Mark Levinson 9 - Amplifier-Noise from trafo

[lars on]

Hi everyone,

I have this amplifier to fix.
The problem is noise like a transfo vibrating.
Hear the noise : https://soundcloud.com/user-179997290/bruit-ampli-ml9
When I touch the transfo I don't feel vibrations.
The noise is on left and right and comes when inputs
are connected.

Any help would be great.
Thank's

Lars

 

[lars on]

No idea ?

 

[Brian Roth]

If I understand:

Power amp is QUIET  with nothing plugged in...

Plug something in and it sounds like that crap.

You have cable/connector/whatever problems!

Bri

 

[lars on]

Thank's Brian for your help.
If it was as simple as that I would have solved the problem.

The thing is this noise is audible from inside the case
with nothing connected. And when I plug in & out
I can hear the same noise but louder in the speakers.
I'm sure it comes from the trafo.

What I want to know is if the trafo is dead or if a faulty component
is causing this bad behaviour of the trafo.

 

[abbey road d enfer]

Thank's Brian for your help.
If it was as simple as that I would have solved the problem.

The thing is this noise is audible from inside the case
with nothing connected. And when I plug in & out
I can hear the same noise but louder in the speakers.
I'm sure it comes from the trafo.

What I want to know is if the trafo is dead or if a faulty component
is causing this bad behaviour of the trafo.

That's a better explanation of the issue.
Could be something is drawing abnormal current. Maybe a rectifier is gone... Xfmrs don't like asymetrical current.

 

[lars on]

Thank's,

Should I test first the rectifiers on the power supply board ?
Could it be a cap with bad solder to ground ?

 

[abbey road d enfer]

Thank's,

Should I test first the rectifiers on the power supply board ?
Could it be a cap with bad solder to ground ?

It could be. ATM it's quite hard to reduce the field of suspects.

 

[JohnRoberts]

Hi everyone,

I have this amplifier to fix.
The problem is noise like a transfo vibrating.
Hear the noise : https://soundcloud.com/user-179997290/bruit-ampli-ml9
When I touch the transfo I don't feel vibrations.
The noise is on left and right and comes when inputs
are connected.

Any help would be great.
Thank's

Lars

Mechanical noise and circuit noise similarity may be coincidental to mains frequency.

I am not familiar with Mark Levison models but a search for Mark Levinson 9 reveals what looks like a large round (torroid) transformer, and two massive capacitors (?).

Mechanical hum as Abbey suggested could be caused by heavy loading. A good place to start looking is the power supply, is capacitor ripple high or otherwise unusual.

JR 

 

[lars on]

I found one diode 30S6 shorted.
I've to find the same and try.

May be my next post will be : "Sooolved !"

 

[lars on]

Can I replace a rectifier 30S6 with a MUR460 ?

 

[Khron]

I'm quite sure it doesn't need to be a fast-recovery type, but sure, that should work fine.

 

[lars on]

Thank you Khron  8)

 

[lars on]

After having changed the bad rectifier the
noise is lower but still there (100Hz, double of the domestic network freq).
And now something new : I have some intermittent crackles, like a dirty vinyl.
Only on the left channel.
Could it be a capacitor ? Still on the PSU ?

 

[Khron]

"Cold joints" or cracked solder joints are not impossible.

... Or perhaps even some faulty small-signal transistors. One of these days I'll have to go through and replace all the transistor pairs in the mic preamps of my DDX3216 - a few channels are really noisy when passing signal, and that solved the issue on one of the worse-offending channels.

 

[lars on]

Transistors.. the TO92 box you mean ?
Transistors are difficult to check in the circuit right ?

 

[Gus]

is it this?
https://stereonomono.blogspot.com/2019/06/mark-levinson-ml-9-power-amp.html
30amp bridge rectifier

I am not a member of Diyaudio so I can't view the schematic
https://www.diyaudio.com/forums/solid-state/238567-worth-levinson-ml-9-schematic.html


From the write up it sounds like it is balanced in so you would need to match the transistors at the input

Looks like they sell for a good amount so if you are not careful you might make it worse
https://www.hifishark.com/model/mark-levinson-ml-9

Do you have a good DMM, current clamp and a scope?

You need some good equipment to troubleshoot and fix amps

 

[lars on]

Do you have a good DMM, current clamp and a scope?

I only have a good DMM, my knife and my d..k, that why I don't wanna touch to transistors. Just testing.
But I can't test capa of 10000uF value. I'm waiting for the tester...
It is excatly this amplifier. Thank you for the schemo's link.
It is one channel I guess, without PSU and output board.
What I can only do is testing this 2x10000uF electro capa on the PSU,
swap them with new maybe all diodes too...

 

[lars on]

Hi there !

If anybody is still curious with this thread,
I have changed diodes from the PSU board,
changed one regulator (broken at an angle) from the right channel.
I have tested all big capacitors.
But no improvements. Always thhis 100Hz background.
Even worse sometimes there is tiny cracks from the left speaker...
I let you listen the sample (recorded with my phone very close from the speaker,
and I added +16db)
Maybe this new noise can point to something ?

Thank's to curious.

 

[lars on]

Any idea ?