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DB3 BX3 keep blowing up outputs. Please help!

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darai22jp

Active Member
Hello all.
For some reason my dbx bx3 mk1 keeps frying the outputs.
All started last week when the input cables came disconnected from the amp and killed the amp. After some help from some members here, found out the problem. Resulted in the 3.9 ohm resistor in the protector board and 4 outputs blown in one of the boards. After being resurrected worked well for a day and suddenly start smoking again. This time again the resistor was bad and again the outputs were blown. After replace parts it happened again and blew up this time a resistor that is connected to the transformer secondary black cable and that goes to gnd, and blown outputs again.
After replace those parts again all happened again.... I am really clueless... anybody has any idea what can be wrong? Any help will be really appreciated. Thank you
 
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Usually when one finds shorted output transistors, it sometimes backs up to the driver and /or pre driver transistors. Make sure the emitter resistors are good also.

You should have or use a meter and a DBT.
 
Hello.
After finding some time to work on this amp during the weekend, replace a hole bunch of transistors that were blown. Powered on and waited a little bit so it could warm up. While waiting, once again destroyed the ouputs in one of the boards. Probably this is the fifth or sixth time that this happens so definetly is something wrong somewhere else. Any ideas were I can look?I am really out of ideas. Any help will be really appreciated. Thank you
 
First, put it on a variac so that you can see whats going on, just before the current rushes away.
I am sure you have some burnt resistors, and maybe broken connections. It is useless to start
up before you have checked against low impedance and wrong working voltages. Working up
a routine you can quickly find the worst parts before any working voltages are applied to it.
 
Hello all. It has been a while, busy with work and stuff... after blowing the outputs so many ties decided to put the amp Aside for a while after a lot of thinking decided to replace the variable resistors. After install set the current variable resistors at their highest resistance setting just in case, and that did it, this time didn’t blow up and was able to set bias. Seems that those pots were the problem all this time...Running like a charm now. That service manual is quite sketchy since it doesn’t say to verify the variable resistors before powering up. Well, lesson learned. I would like to thank everyone for their tips!
 
Did you measure these trimmers after removing them? Are the wipers, perhaps, not making contact?
 
Hello, seems that the pots were faulty, they were kind of loose while moving with the little screwdriver. After replace them and setting the new to the highest resistance setting did the job.
 
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