Sennheiser SK300 G2 bodypack tx - shared experience - don't do what I just did!

[noreply]

I was (spoiler - still am) trying to 'fix' a Sennheiser EW300 G2 - SK300 G2 body pack transmitter 830-866 MHz

This device had leaked battery corrosion - fortunately - only on the battery contacts (so I thought).

I did a total teardown and cleaned all on the modules after a visual inspection (my first mistake).

Everything appeared to be cleaned - no visual signs of corrosion on main PCB.

So I reassembled the unit - checked everything once again - and then powered - up the unit

NOTHING!!

No power-up - unit appears to be dead.

Well I thought this was too easy - surely it cannot be as simple as cleaning any remains of corrosion and making sure no tracks are shorted / broken.

So back to debugging.

Power seems to be going to PCB

I partially disassembled the unit once again - this time checking the the power is going to the PCB directly - yep! all good here - visual inspection was OK - so I decided to power-up the device by directly 'depressing' the on/off logic microswitch.

I pressed - and within seconds could 'smell' something - KILLED BATTERIES immediately - and then had a closer look at where is the 'damage' occurred 

Interesting thing was that when pressing the on/off button on the assembled unit BEFOR - nothing happened - no 'small' nothing - so I obviously assumed (second mistake) that nothing critical happened - perhaps there was no power going to the main circuitry - because on/off logic microswitch did not engage (button not pressing due to poor alignment?)

BINGO

- yes - the button was not engaging - but when I pressed the button manually it did engage - and 'puff' there was a short somewhere.

MY FIRST MISTAKE was not examining the circuitry under a microscope 

My microscope was not available (on loan to someone) - so I had a real good look at the complete PCB logic circuitry with my 'pissweak' magnifying glass.

Needless to say - my poor vision and the bad magnifier - resulted in missing some corrosion between the tracks neat the LT chip and nearby components.

Green corrosion + green PCB + poor visual inspection = BLOWN components!

Looks like the 'short' caused by the corrosion was sufficient enough to smoke the LT chip and possibly some other components.

I took a pic with my mobile phone camera (better than my eyes - should have used this in the beginning and this post would not exist  ) and could see that there is a 'burn' mark crater on the LT chip - most likely fried.

Dave did a teardown on one of these devices some time ago - there are many variants - but they appear to be using the same power / charging circuitry.

I need to 'fix' this unit - and was hoping that someone WHO HAS HAD EXPERIANCE WITH SIMILAR PROBLEM (blown ltc3401? and I guess possible 'other devices as a result'??)

https://www.analog.com/media/en/technical-documentation/data-sheets/3401fb.pdf

could point me in the right direction with regard to best approach to instigate the fixup procedure - needless to say I will need the LT chip - so any tips of supply here in UK (trying to save time) would be most welcomed.

Like - do a 'blanket replace' of all 'dodgy looking' components??

What worries me most is not to have 'blown' other parts of the board - but I guess not - as the 'main' power would be first to blow (expensive fuse  ) and save the other critical parts of the board.


As I write this post - I managed to find a service manual - so will share this with everyone here - see below.


I have LEARNED A VERY IMPORTANT LESSON

DONT EVER power-up after corrosion cleanup UNLESS you EXAMIN the PCB / mainboard WITH A MICROSCOPE

It is possible to still have minute traces of corrosion - enough to 'short' circuit critical components - and YOU WILL NOT SEE THEM unless you use a MICROSCOPE!

The other really stupid mistake was not to supply extremal power (via jumpers) and simply put the batteries inside the unit.

If I had planned for the worse - some possible remains of corrosion which I missed (I did miss it) - then providing external power with a current limit set at miniscule level - would at least give me some protection at the massive current draw due to the 'short' and shut down the power before any damage!

Well I did learn something here - at a BIG expense to me - blown components!

I hope this 'story' will be useful to everyone attempting to 'fix' PCB's with corrosion - please avoid the stupid mistakes I made 

[m3vuv]

Its like look twice act once,reminds me of a time a good few years ago,a company i was working for had a laser copier/printer that was just serviced and free to a good home (it was big about 4ft square!),so in my car it went,i had it in there a while after puzzling what to do with it,i assumed it was monochrome as it was free,finaly i decided to take it apart with a hammer and salvage some parts,it was only when i took the toner cartriges out i realised it was not monochrome but colour!!,at the time it would of been worth £1K,talk about feeling gutted!!.