB. I suspect these may well be "hidden" antennas in the glass... The signal is filtered and is sent to a TEA6101 diversity switch. This analyses the signal from each antenna, and sends the strongest one on to the receivers. One signal is sent to a conventional TDA1576 FM demodulator, and on out to an amplifier and off-board via the molded socket. 

The second half goes to a TDA5210 UHF receiver/decoder.. what's the betting that's our remote lock/unlock ? Output from that feeds the IC under the heatsink clamp... 

There's also a small switching supply for +5V, and some amplification and filtering of RF along the way. 

Working out where the +12V power goes to the board, brings up the +5V supply... good. Poking round the data lines with an oscilloscope shows no activity ... but is it expecting data from the car? 

A careful visual inspection of the board under the microscope shows no corrosion. Time to remove the heatsink clamp...

I thought this may be hiding some sort of power amplifier, but no, it's a microcontroller. There's also some corrosion around it's 6MHz ceramic oscillator.... Scoping up around the microcontroller shows no activity ... 

Removing the oscillator shows up the corrosion.. See that black line between the two of those long pads? 

It's duly cleaned up ... 

The oscillator put back into place after checking the bottom of it is clean.... It's soldered back in with the hot air tool.

When I removed the oscillator, I also removed a small decoupling capacitor by accident (there can be some collateral damage with a hot air tool!) ... thankfully I didn't lose it, and this is hand soldered back in place.

I don't think the heatsink clamp is actually anything of the sort. I think it's an RF shield. Anyway it's replaced. Powering up now shows some activity on several of the microcontroller's pins..

 So, it's time to reassemble it, and send it back to Nick to see if it works or not ... fingers crossed.

**** UPDATE **** 
 It worked :)