Harman Kardon HK3490/230 blows power-transistor

[dpijl10]

I have a Harman Kardon receiver (HK3490/230) which has blown its end power-transistor (2SD2401 - physical damage). Both fuses are toast also.

This has happend before when a portable device had been connected and disconnected. I replaced the power-amp and its companion 2SB1570 and could not find anything else wrong - replaced the fuses and it was running again, until a few days ago. My son connected his tabled to a line-input and played some music. After disconnecting it, the receiver produced some smoke and died - hence fried 2SD2401 (cracked and hole in it).

Did I overlook something the first time, or is there somekind of designflaw a kind of dc-current after disconnecting something like a tablet? Some dead caps maybe?

Can anyone supply me a schematic of the mainboard (CUP12060) or point me in a good direction? This has happened only to channel 1 (right/red), channel 2 (left/white) has not died on me.

[rhomanski]

Re: Harman Kardon HK3490/230 blows power-transistor

Never plug or unplug anything with the unit on. So many people have damaged their equipment that way. The plug will make a momentary short circuit in the socket and blow something. That's probably why it's only one channel that is blowing out, because the same area of the plug is shorting.

What's worse is if the fuse blows and the user wraps it in tin foil and turns it back on. Saw that once.

HiFi Engine has the HK 3470 maybe that will be close enough. http://www.hifiengine.com/manual_lib...n/hk3470.shtml .

[R_J]

Re: Harman Kardon HK3490/230 blows power-transistor

I would check for poor solder connections in that channel of the amp. Here is the manual
Also check (and adjust if required) the bias for that channel after replacing the transistors.

[dpijl10]

Re: Harman Kardon HK3490/230 blows power-transistor

Thanks for the suggestions and schematics. I've never had any issues disconnecting devices in this way on other receivers/amps, short circuiting an input should not be a problem (if connected to ground that is). This receiver cannot handle it, that has been proven. I will create a buffer to prevent further blowouts. Thanks again.