Electronics > Repair

Agilent E3631A readback issue

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dave the sound guy:
I'm finally having a go at a sick E3631A power supply that I've had on a shelf for a while.  It passes self-test and all three outputs function normally when measured externally (and correctly respond when adjusting the encoder wheel), but the voltage and current readback is very wonky for all three channels. 

I'm proficient at repairing audio equipment, but sophisticated test gear like this power supply is a little out of my comfort zone.  Needless to say I'm up for the challenge and would gratefully welcome any guidance. 

I have the service manual and have been studying it.  The first thing I did was to check bias voltages as described on page 98...everything was in order.  I also checked resistors in the ADC circuit and found none were open.

dave the sound guy:
I wanted to eliminate calibration error as a possibility, so I attempted a re-cal and it mostly failed.  Oddly it did accept calibration on the 6V voltage but yielded no improvement to the readback.

I'm having a little trouble following the schematics and I'm not sure where to start looking for the fault.  I have a second 3631A that I can use for reference however.

Here's a short video that better illustrates the unit's behavior:

SoundTech-LG:
I could be wrong, but that sure looks like a rotary encoder issue. When you turn it, it does not seem to raise or lower the voltage correctly, and jumping around. Common issue with these too! I see the DVM looking correct though, so maybet not...

dave the sound guy:
The encoder definitely works as it should.

I might be on to something.
I opened up my good supply and started to compare measurements with the faulty one.  With both set to the 6V output at 6V I had a look at the FLASH output in the ADC circuit at R29.  The top image is the good supply, the bottom is the defective one.  The signal was also unstable on the bad unit.  Looks like I should start working backwards...

dave the sound guy:
Output of U5.  Top is from the healthy unit.  Note the amplitude difference.

I measured both sides of R4 between U3 and U5.  U3 side is a steady -10 on both, U5 side is -29mV and steady in the reference unit, and -200mV and slightly fluctuating on the bad one.

Bad U5?

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