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

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

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

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

Any help greatly appreciated.

are the power supply caps ok?
try replaceing them.

"chris C" wrote in message
...

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

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

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

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

Any help greatly appreciated.


--
chris C

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.


"D MARSHALL" wrote in message
...
are the power supply caps ok?
try replaceing them.

"chris C" wrote in message
...

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

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

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

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

Any help greatly appreciated.


--
chris C

Thanks to you both,
I had suspected a cap. I'll have a more thorough look at the power supply area of the board.

Other than looking for obvious signs of failure should I be checking with a capacitance meter? I don't have one, but I've been meaning to upgrade my DMM for some time. Will a capacitance meter give a reliable reading of a cap which is in circuit?

I have a very basic CRO adaptor which plugs into my PC's sound card. It's got a 10x probe so I'll have a look for ripple as well.

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.
======

Caps in overly heated (browned) areas are much more prone to failure. An ESR
meter can ususally be used in circuit - in part that's what they were
designed for.

Mark Z.


"chris C" wrote in message
...

Thanks to you both,
I had suspected a cap. I'll have a more thorough look at the power
supply area of the board.

Other than looking for obvious signs of failure should I be checking
with a capacitance meter? I don't have one, but I've been meaning to
upgrade my DMM for some time. Will a capacitance meter give a
reliable reading of a cap which is in circuit?

I have a very basic CRO adaptor which plugs into my PC's sound card.
It's got a 10x probe so I'll have a look for ripple as well.


--
chris C

thanks again,
I have a theoretical background in electronics (communications engineering degree) but I have no experience in repair, so please bear with me.

Would it be wise to simply replace all the caps in the power supply (I'm assuming it's the electrolytics that are the problem) or am I better off throwing the unit away?

I don't know about replacing all the caps, or how you feel about that
receiver, or how valuable you consider your time to be. Lots of people would
fix it even if it required paying a shop. Others wouldn't. Still others
would fix it themselves just because they enjoy a "project". Some people
prefer buying new stuff just because it's new and the receiver isn't. Which
category do you fall into?

Mark Z.


"chris C" wrote in message
...

thanks again,
I have a theoretical background in electronics (communications
engineering degree) but I have no experience in repair, so please bear
with me.

Would it be wise to simply replace all the caps in the power supply
(I'm assuming it's the electrolytics that are the problem) or am I
better off throwing the unit away?


--
chris C