47 volts on output resistors?

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I thought it might ;) Low or even no bias doesn't remove the audio, it just increases the distortion.



Ah, now then grasshopper. Remember this :D



At least its getting there. If you still have a genuine low bias issue on this channel then its probably because of the differing characteristics of old vs new semiconductors... you know what to do :)

I havent tried to bias the amp yet,Im too busy listening to some quality music again:D If I hear one more peep out of that balance pot Ill desolder it off its board and pull it apart to clean it properly. Ive ordered another 2SA747 just in case the MJs dont want to bias plus I want to know if theres any sonics difference? So far I cant tell the difference? The MJs sound great and Im only using my Pioneer test speakers!
 
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I tried the original transistors on the left side and I still couldnt bias the left side? I just tried again with the MJs installed and I still cant bias the left channel. The right good side has 1.76v on the bias trimmer wiper and 380 ohms across the trimmer to get 50ma. The left bias wiper has 1.3v and 380ohms across the trimmer to get 10ma? But the amp sounds fine? Why do all the weird things happen to me? :hypno2::confused::D
 
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OK :) You enjoy the amp for a while, lol, you've earned it.

Yeah, thanks for helping me out with this Mooly its appreciated. Ive learned a lot about this driver board and some circuit theory from you that I can apply to other amps as well. It seems strange that the right side is 50ma and the left is 10ma yet the output trannys were nearly the same temperature after an hour of playing music.The right side was warm,the left just a little bit less.
 
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The voltages on and around the wiper of the preset wont be any guide to actual bias current flowing. The determining factor is whether there is enough voltage available between the bases of the output npn/pnp pairs to turn them on. And that figure is a (slightly)variable quantity depending on the devices used.
 
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The voltages on and around the wiper of the preset wont be any guide to actual bias current flowing. The determining factor is whether there is enough voltage available between the bases of the output npn/pnp pairs to turn them on. And that figure is a (slightly)variable quantity depending on the devices used.

Ah,So it might be a lack of current from the main caps board? Ill check it out for a dodgy connection,etc..
 
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Caps measured in-circuit or out? What was the remnant DC voltage charge across them and what are you using for a capacitance meter? I know that one of my multimeters and a handheld LC meter are claimed to read to >20,000 uF but the accuracy is all over the place and readings aren't even repeatable. Let's just say I don't use them beyond 1,000uF.
 
Ah,So it might be a lack of current from the main caps board? Ill check it out for a dodgy connection,etc..

No, he is amplifying a point I made about setting the standing current in post 133. The level you have of 10 mA is not enough to guarantee freedom from cross-over distortion and you need to set this to 50 mA as set out in Section 5 of the Service Manual - the one I downloaded is for AU9900A/11000A by the way.

If there was an issue of stability there would have been difficulty with setting the level to 10 mA. This level is apparently constant and the outputs are cooler than those in the right channel - I see no issues with increasing the standing current to 50 mA. The trimmer wiper is more advanced in travel (and resistance) than in the right channel - whether or not there is sufficient travel left to effect the increase you will know by testing. If there is a problem in the last regard that can be looked at later.
 
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Caps measured in-circuit or out? What was the remnant DC voltage charge across them and what are you using for a capacitance meter? I know that one of my multimeters and a handheld LC meter are claimed to read to >20,000 uF but the accuracy is all over the place and readings aren't even repeatable. Let's just say I don't use them beyond 1,000uF.

The charge is 41v across the caps using a DBT,I measured them in circuit with a Digitech QM1572 capacitor meter. I just measured them in circuit with a Fluke 87V DMM the 3600uf read 430uf and all the rest read a bit higher as well.
 
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I can adjust the bias on the left channel now after I tightened up the screws connecting the caps to those brass bars so the amps working great now,even with the uf being down on the main caps? Ill connect the amp up to my Timewindows and have a good listen before I buy some more main caps.What an epic this has been but its not over till the fat lady sings so Im going to listen to Mama Cass when Im all hooked up to the good gear.Yay!:D P.S. Thanks heaps you blokes,its always best Ive found when I can bounce ideas off of others when the going gets tough. Ive learned heaps from this little problem.Ha! Thanks again fellas,I appreciate it.:drink:
 
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Sounds like you are having fun with it (at last :D)

I wouldn't to much over those large value caps if your only test is measuring values in circuit. An easy test... just tag another 1000uf (whatever you have) across the 'suspect' cap and see if the measured value now increases by that amount.
 
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Sounds like you are having fun with it (at last :D)

I wouldn't to much over those large value caps if your only test is measuring values in circuit. An easy test... just tag another 1000uf (whatever you have) across the 'suspect' cap and see if the measured value now increases by that amount.
Ive got the amp running on one set of speakers and it sounds great! Ill do that test soon, Im still marveling at how good this thing sounds at the moment.Ill have to change those molex connectors from the driver board to the output module,the right channel suddenly stopped working 10 minutes after I set it up properly and as soon as I had it back in the workshop it was OK? So Ive set it back up again after checking it all out and its been good since.:D P.S. I forgot to tell you that I got the right channel going when I found a broken pin on one of the 100uf,80volt electros,I replaced it with a NOS Elna.
 
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I measured that big Superthrough electro cap out of circuit it reads 4700uf instead of 10,000uf. The amp sounds great though? I cant pick anything wrong even though Ive been listening really close for a while now.Ive got a dodgy right RCA input plug on the Tuner input but other than that its good.I dont think Ill be re-installing the original outputs 2SA747 & 2SC1116 because the MJ21193/94s sound just as good.The left channel wasnt biasing because the negative terminal on a big filter cap had a stripped thread on one of the screws when I over tightened it. That was what was making the voltage change all the time and why I couldnt bias it past a certain milli-amps.
 
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I'm pleased to hear its still working OK :)

I wouldn't condemn the big cap based just on a simple reading. Large caps that have been run at high voltage soak a lot of charge into the internal dielectric and that means that hours or days or weeks after they were last used, the terminal voltage is far from zero. Discharge the cap and you will see the voltage creep back up again. That can throw the reading depending on the specific meter and measurement method used.
 
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I'm pleased to hear its still working OK :)

I wouldn't condemn the big cap based just on a simple reading. Large caps that have been run at high voltage soak a lot of charge into the internal dielectric and that means that hours or days or weeks after they were last used, the terminal voltage is far from zero. Discharge the cap and you will see the voltage creep back up again. That can throw the reading depending on the specific meter and measurement method used.

Yes but the other three caps read from memory,7600uf, 7800uf and 8200uf? When they were new they all read about 9600uf/9800uf. Its weird that it sounds better now than it ever did before.So I dont care what the readings are now, as long as it sounds good thats the main thing! :D
 
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