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stevey88
Samba Member
Joined: January 16, 2008
Posts: 1317
Location: Fremont, SF Bay Area
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 Posted: Wed Apr 18, 2012 8:24 pm Post subject: AFM Pig Tail Schematic |
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crazyvwvanman is kind enough to send me his spare pig tail for tracing the circuit used. Here is the schematic ( corrected ):
Basically, it is an integrator circuit that filter out the noise generated by the wiper of the AFM potentiometer. When the wiper has a bad contact and got no output, the output voltage of the op-amp will not drop suddenly.
You can get a better picture if you view the schematic in the Forum Gallery.
_________________
Steve
87 Westfalia full camper 4 speed
Last edited by stevey88 on Thu Apr 19, 2012 7:53 am; edited 2 times in total |
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jmranger
Samba Member
Joined: January 14, 2010
Posts: 701
Location: Quebec
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| Posted: Wed Apr 18, 2012 9:28 pm Post subject: |
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The collective wisdom/expertise/kindness of Samba members never stops to amaze me. Thanks Steve & Mark.
Are questions/comments allowed? 
I actually tried to reverse-engineer the circuit from the available pictures and the chip datasheet (some links in the original discussion here) but I stopped at what you can see here.
The main difference is that I have pin 2 & 3 reversed compared to your drawing - I have the wiper signal coming into the (+) side.
Obviously, I didn't have the R/C values. Are the caps in uF?
Also, was yours drawn from an 025 906 302 or the revised 025 906 302A?
Last question, you confirmed that the bias input (pin 5) is floating. Correct ?
Although I could draw it fairly well, I didn't understand it much. Haven't played with OpAmps in a long time, and the output picked between two resistors is a design pattern that doesn't ring a bell
Anyway, thanks again.
JMR |
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stevey88
Samba Member
Joined: January 16, 2008
Posts: 1317
Location: Fremont, SF Bay Area
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| Posted: Wed Apr 18, 2012 9:55 pm Post subject: |
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imranger,
I screwed up. The op-amp symbol used in the orcad schematic capture software has the non-inverted input on top and I should have noticed that. The schematic is now corrected. Thanks.
The part I used to trace the circuit is 025 906 302 not A.
The bias pin is NC.
All caps are in uF.
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Steve
87 Westfalia full camper 4 speed |
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Wildthings
Samba Member
Joined: March 13, 2005
Posts: 50337
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| Posted: Thu Apr 19, 2012 11:08 am Post subject: |
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| Never really figured out what the harness was supposed to do. I bought one when my Multi was fairly new as recommended by a dealership mechanic and it did nothing to fix my symptoms. The only thing it has ever done for me was make a convenient place for me to install a capacitor across two of the pins. In a few tries I was able to hone in on the 22uf size capacitor as that which worked the best just as others did. Hard to understand how VW engineers failed to come up with a fix that would work as well as a single cheap capacitor. |
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