I dont want to overload the KX thread so i started a fresh thread for my problem!
Finally i found some time to assemble one of the KX-boards that were lying around for some months already.
I had a really hard time because i don´t have any experience with these small smd parts. I really should have practised with some trash parts before risking the damage of expensive components.
Anyway, i managed to solder the parts on the PCB somehow.
The two biggest mistakes were :
a) Mounting LT1013 instead of LTC2057!
( dont ask me why, i had them both lying in the component-box for KX boards and totally forgot that i bought the LT1013 for some future project. And they are ( of course ) not pin compatible)
b) dropping the assembled board on the floor while i was trying hard de desolder the LT1013´s
Actually i discovered the wrong Opamp after i tried to power up the board and Zener Vref was some millivolt.
Then i replace them with the correct LTC2057 and after powerup i could measure 7.15xxxxx Volts.
That looked promising but the output was still unstable after the 4th digit and so i did some more inspections.
I changed two of the film capacitors ( only suspected visible damage ) and reflowed some components that appeared to not have proper soldering. I also found some very tiny traces of solder shorting some tracks on the PCB and removed those.
After that the output was down to 7,09xxx Volt but stable to the last digit ( 6.5 Digit meter ).
But even after one hour of operation i could not feel any warming of the LTZ1000 casing.
I then found out ( by measuring the potential of some points ) that no real current is flowing though the heater ( only 1 mV drop across pin 1 and 2 of LTZ, heater has around 300 Ohms -> 3 microampere current through heater ? ).
Here is the circuit from AN42 with my measurements :
Sorry, unable to do inline images. Pic is attached now!!!
Yes, my knowledgement is still pretty limited. I am studying a lot of books right now to catch up.
But it seems to me that the zener part of the LTZ is working -> 7.09 across zener and base-emitter of Q1 of the LTZ.
But the temperature control part seems like having problems. there is 550 mV across - and + of the opamp. So that opamp should be saturated right?
Meaning the output of the opamp is correct?
Why is the voltage drop across collector emitter of Q2 of LTZ only 41mV?
So should i suspect the Q2 of LTZ is damaged?
Any suggestions?
They are all very welcome!!!
Thank you very much
Joerg