Harman Kardon AVR 645

[Arun J]

I have a Harman Kardon AVR 645 7.1 audio receiver. My problem is that when I connect my MISSION MVC CENTRE SPEAKER to the centre channel of HK, amplifier transistors blown up. (2SA1986 and 2SC5358 ). It works fine when I connect to all other channels of the amplifier. I came across this incident two times now.

When I got my receiver back after 2nd repair, I never used to connect speaker again. Can anyone help me by saying what would be the reason for this damage?

I have also Kenwood surround sound speaker system. When I got my receiver first time, I had connected centre speaker of KW to my HK's centre channel. HK got damaged and I had to take the system to repair shop. After a long while I got spares and repaired it. When I got it back, I thought it was a problem with KW speaker. So I bought new mission speaker and connected. Again my HK damaged. Now again I got it repaired and I am afraid to connect the centre speaker again.

The amplifier works fine if centre channel is not connected. It damages when the circuit completes with speaker connection. I remember I could hear sound from centre channel for a second before got damaged.

I am told that, the service person didn’t find any other component damaged when transistor replaced.

I appreciate if anyone could help me by giving some points to look into the circuit.



Thanks

 

[BradtheRad]

Uncanny for it to happen to two units, and with different speakers.

Long shot ideas...

1.

Do settings for the center channel allow a connection in a way that generates internal feedback? Example, by inadvertently feeding monitor output to monitor input. It might be by direct path or roundabout through equipment. Then when you pressed the monitor switch it would generate a strong signal, most likely high pitched, above audible range. If you have a center subwoofer it might not respond. But the strong signal would destroy output transistors.

2.

Or did you use speakers with a very low resistance? 4 ohms? Or speakers in parallel? Thus overloading the center channel transistors?

3.

Or could one part of the speaker crossover be shorted? The speaker might be fine for one frequency range, yet overdraws the amplifier at another frequency range.

 

[Arun J]

Many thanks for your reply. I would like to give you reply to your questions.

1.Do settings for the center channel allow a connection in a way that generates internal feedback? Example, by inadvertently feeding monitor output to monitor input. It might be by direct path or roundabout through equipment. Then when you pressed the monitor switch it would generate a strong signal, most likely high pitched, above audible range. If you have a center subwoofer it might not respond. But the strong signal would destroy output transistors.
------ I have configured distance (in feet) and volume to all 7 channels in same range. Disappointingly it doesn't need a signal to damage. It simply damages when we switch ON the system after setting the speaker connection to centre channel.

2. Or did you use speakers with a very low resistance? 4 ohms? Or speakers in parallel? Thus overloading the center channel transistors?

-- Both Kenwood and MISSION speakers are of 8 ohms. (HK recommends to use 8 ohm) Both are working fine with my Kenwood rack system as well as in all other 6 channels of HK

3.Or could one part of the speaker crossover be shorted? The speaker might be fine for one frequency range, yet overdraws the amplifier at another frequency range.

--- Seven-Channel Surround Modes
Power per Individual Channel
Front L&R channels:
75 Watts per channel
@ <0.07% THD, 20Hz–20kHz into 8 ohms
Center channel:
75 Watts @ <0.07% THD, 20Hz–20kHz into 8 ohms
Surround (L & R Side, L & R back) channels:
75 Watts per channel
@ <0.07% THD, 20Hz–20kHz into 8 ohms

You can see both centre and LR channels shares same specification. Both speakers work fine in L / R channels, but fails when connect to Centre.


Please see the centre channel circuit diagram.



I thank you for your time and interest though.


Thanks