BM6000 volume motor voltage offest

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Christian Christensen Posted: Thu, Oct 22 2015 10:14 PM

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I got myself a dead BM6000 ( 80s version )
I loooove the broken and dead stuff as I love to be Jesus and give them life. ;) 

First I  had a visual look, and it was leaking caps everywhere.
Even the biggie smoothies in the power supply where leaking, never seen those bigges in 80s amps to be leaking.

So took the decision to recap the power amp and the power supply before error tracking it.

What I found was that the 5 volt regulator 7805 was dead. - replaced and the receiver came to life.
The 7805 is giving the amp idle supply of 5 volt, which means it is working 24/7, it kinda make sence that it will die at some point early. - seen the the right  in first image bellow.
Actually I could see that it has been replaced before., as it was a lot of cooling paste mess around it.



But changing volume didnt work, the motor hardly moved.
( the volume potetionmeter is by belt driven by a small motor )
I found that the drive amp to the motor L378 was burning hot with in seconds, and the PCB was black around it. I replaced the 378, seen in the middle of the image above

took the motor out and and removed the parts in the gearbox,
Cleaned everything.
But I didnt really know how to deal with these small motors.
I got the motor working smoothly again buy running it on a 9 volt batteery as I sprayed tons of alcohol into it. My purpose what to losed up the old oil after awhile it run smootly and then i poured in lots of sewing machine oil

So, mounted everthing back 
for a moment it seemed like it worked, but afer a while motor went dead again. new op amp hot again.

 So I measured the motor - 0 ohm.
 turning the axel a bit I got 100  ohm

short cut somewhere, so I removed the back end of the motor.
The rotor is diverted into 3 pieces around the rotor 120-120-120 degress. I could barely see the gaps between as things are to tiny, but could imagine *** there in the gap
.The red arrow is pointing on the cap between the sections.

assmble again. now it became better. I measure mostly 100 ohm depending on axel posistion, sometimes 60 ohm and a few positions 30 Ohm

Now..I have no big experience of motors, so my knowledge is low.

Back assmebling everything again.
And the dead amp is working again.

Question 1. I am worried about this small motor. is there anything more I can do to make it better ?

My worries is that it will shortcut again, and blew the drive amp so I put in a 315mA fuse

Time for test run and after reading the schamtics I realized what the trimmers are for and adjusted them.

So basicly the dead BM6000 was risen from the dead and is alive again :)
But I did voltage measure on the motor while testing colume up and down and the preset buttonWhile using the up and down buttons, I can read that the motor gets appropriate voltage and voltage goes down to zero.

but when using the prestbuttons, which are givers to a regulate system, based on the third section of the volume potetionmeter., the system leaves a voltage offsett everytime I have used the preset., which automaticly also goes into function if I turn off the BM6000 into standby mode.

The voltage offsett can be anything between -2 to + 2 volt.. Not enough to make the motor move.
In a perfect regulate world the motor should stop at the exact place and the voltage offset on the motor should be zero.

Question 2, is this normal with a slight voltage offsett ?

 is the motor bad, maybe it should move on 2 volt until it slides slowly into zero offsett ?
or should the motor not be able to move at 0.5 volt to 2 volt and will stop at an non exact place and leave an offsett in the regulate system ? 

Anyone with higher experience of regulate systems and small motors to give me knowledge and advise in this situation ? 

this means that as BM6000 always moves to pre-preset when you turn it off there will be a constant small load on the drive amp 24/7 

Is this to be worried about, or should I just be happy and use my BM6000 that lives again ?

Christian 

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sonavor replied on Fri, Oct 30 2015 12:22 AM

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Excellent pictures and nice job of resurrecting the volume motor. I had one that I had to take apart and clean as well but it wasn't in quite as bad a shape as the one you just fixed. I can't really answer your questions about the voltages on the preset right now as I don't have one on the bench I can measure. It has been too long since I worked on the one I have that is working. The Beomaster 6000 Type 225x (from the 80's) is a really nice receiver. Like the Beomaster 8000 it controls the Beocord 800x and 9000 cassette decks along with the Beogram 800x series. The remote control functions are really nice. I have my Beomaster 6000 paired with a Beogram 8002, Beocord 9000 and a pair of MC120.2 speakers. I also have a pair of Yamaha NS70 acoustic suspension speakers on the second channel. The Beomaster easily drives both.

-sonavor

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sonavor replied on Fri, Oct 30 2015 12:23 AM

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One more thing...I'll bet the Beomaster 6000 sounds great driving your M75's.

-sonavor

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Christian Christensen replied on Sat, Oct 31 2015 6:32 PM

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Thanks for the complimants.
I do also have a pair of  MC120.2. they are used to give full quality of a full set Telefunken Akusta set.
A fantastic combo

Christian