Products > Test Equipment
Keithley 2400 SourceMeter review and teardown
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kawal:
Openloop thanks for helping :

OP AMP voltages :
Pin 2 0V = FCOM
pin 3 -2.4V Removed the diodes from the schematic  - there is no input diodes
pin 4 -1.95V  Negative supply
pin 6 -1.2V same as output transistor bases
pin 7 4.2V  Positive supply 
R562 0.1mV . 2mA OK
R563 4.1mV !!!! 8.2mA Hmmm  So where is the current going ? this is with output disabled
 BTW why is the -240V on the output when the output is disabled ? there should be a relay for the output correct ?

Meter displays ---.--- when output is disabled and -215.something V  when output enabled. on high range and -21.something V in low rage



openloop:

--- Quote ---pin 3 -2.4V
--- End quote ---
Damn, I thought it was +

It changes things. Apparently output stage does exactly what it's told to.
It's control (a.k.a. "maindrive") that, ahem, driving it down!  :-DD

OK, I need a minute to think about it.  ???
kawal:
Open loop
 Your comment about FCOM being the output is helpful

I measure from FCOM to power amp stage output and that is 0V
I measured from FCOM to output positive terminal that is -5V HMMMMM something not right i would think

 
kawal:
Ok things just got worse .

U660 AD7849 is hot and not changing the current setting.
U661 is warm but not hot.

NIDAC is at 40mv
openloop:

--- Quote ---I measured from FCOM to output positive terminal that is -5V
--- End quote ---

God knows what kind of protective junk they have there. Smells like a rabbit hole.

Much more interesting is to see why maindrive is bad.

Can you check the voltage at TP213? It's straight down from U500, closer to the middle of the board.
That's combined control point driven by +/- Voltage control and +/- Current control.
They are merged together through 4 diodes: Q209, Q211, Q213, Q215. They kinda form a rectangle with TP213 in the center.

Can you measure voltages on them too to see which one is conducting?
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