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Old 4th June 2008, 05:08 PM   #1
Mywing is offline Mywing  Italy
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Default 400A, emitter resistors and bias: I would learn

Well guys,
I was not able to find a technician who wants to get his hands on my 400A. So if any of you is willing to help me through this forum I can try to make myself small operations necessary.
As Nelson suggested me, I checked the bias through the temperature and everything seems ok. But I would check also with a millivolt meter.
Nelson says: "You must monitor the current
draw of the channel, best done by reading
the voltage across one of the emitter .33 ohm
resistors on the output stage. Since it takes
time for the ampto warm up and the bias is
somewhat temperature dependent , You want
to adjust the value of the bias very slowly,
and then wait while monitoring the current draw.
If the channel is running cold, then you increase
the bias above the existing value by a little bit.
Then you wait.. "
In the second attachment to this R-K Rønningstad's thread http://www.diyaudio.com/forums/showt...postid=1528915 # post1528915 has shown:
"The output transistor emitter
resistors are exposed along
each of the heatsink output channels ..." .
But with my inexperience I are not exactly sure have identified the resistors. Do I believe those suitable with a small red arrow in the attachment, ... or not? But why in a channel it misses one of them (red circle)? (See next attachment)

Thanks,
Nino
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Old 4th June 2008, 07:27 PM   #2
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They are the Big resistors, all in rows right next to each power
transistor.
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Old 4th June 2008, 07:51 PM   #3
Mywing is offline Mywing  Italy
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Sorry,
I've forgotten the pic.

Then I was wrong...
The right ones would be the light blue near the small arrowed... that's right?

Talking talk about electrolytics as you can see in the attachments there are in each Pcb an orange 470uF/16V cap and two blue ones of which I do not see the value. They should be 4.7uF/50V, someone can confirm?
(Unfortunately, I have three 400A schematics and all different among them)
Thanks
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Old 4th June 2008, 08:07 PM   #4
Mywing is offline Mywing  Italy
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Now the last question by now:
In this second pics you can see a blue axial cap (I think electrolityc, located in low in front of the back panel) of which, watching the schematic, I have not succeeded to discover the value. Have you any idea?
Other suggestions are greatly welcome.
Thanks so much again.

Sorry if I said nonsenses or annoyed you, but I would learn...
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Old 4th June 2008, 09:32 PM   #5
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Quote:
Originally posted by Mywing
Talking talk about electrolytics as you can see in the attachments there are in each Pcb an orange 470uF/16V cap and two blue ones of which I do not see the value. They should be 4.7uF/50V, someone can confirm?
Just get some 47 to 220 uF at 50V, and don't worry about it


Quote:
Originally posted by Mywing
In this second pics you can see a blue axial cap (I think electrolityc, located in low in front of the back panel) of which, watching the schematic, I have not succeeded to discover the value. Have you any idea?
That capacitor is power supply for the display I believe. Try
a 1000 uF at 35V for that.
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Old 5th June 2008, 01:02 PM   #6
Mywing is offline Mywing  Italy
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Thanks for the help Nelson.
Quote:
Originally posted by Nelson Pass


That capacitor is power supply for the display I believe. Try
a 1000 uF at 35V for that.

Then since this cap is only display supply's, could I avoid to replace it or you recommend me to do however?

About the emitter resistors I understood that they are the light blue in row, every in correspondence of a transistor. But in the center of the row I see a small brown resistor. The same on the other channel. Do you have an explanation for this? Or they broken resistors replaced by the precedent owner?
If it is so, could the fact that is different from the other ones influence the stuff's work?

Please guys, don't leave alone Nelson to help me...
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Old 5th June 2008, 04:19 PM   #7
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Quote:
Originally posted by Mywing
Then since this cap is only display supply's, could I avoid to replace it or you recommend me to do however?

About the emitter resistors I understood that they are the light blue in row, every in correspondence of a transistor. But in the center of the row I see a small brown resistor. The same on the other channel. Do you have an explanation for this? Or they broken resistors replaced by the precedent owner?
If it is so, could the fact that is different from the other ones influence the stuff's work?
If the capacitor is working, then you can replace or not as you like.
It should not affect the sound.

The middle resistor is the bias resistor for the drivers. You can
ignore it.
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