Help re HP 6644 PSU (solved eventually) - Page 1
Forum section: electronics / testgear

author: VK5RC
author_rank: Unknown
timestamp: March 01, 2014, 01:49:24 am
content: Can someone please check my 'thinking' re a 'slightly funny' old but new to me HP 6644A PSUAt switch on, it displays -0.3V, this is measureable at the output terminals even when output is "disabled"!If the "protection reset" button is pushed when the output is enabled it then appears to work normally, correct voltages and currents are displayed as are being delivered to the output terminals. If a decent load eg 10ohms is present at the output, this voltage both at the terminals and displayed either vanishes or is greatly reduced (eg to 1mV or so) and it doesn't go into protect mode as it does most other times unless there is a load.The circuits (x2) for the major output controlling transistors is attached below.From my understanding the voltage is controlled by 7 identical NPN power transistors (XQ301, XQ303, XQ305, XQ307, XQ201, XQ203 and XQ207), the part list doesn't include a 'real' part number but looking at the photo on Agilents website, the replacement part is a MJ 15003G, a NPN 140V 20A power transistor, which seems appropriate. There is also another identical power transistor (XQ205) which I think is used to pull the output low (on diagram DP = down programming), presumably active on "protect' "output disable' or reducing the output voltage.I think someone in the past in my unit has replaced some of the power transistors, in my unit the 7 transistors have been replaced with power FETs MTM13N50E, but XQ205 appears to be an original HP part (presumably a silicon power NPN similar to MJ15003).Do the symptoms fit the potential voltage drop that could be induced by the difference between the FET type NPNs (the 'incorrect replacements) and the more conventional NPN (pull down XQ205)?I ask before going through the task of replacing all 7 of the power NPN.Any comments appreciated.

author: Rigby
author_rank: Super Contributor
timestamp: March 01, 2014, 03:54:11 am
content: I wish I had an answer for you. My much older HP supplies can get a negative voltage (or rather, inverted voltage) when the voltage is > 0 while the current = 0, but your supply is a much newer design.

author: megajocke
author_rank: Regular Contributor
timestamp: March 03, 2014, 08:51:02 pm
content: The schematic looks like it was designed to support both BJT and MOSFET output transistors. Most likely, the base driver MOSFETs would be installed for standard BJT outputs or the 0 ohm resistors would for MOSFET output transistors.Which ones are installed in your unit?

author: VK5RC
author_rank: Unknown
timestamp: March 03, 2014, 11:22:50 pm
content: The load resistors are under the heatsink, so it will take me a while to get to them, thanks for the comments, when I was looking at the underside of the main board after my first post, the soldering looks too good to be a repair job on the fets. So I think you may be right re the circuit being fet capable.

author: megajocke
author_rank: Regular Contributor
timestamp: March 05, 2014, 10:53:34 pm
content: Load resistors? I was thinking of R230 and its friends. But in any case, what you have measured is unlikely to be caused by this part of the supply or to even be out of spec. What is the short circuit current when the output is disabled?

author: VK5RC
author_rank: Unknown
timestamp: March 06, 2014, 05:50:43 am
content: Bizarrely, when the output is 'disabled' from the front panel, the voltage display reads - 0.3 v and this is measureable at the output terminals, it appears to be coming from a high impedance source as a load of a few ohms will reduce this issue to a few millivolts, if you enable the output it then goes into ptotect mode and the - 0.3v is still present!

author: megajocke
author_rank: Regular Contributor
timestamp: March 06, 2014, 07:21:20 am
content: Such behavior seems to be quite common for supplies that try to become high impedance when disabled rather than a low-impedance 0 V source. I don't know whether it's normal or not for this model though, but is it really causing any problem?

author: VK5RC
author_rank: Unknown
timestamp: March 06, 2014, 08:57:24 am
content: Not really, I think, I suppose I am a little concerned that reverse voltage may damage some circuit later on but I think It's better to leave it as is, if I start replacing lots of major components I think I might generate a big problem from a small one, I appreciate your time to help, thanks

author: megajocke
author_rank: Regular Contributor
timestamp: March 07, 2014, 06:53:41 pm
content: The spec sheet I can find suggests the programming accuracy is +-4.1 mA, so your measured off-state current of about -100 µA (1 mV over 10 ohms) would be within this specification if output disabled is taken to mean the same as programming I = 0 A.I'd suspect the negative voltage when the output is turned off is caused by the OVP sensing comparator circuit, providing a negative output current through R703 as shown in the attached schematic. The current would be about -170 µA so with OVP set to full-scale so seems to match your measured value.Assuming the supply actually updates the OVP setpoint while in disabled mode, you should be able to see the amount of negative output current change proportionally to the OVP setting if this is where it is coming from. If the negative voltage starts to go above a few hundred millivolts the reverse polarity protection diodes will start to turn on however, so you need a resistor value of suitable size to give a measurable voltage while not exceeding 100 mV or so. Or you could use the current range of a DMM.

author: VK5RC
author_rank: Unknown
timestamp: March 08, 2014, 11:25:42 am
content: With a 2.2KOhm resistor across Out+ to Out-, the output (when "disabled") voltage is between -0.09V and -0.03V,The current that flows across the output terminals even when they are disabled IS related to the OVP set voltage, but not all the current flowing down from U603 U608 (via R703) ends up out the output. The current down R703 is about 100uA at 66V OVP.Table of resultsOVP setting Displayed V measured output I at terminals V across R703 (0V=-Out) V across R657 (0V=RMINB)66V -0.09V -35uA +2.91V +0.018mV44V -0.06V -22.9uA +1.94V22V -0.03V -11.3uA +0.97V +0.018mVNote in the 6644PSU C690 and C695 aren't populated.Any comments appreciated

author: megajocke
author_rank: Regular Contributor
timestamp: March 09, 2014, 09:10:11 pm
content: Now I'm not sure, but reading your first post again I get the impression you are having trouble with protection tripping when it shouldn't. Is this really the case?

author: VK5RC
author_rank: Unknown
timestamp: March 09, 2014, 11:16:14 pm
content: Yes, whenever the output is "enabled" by pushing the Output On/Off button the protection triggers.At switch on (output disabled) the output is the -0.3V (at the terminals), if the output is enabled it goes into Protect no matter what settings of OCP OVP V or I. It then displays 0.679V (and this is at the terminals)If when the output disabled light is off i.e. output enabled and its in Protect mode, and you push "clear protect" it then works perfectly, voltage reading 0.001V )as at terminals.If you then disable output (by Output On/Off button) it displays -0.3 V (at terminals), if you then enable output it goes back into protect mode, which can be cleared.Thanks again for your help.

author: VK5RC
author_rank: Unknown
timestamp: March 10, 2014, 03:00:17 am
content: I think I am getting close, I had been lead astray by the negative voltage ASSUMING that was the strange thing, following the "OV at turn on" fault finding chart, using a DC scope, i have found a short duration 70V or bigger spike upon switching the output on!I will follow this up, there is a troubleshooting table to help, it may take me a while, megajocke, thanks for your help.

author: VK5RC
author_rank: Unknown
timestamp: January 10, 2015, 07:10:50 am
content: Found the issue after several false starts and a period of building enthusiasm!Ultimately I think someone had altered the PSU, but done a tidy job so I didn't suspect initially.A 120V 8uS pulse was present in the output upon enabling the output, this would trigger the over voltage protection, this spike would not occur upon cancelling the OV protection and then the PSU would work well.According to the manual the sequence of events between normal output disable/enable and OV protection clear should be very similar, so theoretically the spike should occur on both events.An output electrolytic capacitor was missing and replacing it fixed the problem!!!!!!! ARRRRGGGGHHHHHIssues that lead me away from finding it earlier- the PCB is used for multiple versions of similar PSU so a large number of components are normally not populated, perhaps about 10% of components.- the schematic in the service manual is also for multiple versions and many components shown on the schematic are not present, you need to refer to the Parts List, necessitating a redraw of the schematic.- the output transistors had been changed from BPT to MOSFETS with different bias resistors, not found in any versions of the manual nor in updates.- early on a clip slipped from an IC pin and I shorted the gated -15V supply in one of the op amps, so I created another problem (x2 actually)!!!Got there eventually but learnt a lot in the process. Thanks to the forum for help.