EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: wictor on November 27, 2017, 05:01:47 pm
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Hi,
Here comes yet another repair story...
I had following problem with my power supply: All the outputs were working and voltage and current could be set in limit mode. But voltage and current measurements were not working. Display was showing usually over limit values and sometimes when it was not on the limit, all values were fluctuating a lot.
By reading the forum I quickly realized that measuring the ADC part can be tough, since it's based on current and it is multiplexing couple of channels. I measured that muxes were ok. I measured all the resistors and input voltages, ADC references and those were ok. For opams I decided to build test bench for measuring their performance and see if any of those is faulty. I knew that it would be easier to just change all opams, but I didn't have those and I would need to order them and wait some time to get them. And problem could still be somewhere else. So I built voltage follower circuit to breadboard and tested OPA27(U6) and it was behaving ok.
AD711(U8) was not behaving so nicely. I had following setup for opamp: +10V positive supply, 0 negative supply and test signal to non inverting input was square wave 6V p2p with 5V offset. Following is picture with 10kHz input:
(https://www.eevblog.com/forum/repair/hp-e3631a-voltage-and-current-measurements-problem-solved/?action=dlattach;attach=374282;image)
Channel 1 is input(pin3) and channel 2 is output and inverting input.
And with 100kHz:
(https://www.eevblog.com/forum/repair/hp-e3631a-voltage-and-current-measurements-problem-solved/?action=dlattach;attach=374284;image)
I managed to find TL071 for testing and after replacing AD711 with TL071, power supply display is working normally again. Now I just need to order new AD711 to fix it properly.