marantz sr-50 humming

[chris C]

My Marantz SR-50 has developed a 50Hz hum thru both channels. I
started as a fairly quiet hum and has gotten worse over time. Th
unit still works otherwise.

Looking inside I can see that a few components in the power amp stag
have obviously become quite hot at some stage. There is a brow
discoloration on the PC board underneath two pairs of small transistor
(Q701, 702, 703 & 704). These transistors become quite hot to touc
even when the amp is not loaded.

D810 (zener I think) on the power supply has also been quite hot a
some time (dark brown colour on PCB)

My multimeter gives a reading of 39V across both of the 4700uF powe
supply caps.

Any help greatly appreciated

 

[D MARSHALL]

are the power supply caps ok?
try replaceing them.

 

[Mark D. Zacharias]

It probably is a bad cap, though probably not either of the two big ones, or
you would have measured less DC on one than the other. A scope would be
useful. You could see big ripple, if any, on a cap where there shouldn't be
much. That's my suggestion. A bad cap, maybe 47uf to 470 uf, in a regulated
power supply area.

Mark Z.

 

[Jerry G.]

You have bad filter caps in the main power supply. Most likely due to
the age of the unit, I am sure that many of the decoupler and bypass
caps are also worn down.

Jerry G.
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