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Old 26th October 2013, 12:59 AM   #1
diyAudio Member
Join Date: Oct 2013

hi to all this my first post...

I need some help with 1 of my Yamaha power amp PC 2002M sr no. NN01138 the amp comes out of protection on ch. 1 A & B ch. B works ok but ch. A not passing the audio signal with no VU meter movement the PDA borad works in my other amps and tried 2 other PDA broads same problem the input wiring is ok too.

The power stage seems to work the bias is 11.0 mV the DC offset is ok

I checked all of transistors and resistors and diodes all seem ok also check the driving 2238 and 968B & 914 transistors

I have the service manual also not sure where the audio signal leaves the PDA board to the DA board the problem may be before the 2238B and 2238B transistors??? Thanks
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Old 26th October 2013, 12:02 PM   #2
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east electronics's Avatar
Join Date: Jan 2006
Location: Athens GREECE
There is only a few real reasons to activate the protection in audio amplifiers
1) dc offset
2) excessive temperature
3) often overdrive
4) other type of failure not especially related to the amplifier it self

A Yamaha PC 2002 will suffer from a number of issues that actually focus on parameter 4
--Poor soldering on any of the vertical boards
--Loss of secondary power supply
--loss of ground in any of the front boards or any other place
--Dried capacitors in any stage of the amp with first suspects near the protection circuit and the auxiliary power supply
--a broken pcb

A general inspection based on the above will probably present this and other types of problems . Work with a method take one step at a time

REMEMBER !!!! big capacitors hold voltage inside of them for some time after shut down .That can confuse your measuring very easy plus that remaining voltage will be enough to produce a big damage if you short something while working .

Kind regards
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