BMW Logic 7 sub amp issue

[koerneka]

(I tried to post this once but apparently it didn't go through )

I was given a Logic 7 rear (sub) amp to look into, as I am familiar with component-level repairs. The bass of the BMW Logic 7 system cuts out after being on for a few seconds. This is apparently a common issue with 7-series BMWs. A new amp is over a grand from the dealer.

I cracked the amp open, half expecting to see bulged caps. However, I haven't seen any. There are a couple 25V that are kind of suspect (Samwha, SME). There are a bunch of tall KME 25V 6800uf caps and a couple of short squat Rubycons. I think the tall caps drive the subs and they look good.

I might be in over my head, but if anyone has any guidance, I would appreciate it!

[999999999]

Re: BMW Logic 7 sub amp issue

I know nothing about this amp, but I'll throw a few ideas out there... since the bass works for a short period before cutting out my random (since it could be true for some amps but I've no knowledge of this amp's circuit) suggestion would be leaking driver transistors right before the output transistors, that they soon enough cause an overcurrent shutdown condition. It might help to search for info about what kind of protection circuits it has, possibly also overheating but unless the heatsink jumped off I wouldn't expect overheat shutdown in only a few seconds.

However if it is common you might find someone elsewhere on the 'net that has traced the circuit, found the fault, and done the repair.

If all else fails do a few common things. Reduce input signal to zero and see if amp stays on with speakers disconnected. In steps, raise input signal very slightly and measure for any output (voltage, to confirm proper operation before it's high enough to even drive the speaker(s) to an audible level). If it made it this far, reconnect speaker(s) and repeat the above.

I would not suspect bad caps to cause this if none show signs of failure. If all else fails, you'd remove the output transistors and see if they are ok (Google search for type, mosfet or bipolar junction transistors), and leave them out and power up the amp with a low level audio input (playing), measuring at the transistor base or gate pad on the PCB for half the waveform of the expected output if it were working properly (lowest voltage scale on a multimeter will at least get a change in reading).

Repeat the test for every output transistor used.

If no luck yet, now repeat the tests with no audio input signal and also measure - there should be a marked reduction in signal reading at the base or gate pad, ideally approaching zero. If it isn't approaching zero you pull the driver transistors off the board and repeat all the above, checking if they are working still and measuring at their base for an even smaller drive level.

If these things seem beyond your ability, your best hope is to keep hunting BMW or electronics forums till you find an owner who has fixed one themselves. Also check for delaminated PCB traces or cracked solder joints that might have caused a break in some subcircuit when it heats up slightly due to current flow. Places to start looking are where prior heat has darkened the top or bottom of the PCB and around larger resistors and transistors.

After it cuts out can you turn it off and immediately back on (or after less than 6 seconds or so) and have it work again for a few seconds? If not, how long must it stay off before it will work again?

That's all the ideas I have at the moment.