Fixed: Keysight (Agilent) E3640A linear PSU repair error 609 (ADC error)

[coy]

Hi,

I got a Keysight E3640A linear PSU for repair. It boots up successfully and all keys menus etc. are working. But output is disabled and it states it detected error 609.

Apparently it is a newer build, not the early Agilent labeled version from 2000s.  My PSU has a serial number of MY56xxxxx and the components are dated 2014 / 2015.

I was not able to find any schematics for this version. The only schematics I found are from the above mentioned 2000s Agilent version, but it looks like a lot has changed since then.

Does anybody happen to have a schematic for this newer version?

From the newer Keysight manuals which cover the later versions with serial MY53xxxxx (but not MY56xxxxx unfortunately), it is mentioned that error code 609 means that the ADC reported some errors back to the CPU. So my hope is I can track down the issue beginning from the ADC. Somebody already tried to change it apparently, there is an AD7192 with datecode 2241 soldered in quite badly - hopefully it is the correct part at all.

For now my plan is:

- Clean up & resolder the ADC
- Check supplies to the ADC
- Capture communication between the ADC and CPU

[openloop]

ADC error most likely is a red herring.

Check power rails on voltage/current op-amps. Common failure point.

[coy]

ADC error most likely is a red herring.

Check power rails on voltage/current op-amps. Common failure point.

Thanks for pointing that out.

I did my test steps and checked communication between the µC and the ADC, and indeed it turned out the ADC has the error bit set.

Apparently the analog input AIN1 is too high (5.2V - which is above the specs).  Have to track down now where this is connected to.

[coy]

I managed to repair this PSU.

The root cause was that the TL074 (U127 in the old schematics, U7 on my board), which is connected to the sense inputs, died for any reason, generating full negative output on pin 8 (-14V), causing erroneous input to the next stage(s) and finally to the ADC (AD7192), causing the ADC error.

After replacing the TL074 and recalibration everything looks fine now.

Thanks for your help, glad I've saved this from being scrapped.