Faulty Harman Kardon AVR 230

[overall feedback]

This is repeatable. Touch the D5V pin with a DMMeter and the amp starts.
That narrows it down to a tiny area. Bad 5V supply probably.
7805 IC and a couple of capacitors. All easy to replace.
Are there common 7805 failure modes?
Finally feel confident about the problem.

Best wishes
David

There are also c900, c894, c895 near controller.

 

[Dave Zan]

The user manual explains a "reset" procedure. This is actually a software reset.
I tried this very early without success.
The service manual does NOT mention it, but inspection of the circuitry shows there is a switch to do a hardware reset.
A hardware reset seems like it should be more complete so I tried it.
Not sure if it made a difference, as OF noted, the circuit is a bit odd.
Also keep the amp powered up all day.
The problem has not recurred.
I suspect that the cause was an electrolytic capacitor that went bad while the amplifier waited for the bad battery to be fixed and that it has reformed now that the amp has been powered for a while.
That is a bit unsatisfactory - anticlimactic and inconclusive.
But at least it works at the moment.

Thanks everyone for ideas and help.
If any of you make it to Canberra then PM me and I will buy drinks!

Best wishes
David

 

[Dave Zan]

Ok, exactly where please ?

Amp did not work today.
Probed the D5V line on the circuit board with the 7805, that's CUP11651 for those with a manual.
Measured 5.6 V and the amp still did not work.
Probed the same line where I previously had, on an intermediate board that transfers the power to the front board, CUP11581.
Amp immediately worked.
So perhaps physically bad power connection.
That makes sense, it had occurred to me that it was odd that a meter probe of 10Mohm/few pF should trip the state when there was already a pile of load and capacitance on the line.
Also makes sense that the connection may have oxidised and deteriorated while the amp sat around, just as I previously suspected a capacitor had.
Will check more, I really hate intermittent faults.

Best wishes
David

 

[Mooly]

Are you sure its the meter lead causing it to start up ? If you poke it all with a plastic pen.... you see where I'm coming from...

Very very rare but... is the crystal or ceramic resonator used for the uP clock OK. Sometimes a crystal can fail to "start" in a given circuit... until something spikes it into action.

 

[Dave Zan]

Are you sure its the meter lead causing it to start up ? If you poke it all with a plastic pen.... you see where I'm coming from...

Excellent idea. Especially since it restarted on a repeat test when I touched the 7805 board, unlike the first time.

Very very rare but... is the crystal or ceramic resonator used for the uP clock OK. Sometimes a crystal can fail to "start" in a given circuit... until something spikes it into action.

I wondered about some such possibility. I will check this.
So slow when I have to wait indefinitely to repeat any test.
It defeated the factory authorised service department but DIYaudio will win!

Best wishes
David