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Old 18th December 2009, 03:19 PM   #1
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Default Bias Cap??? Size???

Studying the Schematics for a Denon AVR-887/2307 surround sound receiver, I noticed something i have not seen before. a large Electrolytic capacitor across the BIAS circuit. If you look at the attached schematic. notice C102FL a 220uf 35V capacitor. I would guess it is there to stabilize the bias circuit??

Now what i found interesting is that in this receiver, there are 7 amp channels. the main Left/Right channels have 220uf where as the other channels only have a 10uf cap in this location. So im curious as to why the difference? what effect would this have? While i am in there repairing a channel should i change all of them to match? How cirtical is the quality of this cap in this location?? etc.



ZC
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Old 18th December 2009, 03:23 PM   #2
h_a is offline h_a  Europe
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That cap filters the VBE-multiplier, means it slows it down. Fast changes in temperature detected by the VBE-multiplier are absorbed by this cap; it gives a more gentle temperature control.

I don't think you need to match the caps, and the quality does not matter particularly as these act on signal that is thrown away, so to say.

From size I've seen everything from none at all, to hundreds of uF.

Have fun, Hannes
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Old 18th December 2009, 03:51 PM   #3
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AHHHH Ok that makes sense now. and the main channels would be expected to have larger speakers and be run harder than the other channels so they put a larger cap there. ok.


Thanks


Zc
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Old 18th December 2009, 08:24 PM   #4
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I have seen anything from 100nF to 220uF in that position.

I would have thought it passes any AC quickly around the bias circuit.
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Old 19th December 2009, 01:15 AM   #5
tomat is offline tomat  Indonesia
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is it will be any difference if i using bipolar caps for that?,or it must be use polar capacitor for that application?
thanks
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Old 19th December 2009, 10:03 AM   #6
h_a is offline h_a  Europe
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You don't need a bipolar cap in this position.

Quote:
I would have thought it passes any AC quickly around the bias circuit.
Sure, it also acts on the audible signal if large enough. But I honestly don't have much clue why one would want to bypass the VBE-multiplier for AC (except for bias stability).

Have fun, Hannes
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Old 19th December 2009, 12:00 PM   #7
AndrewT is offline AndrewT  Scotland
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it might be worth experimenting with the bootstrap capacitor (47uF), since it may be a little on the low side.
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Old 19th December 2009, 12:33 PM   #8
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Quote:
Originally Posted by AndrewT View Post
it might be worth experimenting with the bootstrap capacitor (47uF), since it may be a little on the low side.
Suggestions for upgrading?? what size?

These Receivers seem to be prone to have problems with the amplifier stages. I see a lot of them broken with the same problems. I have 3 of them in shop right now all with one channel blown. and when they blow...they really blow. both output devices, bias transistor, emitter resistors, base resistors, bias pot, diodes and zeners, and the bias cap! pretty much takes out everything LOL! I plan on keeping one of them to run a small surround sound system so I am looking for stability more so than the audiophile type upgrading. this has a very small transformer and heatsink for 7x 100watt channels so im limited to begin with anyway.

I wish i understood the failure mode. one of the receivers the owner which i believe said that the unit had been hooked up and running for over a year just fine. then one day he turned it on, heard a pop and it went into protect and that was it. So I am wondering if it is a problem with defective parts? or undersized caps, or some common failure part.



Zc
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Old 19th December 2009, 12:35 PM   #9
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Oh yeah, Output devices are 2SC2590 and its counterpart. I think 2SB1647 if i remember the number the number correctly???
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