Audio Systems: Old Harmon Kardon HK775 Audio Reciever, output transistors, long periods of time

Question

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Followup To

Question -
Hello Cleggsan,

I have an old Harmon Kardon Model HK775 Integrated Amplifier in beyond excellent condition.
Unforunately, the left channel doesn't work. I've tried different sources such as Turntable, DVD Player, Cassette Player and Tuner with no luck.

It's been sitting around for years in the living room and gets dusting regularily.

An idea on what it could be?

I am willing to open the cover and buy new part if necessary.

thanks,

Enu




Answer -
You must determine what the problem is.  Could be a bad amp; could be a bad speaker or wire.

Do this:  On the back of the amp, pull the speaker wires and switch the right with the left and left with the right. If the problem remains in the left channel you have a bad amplifier; if the problem switches it is a bad speaker or shorted speaker wire.

IF: Amp is bad let's give it a 90% chance the output transistors need replaced.  This is a very common failure and is aggravated when the set gets old and sits around unused for long periods of time.  Replacing the output transistors is not difficult for a seasoned tech with soldering skills, but for an inexperienced person it can be a challenging task.

Let me know if you would like more help.

Cleggsan



Hello Cleggsan,

Thanks you for the super quick response.
I think I am up to changing out the output transitors.  Where Can I find one and what would this thing look like?  I've put together quite a few electronic kits together, I have a good soldering station.

I'll open the cover this weekend and I can send you a digital photo of the inside if you possible help me identify the transition at fault.

thanks again

Enu  

Answer
The outputs are likely mounted on a heat sink (for dissipation of the heat; let's them stay cool). They must be removed, which generally means the heatsink must be pulled and then the outputs unscrewed and unsoldered.

The heat transfer washer and silicon lubricant  is very critical; if not done properly the transistors will burn out again soon.

By the way, I am wondering if you have the right model number; the 775 is a mono block brute amplifier.  See:

http://www.diyaudio.com/forums/showthread/t-86584.html

and

http://cs.georgefox.edu/~mvarney/HK775%20sm.pdf

http://cs.georgefox.edu/~mvarney/hk_interior.jpg  is a color jpg of the chassis with heatsinks on either side.

Happy Repairing.

Cleggsan

PS: Let me know the new model number is you can.