EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: czarlz on June 28, 2024, 11:01:37 am
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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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Hi!
Check the ripple on C1105, C1106.
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Problems solved!
While fixing the drivers' regulators I had to unscrew the whole back panel, the ground connection between the back panel and the terminals was therefore not made. This is the place where the reference ground and the power amps outputs connect. I was missing this!
Second issue with volume and input selector not working: by mistake I inversed one of the FPCs, precisely the one which was sending over the data from the MCU to the 2x R2A15220FP, which are the input selector and volume ICs.
Problems aside, this is the most terrible to fix piece of audio equipment I have ever had in my career. Very badly engineered, I wonder whether on purpose?