CS 800X one channel problem

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CS 800X one channel problem

Post by albert87 » Wed Oct 28, 2020 6:30 am

Hi all,

I was asked to help repairing a CS 800X, it was previously used by many sound rental and have been repaired a lot of times by different people. The amplifier had mix and match transistors installed except for the 71180 and 81180 so I have to replace all of them with MJ15024 and MJ15025. I also replaced the 6018 and 6019 transistors with MJE 340 and MJE 350 since they too are replaced by the previous repairer. The white big resistor 0.33 ohm at R112, R114, R116 and R117 was also replaced with higher rating and had to be changed back to its original along with the 1k resistors beside it. R120 330 ohm was also burned badly and some of the ceramic capacitor C107, C108 has lower rating than written and had to be changed. The bridge diode was also blown and had to be replaced.

So with all things looks good I check for the voltages, however I notice that at channel A the bias voltage for all the transistor is way higher than channel B. I took apart all the final transistors including the 81180 and 71180 leaving only the MJE340 and MJE350. Now the part that puzzle me are the bias voltage of channel A MJE340 and MJE350 are 5.4v and -5.4v while channel B are 1.8v and -1.8v. I tried swapping the pre amp board but it still read the same. I have tested CR100, CR101, CR105 and CR107 with multimeter and they all was flowing one direction only so possibly not faulty. Does anyone have any idea or encountered something similar before?? I would like to ask for suggestion or opinion if any regarding this matter.

Thank you in advance.

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Re: CS 800X one channel problem

Post by Enzo » Thu Oct 29, 2020 4:33 pm

My spider sense tingles for an open resistor.

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