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Old 8th June 2006, 05:53 PM   #1
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Default Differential Dilemma

Hi,

What is going on here?
It is an input stage to the c200.1 amplifier from ampslab http://www.ampslab.com/c200qc.htm

I have spent most of the day looking for PCB mistakes and trying to work out what is going on here and have found no solution yet.

about 27mA in the RHS collector which i think is too much
The CCS seems to work fine on the emitter.
Somthing wierd is going on in the right hand side differential pair transistor.
The lack of current in the left hand side is causing a large DC offset at the power stage output which is appearing at the 10K resistor (-58.3v).

Any ideas as to what might have happened?

Here are my measurements.
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Old 8th June 2006, 06:05 PM   #2
ilimzn is offline ilimzn  Croatia
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At the very least the RHS transistor in the LTP seems to be dead (C-B junction) and the LHS transistor's E-B junction seems to be working as a zener diode.
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Old 8th June 2006, 06:13 PM   #3
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Thanks for reply,
Ill try and replace them with fresh transistors, see if it helps.

Cheers
Craig
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Old 8th June 2006, 06:17 PM   #4
poobah is offline poobah  United States
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I agree... right transistor is
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Old 8th June 2006, 06:59 PM   #5
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I am such and idiot, its always the simple things you forget. I was about to change both transistors when i noticed the RHS one was in wrong way round......

Anyway i replaced it with a new one the correct way round and now all measurements are as described on the ampslab site

I still get a full supply offset at the output though but now its +ve supply not negative supply
I guess ill be able to sort it out from here, thanks ppl.

Regards
Craig
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Old 8th June 2006, 07:04 PM   #6
poobah is offline poobah  United States
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Actually... you had the transistor right... the board was backwards... and UOY were smart enough to find it!

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Old 8th June 2006, 07:48 PM   #7
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Dammit i just wrecked the whole thing, was retesting and shorted somthing with the probe that took out half my driver transistors, and half of my power transistors which now cost a lot to replace (Mj21194's).

It was going so well, I am so.. so.. so pissed off!


Craig
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Old 8th June 2006, 08:35 PM   #8
poobah is offline poobah  United States
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