AVR25 II just shuts down

Start with the manual and carry on from there. Television to AVR, but powered speakers don't sound like a good idea. Your AVR is the amp.

Harman Kardon AVR 25 II Owner's Manual - ManualsLib

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Nope. You get one component OUTPUT (monitor) and two component INPUTS (Vid 2/DVD) but no HDMI video output.

If you can input and switch HDMI video directly at your display from your cable/SAT etc. you should be fine, just run all the audio through the AVR-25 through the coaxial/optical digital inputs, L/R analog RCA or the 5.1 direct inputs.

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Harman Kardon HK3490 Service Manual luck for you

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Can say I've seen this on a H/K. First swap the speakers left to right and right to left if the sound is low on the same speaker its the speaker not the receiver (could have a driver out). Could also be a dirty output relay.

Is the sound good other than it being low?
Is it that way with all inputs?

They just overheat, ours does, have plenty of circulation and a fan behind it if its warm.

press the off/on buttun and keep it pressed in while pressing the standby button... it should fix it

hey i also had this problem, i don't know the name of these but they are basically plugs that you have to block output from the front rear and center speakers of the white/red cables... sry for bad description but its on the left side when looking at the back of the receiver, while all of the other speaker wires should be hooked up to the right with the twisting knobs... it took me forever to find this out but yeah it was a simple fix