Hey everyone,
I noticed that there are some people experience here with this piece of equipment.
The story is like that:
1 ) I bought it broken, shutting down soon after power up (protection)
2 ) I found broken C1016 (100uF/35) in the feedback loop of the SBR channel, causing leakage and DC offset
3 ) Then I found broken Q1096 (2N5551C) == no +LB voltage, i.e. no positive rail for the amp drivers
4 ) The amp starts no without any problems, but..
5 ) There is a 100 Hz humming noise in all the channels
6 ) No sound from any of the channel
7 ) The sound gets through the headphones though (without 100 Hz noise)
8 ) Changing inputs takes no effect (I can see them changing on the screen only). By default AV1 is the only one which passes the signal.
9 ) Volume level for the only working input in combination with headphones seems to be fixed, changing it takes no effect.
Seems to me that I have two separate issues:
1 ) Ground issue
2 ) Input selectors & volume control ICs issue (IC741 & IC742)
The thing which I find totally bizarre is the ground connection within the power amps. If I look at the schematic, the input stage (+ drivers) has a separate ground from the power stage ground. The input stage ground is common with the chassis and all the input RCA connectors, which I find OK, but then the power output stage ground (also speakers' "-") and the main supply middle/neutral point has nothing to do with the "signal/reference" ground. They put some components like R1310 (marked with purple in the schematic attached) to tie them together, but they are "no use". If I measure resistance between the signal/reference ground (marked with yellow) and power amp output terminal "-" (marked with solid green), I read 20 kOhm. How can the amp's input&drive stage work correctly with the power stage if they have no common ground?
Will be super grateful for any of your thoughts.
BR
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