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bekim 3rd December 2007 12:18 PM

Luxman C-120a Mark II problems
2 Attachment(s)
I couldn't find any related answer before posting.

I bought broken pre/amp M/C-120 mark2, and on preamp were 2 fets in each channel dead. I have replaced the fets, Q201, 202, 203, 204 -- 2SK170 2 per channel and all electrolytes except black gate ones. In power supply I had to change all electrolytes, transistors as well because they were overheating and all biasing diodes KB-265 - double diodes with two 1N4148 in series, many of diodes where below the original ratings. All soldier I changed with new one, due the age of component this was required and was necessary.
The problem is, when preamp is powered, a DC offset can be detected only in right channel. I checked on trimpot as well replaced them with new ones, and I have same problem with right channel. All fets and other trannies are checked with multimeter and component tester, one by one. When tone bypass is pressed, dc appear on scope liitle bit shorter then when is off. One guy on told me that, on TP should be less 0volts but, below 20mV is ok. This should be checked after 15minutes, which I did. Is very difficult to do this in case voltage is very "jumpy" when you try to fix this.

C-120 mark 2 isn't same like previous model (without mark 2).
zener diode 4.0v is replaced with resistor 270kohm; C111,112,123,124 are 2.2uF block not electrolytes as can be seen on schematics.

anyone had similar problems with c-120 mark 2....
suggestions are welcome..

bekim 3rd December 2007 12:20 PM

second part
2 Attachment(s)
original files are better quality than this, but cannot post them, because of size

bekim 4th December 2007 06:41 AM

forgot to mention..

output relay is "grounded" and after 5-7 seconds will open...
How can DC appear in output if output is already gnd??

caesar148 5th December 2007 01:04 AM

I see DC blocking capacitors in the output. There should not be any DC output under all circumstances. This indicates the capacitors are either shorted internally or externally at the traces.

bekim 5th December 2007 06:52 AM

yes, I manage to isolate problem - flat amp. Even though I changed everything still dc appears. But, if preamp is left on for some 30 minutes or so, and then is truned on/off dc will not appear. And, if is left off for hour and is turned on, still same problem.
is very simple design for preamp and big problem.

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