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Marantz 140 - BIAS not stable

L

labatt242

Active Member
Hello !

My Marantz 140 seems having issue regarding the BIAS adjustement on the left channel. It lose his adjustement every 2 or 3 months (goes to +50mv). Yesterday, I was worried of the temperature because the left side was hotter than the right side. After adjustement, the left side BIAS goes between 0mv and 15mv and the right side stay at 10-10.5mv.

Any ideas?

Thanks !

Guy
 
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labatt242 said:
Hello !

My Marantz 140 seems having issue regarding the BIAS adjustement on the left channel. It lose his adjustement every 2 or 3 months (goes to +50mv). Yesterday, I was worried of the temperature because the left side was hotter than the right side. After adjustement, the left side BIAS goes between 0mv and 15mv and the right side stay at 10-10.5mv.

Any ideas?

Thanks !

Guy
Click to expand...
I would start with replacing those single turn pots with multi-turn units. Even if the problem is deeper replacing those are a good idea anyway.
 
creeve said:
I would start with replacing those single turn pots with multi-turn units. Even if the problem is deeper replacing those are a good idea anyway.
Click to expand...
I think I found something.

From a 2275 post : https://ca.mouser.com/ProductDetail/Bourns/3386H-1-202LF/?qs=sGAEpiMZZMvygUB3GLcD7rufck0TIBKy76Ho9t96FVo=

I did a little search on the multiturn and I found this : https://ca.mouser.com/ProductDetail/Bourns/3296P-1-202LF/?qs=sGAEpiMZZMvygUB3GLcD7sSg8po3Zpo34tTvns3IJZ8=

Any thoughts on the 3296P-1-202LF?

EDIT : Just found this one : https://ca.mouser.com/ProductDetail...=sGAEpiMZZMvygUB3GLcD7v/2K2JTtKgb0JoFJX7tGLc=

It seems a copy/paste from the 3386H-1-202LF but multiturn
 
labatt242 said:
I think I found something.

From a 2275 post : https://ca.mouser.com/ProductDetail/Bourns/3386H-1-202LF/?qs=sGAEpiMZZMvygUB3GLcD7rufck0TIBKy76Ho9t96FVo=

I did a little search on the multiturn and I found this : https://ca.mouser.com/ProductDetail/Bourns/3296P-1-202LF/?qs=sGAEpiMZZMvygUB3GLcD7sSg8po3Zpo34tTvns3IJZ8=

Any thoughts on the 3296P-1-202LF?

EDIT : Just found this one : https://ca.mouser.com/ProductDetail/Bourns/3299X-1-202LF/?qs=sGAEpiMZZMvygUB3GLcD7v/2K2JTtKgb0JoFJX7tGLc=

It seems a copy/paste from the 3386H-1-202LF but multiturn
Click to expand...
Go with the 3296P-1-202LF. They both will work but this one will drop right in and have the adjustment screw properly oriented.
 
My experience is pretty limited, but I have had the problem of wobbly bias a few times and two suggestions have worked at various times. A flaky diff pair can cause this and a flaky bias transistor can cause it. If your trimmer fix doesn't work, might want to consider these two areas.
 
Make sure you measure the resistance on the old so you can preset the new one roughly to the same postition before you power it up. And use a dim bulb just in case.
 
Steven Tate said:
My experience is pretty limited, but I have had the problem of wobbly bias a few times and two suggestions have worked at various times. A flaky diff pair can cause this and a flaky bias transistor can cause it. If your trimmer fix doesn't work, might want to consider these two areas.
Click to expand...
Steven is spot on with this. In fact it may be a good idea to do all of it while you have the amp board apart, it's not fun having to go back in there a second time. Those transistors and the pots should cover almost all of the bases.
 
Steven Tate said:
My experience is pretty limited, but I have had the problem of wobbly bias a few times and two suggestions have worked at various times. A flaky diff pair can cause this and a flaky bias transistor can cause it. If your trimmer fix doesn't work, might want to consider these two areas.
Click to expand...
I thought it could be the BIAS transistor but didn't know about the trim. BIAS transistor is H709?
 
I succesfully changed the trim resistor, the BIAS transistor and both drivers transistors for TIP29C and TIP30C (was planned on my project).All turn on correctly except I cannot set the BIAS at 10mv. At full CW it goes at 0mv and at 3/4 it goes at 1.9mv maximum..... Something is odd here... any ideas?

EDIT : I installed the old drivers transistor and now I have a stable 10mv BIAS!!! It seems the drivers transistors TIP29C and TIP30C are not the best/right replacement for Marantz 140. What is the best replacement for these transistors? I have plans to do a clean job of my Marantz 140 !
 
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I have used KSA940, KSC2073 with success. Be sure to get them from a reputable supplier.
The Vbe of the 'bias' transistor can have a big impact on the idle current, generally speaking, higher Vbe means higher idle current.
I made these Vbe measurements a while ago using an Atlas Peak DCA55:
2SD669=0.68V
MJE243G=0.70V
2SD612=0.70V
2SC495=0.72V
KSC3503=0.76V

Tom
 
catrafter said:
I have used KSA940, KSC2073 with success. Be sure to get them from a reputable supplier.
The Vbe of the 'bias' transistor can have a big impact on the idle current, generally speaking, higher Vbe means higher idle current.
I made these Vbe measurements a while ago using an Atlas Peak DCA55:
2SD669=0.68V
MJE243G=0.70V
2SD612=0.70V
2SC495=0.72V
KSC3503=0.76V

Tom
Click to expand...
Hi Tom,

Thank you for these informations. I think I will go with a Atlas Peak DCA55 that seems a very usefull tool.

My Marantz 140 played during at least 5 hours yesterday and all seems fine for now !
 
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