Electronics > Repair
Marantz SR5200 amp damaged center channel + DAC issues
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poot36:
Well I have received the replacement parts and have installed them.  The center channel now works and the voltage measurements are very close to the working channels.  I have also found out the hard way that when testing dual rail amps on the bench to make sure both rails are connected or the speaker likes to start smoking and that is the weardest smell I have sniffed to date.  The amp still works after this incident so that is good.  I think what saved it was my current limited 0.5A max test supplies from old printers.  I still need to figure out the digital section of the amp but that is going to be another topic in its own right.
coromonadalix:
You may had an voice coil problem on the speaker, some deformation, and the wire enamel began to short to the metal ...
poot36:
I know that when I was trying to attach a audio source to the amp that at minimum the ground rail got disconnected and at some point the amp started motorboating.  After I unplugged the power supplies I found out that the negative rail had become disconnected.  I suspect that the full ~30V from the power supply in dc got applied to the speaker coil and smoked it.  I don't really care because I got the speaker for free.
poot36:
Well I just did a full power test in the amp and that was a success so I think this issue of the amplifier is fixed.  I did notice that there is around 0.063V dc output on the center channel with no speaker attached and no signal input compared to around 0.030V dc on the other channels.  Is this because my emitter resistors are 5% tolerance?  It does not trip the protection circuit so I am not sure that I should worry about it.
coromonadalix:
You dont have any bias adjustments for the center channel, it is an very basic amplifier

The important parts are between the bases pins of the output transistors, 5 resistors 3 capacitors.
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