Harmon BDS280 failed trigger output

[debruina]

I am working on a Harman Kardon BDS280 that is not powering up its subwoofer. The subwoofer is connected with a mono 3.5mm plug in the triggerport so when the BDS starts the subwoofer powers up

Normally the BDS outputs a 12V when powered up but in this case the voltage was missing.

So i checked the schematics (see image) and found out that Q19 was blown. I replaced it and everything started working again. After that i removed the trigger plug cable and when i checked it again the 12V was missing again. I checked Q19 and found out it was blown again.

Inserting or removing the 3.5mm plug from the sub causes a temp short in the circuit depending the angle. I first thought the 3.5 mount was bad but others have the same problem. Now i know you should not remove cables when its powered up but i can't believe it's this easy to kill a BDS280

Anyway, what am i missing here? The only thing that should protect this circuit is the F201 sell healing fuse.

[budwich]

Re: Harmon BDS280 failed trigger output

maybe the "self healing fuse" is "fully healed"... meaning it isn't tripping any more (or isn't tripping fast enough) and basically the q19 is protecting it from further "damage". :-) try testing the fuse with a 9v battery (and small resistor - 2-3 ohm). since it is "resettable", it shouldn't harm it assuming you keep the current under the rating which is quite high so it shouldn't be a problem.

[stj]

Re: Harmon BDS280 failed trigger output

jack plugs have that problem as you withdraw them the tip and any rings drag across the ground contact.
thats why they should never be used for power.

[debruina]

Re: Harmon BDS280 failed trigger output

maybe the "self healing fuse" is "fully healed"... meaning it isn't tripping any more (or isn't tripping fast enough) and basically the q19 is protecting it from further "damage". :-) try testing the fuse with a 9v battery (and small resistor - 2-3 ohm). since it is "resettable", it shouldn't harm it assuming you keep the current under the rating which is quite high so it shouldn't be a problem.

I tested the fuse and it looks okay, Put 5 volts on it, 5 Ohm resistor in serie and the current drops to about 0.100mA. Removed power and the fuse resets. I dont see how the transisor Q19 gets damaged. It can handle 2A and the fuse should trip at 0.7A.

I am going to replace the fuse anyway, maybe its not fast enough.

[budwich]

Re: Harmon BDS280 failed trigger output

As your testing suggests, the fuse seems to work but of course, speed is of the essence. As your "findings" on the "plug pull" show, it is more likely a momentary ground which causes the "spike"... which does mean that the current could rise to a larger than rated current for Q19. The spec on the fuse speed seems to suggest its reaction is in the area of .1 seconds which seems pretty fast but semis may not even like that sort of "pulse" of over current.

[debruina]

Re: Harmon BDS280 failed trigger output

I measured the normal operating current between the receiver and the sub. It's only 5 mA so i put a 1K ohm in serie of the trigger line. This will prefent damage to the transistor. I don't understand why it is made like this. The supply voltage comes directly from the powersuppler (+12V_D). Btw it only happens on the side of the sub. The sub had a stereo jack with one side for audio and the otherside the trigger. The receiver however only has a mono jack. So you can remove the plugi from the receiver while powerup without a problem.

Anyway I modified the board a bit. Cut wire 5 (trigger output) and put a 1K resister in between. Works like charm.

1ste image Q19 with a hole in it :P
2th image: modification