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 Forum index » DIY Hardware and Software » The Repair Shop
Repairing Mackie CR1604VLZ, burnt transistor?
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ma_headphone



Joined: Feb 01, 2010
Posts: 3
Location: Finland

PostPosted: Thu Nov 30, 2017 1:42 pm    Post subject: Repairing Mackie CR1604VLZ, burnt transistor? Reply with quote  Mark this post and the followings unread

Hi!

I've been replacing the ribbon cables on my Mackie CR1604VLZ to fix the well known issues related to them. After replacing them I heard generally a big improvement in the sound.

However, I did a mistake during the repair. During the replacement I inverted one of the ribbon cables, and I saw smoke coming out of the power/preamp board. I immediately turned off them mixer and fixed the cable direction.
The result of this is that, when I power up the mixer, the 1st channel does not work anymore. There is also a loud buzz on the master, but all the rest seems ok. If I enter through and insert point on the 1st channel (thus bypassing the preamp) I can hear the sound, and I can also use the EQ. My suspicion is that there is an issue in the preamp/input section.

I checked the PCB and I think I found a fried PNP transistor. I checked the legs and it seems like 2 legs are shorted. I checked the legs on the transistors on other channels and there is 7K Ohm resistance between the legs.
I marked in red the PNP which I'm talking about. I'm not sure which of the 2 it is on the schematics, I guess the left one.

How shall I proceed, shall I just try replacing this transistor? Is there some other check I should do, maybe there is something else broken I should replace before turning on the mixer again?

The PCB has both SMD and through-whole components, I think I should be able to replace this PNP on my own, even though I'm not a soldering guru.

Thanks in advance!


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Grumble



Joined: Nov 23, 2015
Posts: 1294
Location: Netherlands
Audio files: 30

PostPosted: Thu Nov 30, 2017 11:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just my two cents:
1st: I would write down which wire went to which point on the pcb when you actually connected the ribbon cable the wrong way, in this case you might easily see where you might expect problems one side or the other.
2nd: If you replace a transistor in a configuration like the cicuit shown here (a current mirror) be sure to use transistors that are matched as close as possible.

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