HP 6632B power supply / help needed - output voltage shown higher that measured - Page 1
Forum section: electronics / testgear

author: urosg
author_rank: Unknown
timestamp: March 07, 2014, 08:23:39 pm
content: Hi,I just scored one of these units on ebay for a resonable price.The first thing I tried to do was of course some basic load testing with a 100W resistors.The problem is that for example when I use the 4R7 resistor and set output voltage to 4.7V the voltage on the unit shows 4.7V but is measured 3.7V output current is shown and measured at 800mA which corresponds to the output voltage and hooked up resistor.If I limit the output current to 1A and increase output voltage the current is again read correctly, the voltage measured on an external DMM is measured at 4.7V but on the unit is shown a little above 6V.Does anyone have any clue where to begin investigating this problem or perhaps if someone already had a similar problem and advice will be appreciated.Re, Uros

author: VK5RC
author_rank: Unknown
timestamp: March 08, 2014, 12:49:23 am
content: Just a silly thought, the sense voltage internal / external button (i think it has one on the back) is it in the correct position?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 08, 2014, 01:27:43 am
content: Assuming you have the sense wires correctly wired, it sounds as if the voltage needs calibrating. Head over to the Agilent site and get the service manual - it gives the procedure for calibrating the unit. Basically, you ask the unit to perform a voltage calibration, it outputs two voltages which you read with your DMM and feed those values back to the unit.

author: urosg
author_rank: Unknown
timestamp: March 08, 2014, 05:43:10 pm
content: The unit does not have a internal/external sensing switch just the external sensing screws, which should be bridged with a factory delivered bridge but since it's a used unit it doesn't have it, but I bridged it with a cable.I did the "Voltage programming and measurement calibration" where the shown values on the unit and the DMM matched but did enter them according to the manual in the unit, but didn't help.The problem persists, when the unit is under load the output voltage is lower than set, but the current limiter works.So if I set output voltage to 4.7V and current to 1A and hook up the 4R7 resistor the voltage on the unit shows 4.7V (measured 3.7V) and the current shows 800mA, when I increase the voltage it goes up to 6V (on the unit but measured 4.7V) and the current limit sets on at 1A.According to the service manual to be able to complete all of the calibration I would need 0.1Ohm 15A current shunt?Is it possible that there would be a hardware problem in the unit?

author: sync
author_rank: Frequent Contributor
timestamp: March 08, 2014, 05:45:52 pm
content: Did you measure directly at terminals of the power supply?

author: AndyC_772
author_rank: Super Contributor
timestamp: March 08, 2014, 08:18:04 pm
content: It sounds like a large error, far more than I'd expect from the unit just having drifted out of calibration naturally. I hope you've not now set it out of calibration by trying to adjust it.Make sure you measure voltage at the sense terminals at the back of the unit, as they're the ones at which the voltage is controlled.There are schematics available in the service manual, so you do at least have a decent chance of repairing it if necessary. I had an op-amp fail in one of my 6632B's, and it didn't take too long to locate the fault and replace it. (No idea why it went bang in the first place, though, but the circuit seemed perfectly OK after the repair).