fixing stereo receiver with burnt resisters.

Thread Starter

audio_cat

Joined Feb 20, 2023
3
Hi all,

I have a Harman Kardon receiver hk670 that has the left channel not working and burnt 2 of my speakers, unfortunately. I opened it up and found two resisters(R450 and R470) had brunt and replaced those. After I replaced them, I turned it on and R470 seemed to be ok? but R450 burnt right the way. So I thought there could be something that went in to the resister that went wrong, so I should find out and replace those too. Also, C418 seems to be shorted too, do i need to replace it or it is caused by something else?

I am very new to this and would love to hear if people here can give me some help.

Here are the photos and documents, I have the full schematic but the file seems too big to attach on here.
If we needed here, you can find it here:
https://elektrotanya.com/the_harman_kardon_hk670.pdf/download.html

Thank you so much.

Auguste
 

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panic mode

Joined Oct 10, 2011
2,793
there must be additional components that are damaged. they too need to be replaced or burning will continue. check all parts around it, including transistors Q428, Q424, Q420, Q426, Q422, Q418. btw, those transistors all look very toasty. the leads are completely black - not a trace of shiny silvery surface on the pins. btw, also check for leaky or bulging capacitors. in fact it is a good idea to just replace all large caps with new ones.
 
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Thread Starter

audio_cat

Joined Feb 20, 2023
3
there must be additional components that are damaged. they too need to be replaced or burning will continue. check app parts around it, including transistors Q428, Q424, Q420, Q426, Q422, Q418. btw, those transistors all look very toasty. the leads are completely black - not a trace of shiny silvery surface on the pins. btw, also check for leaky or bulging capacitors. in fact it is a good idea to just replace all large caps with new ones.

Here is a photo on the right channel which is still working. The leads are also all black? Are they all burnt as well or they are just somehow in black color?

When testing the transistors, do we have to take them off? Or we can just test them when still on the board?
 

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MisterBill2

Joined Jan 23, 2018
18,910
The first thing to know is that the burned resistors are not the cause of the problem, but rather the result of excess current flow because something else failed. and since this circuit appears to be a direct coupled amplifier almost any of the components will disrupt most of the voltages if they fail.
 

Dodgydave

Joined Jun 22, 2012
11,317
Ideally you should test them out of circuit, you can unsolder two legs and check them.

The output at the speaker should be zero voltage to ground, there's obviously faulty transistors somewhere.
 
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MisterBill2

Joined Jan 23, 2018
18,910
The output to that speaker should be measured across the terminals, as "ground" may not be involved even a little bit. Consider the typical bridged amplifier.
 

Thread Starter

audio_cat

Joined Feb 20, 2023
3
I will measure the terminals later as I took the Left channel board off now and want to test the transistors.

Would you recommend some videos showing me how to test transistors and capacitors. I have found some but they are a bit confusing, cuz they seem to say one thing is ok in one video and not ok in another?
 

Dodgydave

Joined Jun 22, 2012
11,317
To test transistors put your multimeter on Diode test and measure between the Base /Emitter ( 0.7V) , Emitter/ Collector ( open circuit), Base / Collector ( 0.7V) ,, this should give you an idea if they are okay or damaged, .

NOTE, Npn will have the Base Positive and Pnp will have the Base Negative .

Resistors test on Ohms out of circuit ideally,..
 
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