Logitech Z906 stuck at input 5/aux

[Qlii256]

Since a couple of weeks I've been having problems with my Logitech Z906 surround speaker set-up. It works fine when I leave it without power for a while, but after some use it stops playing any sound and the console shows input 5 and aux are selected. I cannot change input, volume or anything and I hear a "pop" sound and the unit resets. I've done some digging online and this problem can be related to a lot of things, such as blown capacitors.

I've opened up the unit (woofer case), but there doesn't seem to be any sign of damage or bulged capacitors. I was hoping anyone on this forum could help me out. Is there a way I could test the unit or any of the capacitors? I'm not that experienced with electronics. If pictures are requested, I'll happily upload them.

[Qlii256]

No one seems to be able to help me unfortunately. I'm out of ideas.

[TrickyNekro]

Just because the capacitors are not bulged doesn´t mean that they are good. You probably need an ESR meter to start testing them, or if it´s easier and cheaper, then just go ahead and change them...

[Qlii256]

Thanks you for the reply. To test them, do I need to solder them loose or can I test them while still on the PCB? If I were to replace them, where do I start?

[neuweiler]

I've had exactly the same here.. My z906 worked for 20min, then wouldn't turn on anmore unless I gave it a 24-48h break. Then again: 10-20min. I tried everything I could find on the net (except replacing all caps) but nothing helped. I only saw the LED 5 and AUX lit up. I checked serial comm between console and main board: nothing on scope, no reaction when I sent commands from PC instead of console (search for z906 on github).

When I sent a pic to a friend, he told me he'd remove the black snot. He has seen it becoming conductive over time.
So I measured it: over the amp section it was infinite, over some parts of the audio input 1-2M and over the logic section I measured 200k !! 
In a 3h session, I removed all the black stuff. Mostly it was brittle and in some parts sticky (dangerous for SMD). I gave extra care with a needle pin between the legs of U22. Then I put it all together and tadaaaah!! It works fine for hours now

Be cautios though: It's easy to lift/crack an smd component or cut a trace. I used a swiss army knife (scalpel will work too), held it almost parallel to the PCB and when it got hold of the black stuff, I lifted upwards. Sometimes you'll have to try different angles. But NEVER cut downwards (except alongside a capacitor where you're sure nothing is below). Only upwards motions! And take breaks.. the longer it takes, the sloppier and impatient you get.

Unfortunately I still managed to break a transistor: Q8 (to the right of the "Sub out" connector). If anyone could tell me what part it is, I'd be most thankful. Beacuase with that DTS/DD5.1 mode doesn't work and I only get Surround (the "decode" light never lights up). It is connected to Q6/Q7 over R290 and R292. Q6 and Q7 are have written "W1P 17" on them (so likely pnp transistors?). Their base's are connected via the resistors to Q8.

[Luby17]

Hi,

I have similar issue where my 5.1 works for 20 minutes, then gets stuck and there is a cracking sound coming out of the speakers.
If I restart it, then it works for next 10 minutes.
If I cool it down for a while, then it works for 20 minutes.

BTW from where should the "black stuff" be removed?

https://imgur.com/a/jwSUAGW

[neuweiler]

I removed the black stuff from the audio-input and logic area. I left it over the amplifier area.

before:



after cleaning (3hours):


If you could PM me a close-up picture of this area (marked by red arrows) or tell me what is printed on the transistor I damaged (Q8)