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Issue with Luxman M-4000 Idle / Bias

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ficklecycler

Active Member
I've been working on this amazing Luxman M-4000 power amp project for a while now. I got the amp fully intending to rebuild it and made it pretty far. I have done some recapping before and usually it's a paint by numbers affair. Once I hit this issue, I've been stumped. Little by little I'm getting there but warning, I don't have a developed sense of electronics diagnostics so if you are able to help me please speak slowly :) ! Thanks for any insight you can offer.

The issue- As I try to adjust the bias, I can get it within range of 100mA and it stays there for 5 seconds, then little by little it creeps up, creeps up creeps up, I adjust it down, it creeps up until I completely run out of travel on my (new) adjustable multiturn rheostat. It never really settles down. I eventually just turn the amp off and scratch my head! When I did the recap, I made sure to get a rheostat of the same value and the caps are all the same value as original. In a few locations I went up in voltage on caps when there was poor availability of the original voltage.

Main suspect- Like I said, I'm stumped but on board PB-1019 at C411 and C413 I found 2 polar electrolytic of a low value(1uF 160v at ) that I replaced with a film cap of the same value. This is the only instance where I switched from a polar to a non polar as well as a different material.

I didn't do this anywhere else. Would the behavior I described be indicative of going to a non polar film cap, or should I be looking elsewhere? I ask because getting to this cap means taking a lot of the amp apart since it's unfortunately one of the only boards that doesn't just slide out.
 
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DC offset is going to be a DC voltage measured across the speaker terminals. Idle current (bias) is going to be DC mA measured across the empty fuse holder. The manual doesn't get too in depth about setting the idle current. It will drift up until everything reaches equilibrium, just monitor the current and keep adjusting until it settles at 100mA. That's the first way, the other way would be to set it a 100mA when cold and see where it settles without adjusting it. The first way is the safest although may NOT be the lowest distortion. Since you are running out of adjustment maybe the second way is the correct method. Monitor the current to see where ends up, heatsinks should not be too warm.

Craig
 
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It was working but wasn't adjusted to spec when I got it. With a lot of fiddling I got it within range but the original variable resistors were very problematic so I can't say it was acting how it was supposed to either.
 
can of worms then apart from it did adjust just about ..
go over your work very carefully .
not that you need reminding right now but repair before wholesale replacement is always best ..even the best people get it wrong sometimes ..and the best doesn't include myself either .
 
Thanks for the advice, I'll look back over what I did.

So the capacitor that I mentioned going from a polar to a non polar has no effect on what I'm describing?
 
c411 100uf 100v
c413 47uf 50v ..

these are in the pb 1047 board .. this manual i have is wrong so cant see where those caps go .
i wouldn't think they are a problem anyway .
more likely bias diode or bias transistor or a poor solder joint .
 
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