HP E3631A repair

[Dave]

Hey there!

I recently purchased an HP E3631A power supply on eBay. It was advertised "as-is", I decided to risk it, and sure enough, the 6V output was faulty.
The +/- 25V outputs work perfectly, setting the voltage and current, reading the values back, everything perfect, so I assume there are no problems with the digital part of the supply.

At this point, please open the E3631A service manual, as I will be referring to it below.
The 6V output reads back bogus values (-50 odd volts and 45mA current when there is nothing on the output). Nothing changes when I change the voltage and current settings. I have opened it up and checked for visual faults - nothing appears to be burnt or popped. I have poked around with my multimeter and checked various voltages:
The +/- 15V supply lines (for the opamps) are perfectly fine (U25 and U24 outputs; page 127, part B).
The voltage on the smoothing capacitors C48 and C49 (page 131, part A) reads 0V. I have also tried disconnecting the transformer (J10 and J11) and checking the AC voltage, and it appears to be fine, so the bridge rectifier must be the culprit.

But that doesn't explain the bogus voltage & current readback, so I investigated further.
The voltage adjustment and monitoring for the 6V output is done with linear optocouplers (quite a clever way to isolate circuits, while still maintaining accuracy). I have checked the monitoring circuits (page 131, part B) and +6V V MON and +6V I MON appear to read the same voltage (0V obviously) on both sides (measured to their respective grounds). So, the correct voltages are fed back to the ADC circuit (page 129, part A). Because the 25V outputs read correctly, I would say the ADC itself is good. Could it be the multiplexer U4 if faulty?

I need some suggestions from people who have experience with repairing these power supplies. Does anyone know what usually causes these types of problems?
Thanks in advance!

Cheers,
Dave

[free_electron]

if you change the setting of the 6 volt channel ; does the output actually change ?

hit the button to show the setting as opposed to the real voltage and then set it to 3 volts. hit utput on. do you get 3 volts ? ( measure the output )

two possibilities : one of the 74hc4051 multiplexers on the top board (the board that does the +25 - 25 volt and has the cpu on it ) is dead. ( if the output works the mux is the culprit )
or one of the tl074's is fried on the same board ( there's two next to each other. they set vout and iout coming from the dac.

once fied , simply run calibration. all you need is a good voltmeter , a good ampere meter and a load.

scan the forumfor my posts on fixing these machines and how they work. i posted teardown pictures with detailed explanation on how the machine works , including how the dac and adc work.

[Dave]

Nothing changes on the output, because there is no voltage on the smoothing capacitors. The bridge rectifier must be dead.
Would I risk anything, if I try to feed those capacitors from an external supply, just to see what happens with the regulation opamps and check what's going on the monitor lines?
Like this:


I'm picking up replacement multiplexers this week.
I am also going to check those opamps on the top board, I'll let you know what I find out.

I have already read your posts on the E3631A. Believe it or not, they were one of the reasons I decided to pick up this exact supply.

[SeanB]

Bridge could be short circuit ( though normally only one diode goes short and the rest get really hot until the unit either cracks or goes properly short all ways, which leads to a very unhappy transformer very fast) or one of the capacitors is shorted. Disconnect the bridge rectifier and check it with a DVM for continuity, and while it is disconnected check across the capacitors as well. There could be an open circuit, either the bridge, the transformer winding or a connector in that area.

[ben_r_]

So uh... You guys think I need to send my E3631A in for calibration? Agilent said itll be $160 plus shipping Id imagine. Or should I try and calibrate it myself from the very helpful tips in this thread? I dont have a load, but I do have a brand new Agilent 5.5 digit DMM.

[free_electron]

well do a test. take a fat power resistor. connect your multimeter on ampere in series with the fat power resistor and hook it up.
does the meter ampere reading correspond with the psu ampere reading (+/- 1% ) ? if it does it;s ok.

next hook the fat power resistor up to the supply on its own. set a given voltage. verify using your brand new agilent multimeter. if it matches within 1% the supply doesnt need cal.

the calibration is only really needed if you replaced parts in the analog/digital convertor or digital to analog convertor.  the machine can do that all by itself but you need a way to draw 0.1 ampere , 0.950 ampere and 4.95- ampere. a bunch of power resistors get you there.

[ben_r_]

well do a test. take a fat power resistor. connect your multimeter on ampere in series with the fat power resistor and hook it up.
does the meter ampere reading correspond with the psu ampere reading (+/- 1% ) ? if it does it;s ok.

next hook the fat power resistor up to the supply on its own. set a given voltage. verify using your brand new agilent multimeter. if it matches within 1% the supply doesnt need cal.

the calibration is only really needed if you replaced parts in the analog/digital convertor or digital to analog convertor.  the machine can do that all by itself but you need a way to draw 0.1 ampere , 0.950 ampere and 4.95- ampere. a bunch of power resistors get you there.

Thanks man. Sounds easy enough to do, except I know I dont have any "fat power resistors" hanging around. Any suggestions on one I should buy? Maybe from Mouser or Digi-Key as I have to put in orders there soon anyway.

[staxquad]

well do a test. take a fat power resistor. connect your multimeter on ampere in series with the fat power resistor and hook it up.
does the meter ampere reading correspond with the psu ampere reading (+/- 1% ) ? if it does it;s ok.

next hook the fat power resistor up to the supply on its own. set a given voltage. verify using your brand new agilent multimeter. if it matches within 1% the supply doesnt need cal.

the calibration is only really needed if you replaced parts in the analog/digital convertor or digital to analog convertor.  the machine can do that all by itself but you need a way to draw 0.1 ampere , 0.950 ampere and 4.95- ampere. a bunch of power resistors get you there.

Thanks man. Sounds easy enough to do, except I know I dont have any "fat power resistors" hanging around. Any suggestions on one I should buy? Maybe from Mouser or Digi-Key as I have to put in orders there soon anyway.

Agilent specifies a .1 Ohm .01% shunt resistor for current calibration (measuring the voltage drop on the resistor with a calibrated DC voltmeter of 0.01% accuracy and .1mV resolution), that ain't in the category of a bunch of power resistors, more in the category of a resistance standard, so as to calibrate to the accuracy level of the power supply, not below it, otherwise, you ain't calibrating anything.
 

The accuracy of the current monitoring resistor must be 0.1% or better.

like a Cropico CR/0.1 calibration resistor

[ddavidebor]

So uh... You guys think I need to send my E3631A in for calibration? Agilent said itll be $160 plus shipping Id imagine. Or should I try and calibrate it myself from the very helpful tips in this thread? I dont have a load, but I do have a brand new Agilent 5.5 digit DMM.

If it's ok it's better if you don't touch it

[permantier]

I have a E3631A with the same problem, did you identify the problem?, are the multiplexers  faulty,?