BASF D-6075 crackling popping output

[kasputis1]

Hello. I have read a lot of threads here about cracling/popping, but without some help can not identify components whitch slould be checked. I tracked it down to right side power amp by removing preamp jumpers from back, becouse crackling and popping stayed.

Now to the point of creating this thread:
I need help identifying differential pair transistors and driver transistors, becouse they may be causing this problem.

Sorry for posting, but i seen many good members helping newbies and patiently explaining to them.

I attach image of my board.
Also link to schematic (voltage measurements not mine):

hfe-nec-aua-8000e-schematic-unlocked-page-001 hosted at ImgBB

Image hfe-nec-aua-8000e-schematic-unlocked-page-001 hosted in ImgBB

ibb.co

Thanks in advance

 

[chazix]

Sorry for posting

I don't think you should be sorry at all - welcome to AK!

The schematic page is too sketchy for me, in several ways, but the differential pair is roughly in the upper middle, drawn as two transistors in an oval outline.

In your picture, the differential pair is the 5-pin part labeled on the board with B-C-E-C-B. The part should be labeled itself, perhaps with something like A798, which would make it a 2SA798. Such parts do indeed have a reputation for going noisy with time. Once you have the part number, you can search AK (using google is easiest) for how to replace it, usually by using two "siamesed" transistors. Most here would advise against trying to find an "original" replacement on ebay, or from any vendor that you don't know well.

The two parts that are mounted on the board with metal heat sinks are what most would call the drivers. They are less likely to cause noise issues, by enough of a margin that I think you should rule out the differential pair before you worry about the drivers.

 

[kasputis1]

I accidently shorted driver transistors with multimeter. (And in turn they blown couple of output transistors. After replacing blown transistors the thing worked, but popping and crackling stayed. Bias is off ofcourse, but not over the specs, so i guess i have to live with that. I reflowed all board half hour ago and popping went away without me finding out what the probelem was. (I already gave amp some heat by playing loud music)

I guess it will come back so next thing i wuold replace would be differential pair you just identified. I appreciate your time and thank you for knowledge friend.

 

[kasputis1]

Well.. it came back after full assembly..

 

[kasputis1]

Such parts do indeed have a reputation for going noisy with time.

And indeed you were right. You are genius and my savior.

After googling AK about this part, forum is full of information where it fails with same symptoms. And how to replace it.

I picked two ksa992 with nearly identical hFE, zip-tied them with thermal paste and my crackle and popping is long gone. (Wont picture it becouse not very aestethically done)

Anyways, thank you very much for input