Agilent E3640A power supply - problem with power-on state - Page 1
Forum section: electronics / repair

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 17, 2019, 05:01:48 pm
content: I have a problem with E3640A lab supply. After powering up it has 1V on the output (with output disabled, of course). This output voltage drops to 0 once I enable the output or do a reset ("Recall"->"reset" from the front panel or *rst over GPIB). This seems to be a software problem, not hardware, but I am not entirely sure. I read the manual, and I don't see any settings that might lead to this behavior. And I don't see any way to restore factory defaults. Am I missing something? Thanks.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: March 18, 2019, 04:35:51 pm
content: So what happens if you put a load on that 1 volt??? Does it drain off, and go away, or supply the full current of the supply continuously???

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 19, 2019, 02:46:50 am
content: Quote from: SoundTech-LG on March 18, 2019, 04:35:51 pmWhen shorted with an amp meter, it provides 20mA, and the voltage drops to about 110mV, but goes right back to 1V when the load is removed. Remember, this is with the output off and with the default voltage/current settings. When I push "Display limit", I see usual 0.00V 3.000A starting point.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: March 19, 2019, 02:36:01 pm
content: I've had one of those with similar problem that had burned out trace right at the edge of the board towards the front of the power supply. Mine was damaged by previous owner when replacing burned out fuse.I mean, it's a pain to get the board out to just be able to check. It was a very thin trace routed at the edge of the board.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 19, 2019, 08:09:56 pm
content: I also had one of these PSUs and the same problem and found a broken trace from mechanical bending and it killed a "feedback-loop"!It was fixed in a few minutes.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 20, 2019, 05:48:09 am
content: Quote from: Ordinaryman1971 on March 19, 2019, 02:36:01 pmI don't see any thin traces at the front edge of the board. Top or bottom? Perhaps you had a different model.And I don't see how with such a fault the supply might start working fine after turning output on and off.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 20, 2019, 05:50:58 am
content: Quote from: HighVoltage on March 19, 2019, 08:09:56 pmOne more broken trace suggestion! Do you happen to remember what trace it was? What parts did it connect?

author: HighVoltage
author_rank: Super Contributor
timestamp: March 20, 2019, 10:47:41 am
content: Sorry, I looked but I did not document this repair and don't remember the details.It was a sense trace somehow, starting close to the output connection and I found the fault by tracing with an ohm meter.But my PSU was a E3646A, not sure if there is a big difference to yours. The schematic for the E3646A was available, too big for an attachment here. Let me know, if you need it.Also, I had another similar Agilent PSU and it had a broken SMD resistor, also close to the output connectors.Some of these PSU's (may be all) have a crow bar function to shorten the output for protection.But if you have a battery or large capacitor connected at the output, a power cycle trips the crowbar and kills the PSU in a few ms. I had one of these and it also showed similar behavior as yours after the incident. In this case I had to repair the low resistance track that got damaged without visible damage.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 21, 2019, 03:20:04 am
content: HighVoltage, I also have the schematic for the E3646A. It seems to be the only schematic available for this line. At least I could not find any other.I would think that the faults you describe would be present all the time, but mine disappears once the output is turned on. It does look like a software problem. I looks as if the supply is initialized with 1V on the output somehow, and enabling the output overrides that. When I disable the output, this 1V does not reappear.I did some checking though. So far, I don't see any broken traces or faulty components.

author: bitseeker
author_rank: Super Contributor
timestamp: March 21, 2019, 03:48:39 am
content: I checked the service notes for the E3640A, but didn't see anything like this. Could it still be a hardware issue where a control or reference line is in an undefined state at power on due to a bad component (e.g., pull up/down resistor)? Enabling and disabling the output then pulls it to a good state.I've worked on the E3648A and E3631A supplies, but not these single-channel versions. I'm very curious what the problem ends up being.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 21, 2019, 05:13:18 pm
content: Quote from: feedback.loop on March 21, 2019, 03:20:04 amI have repaired many of these different Agilent power supplies and never had a software problem.I would suspect a hardware problem, usually a single component, even if it is intermittent.Does the schematic of the E3646A fit to your instrument (more or less)?

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 21, 2019, 05:51:25 pm
content: I agree with your statement. This is how I would assess probabilities as well. I am trying to keep an open mind, and look at the evidence. And so far I don't see how this can be explained by a failed component.Yes, the schematic for E3646A seems to match E3640A (if you ignore the second channel and mismatch of the numbering of designators).There is a DAC and 4-way multiplexer with precision caps for sample-and-hold and TL074 buffers: voltage ref, current limit ref, overvoltage protection ref and one more ref, which is used as some sort of an adjustable base line for the other three. Everything there seems to work as expected in general, but the voltage ref is sitting a bit higher at power up, but drops as soon as the output is enabled. And it stays there when the output is disabled. So the DAC must be fine since it is shared for all refs, the multiplexer seems to be fine too. The only explanation I have at the moment is that a wrong value is sent to the DAC after power up, and it gets overridden by the right value once the output is turned on. I wish I had some sort of a protocol analyzer to capture the data going to the DAC. Perhaps it is time to get myself one of those inexpensive USB logic analyzers. But suppose I confirm this hypothesis. What is to be done? Can it be a corrupt parameter in the EEPROM (93C66)? Is there a way to fix that at all (copy from a working unit and recalibrate)? Maybe it is too early to worry about that. I need to check that somehow or come up with some other explanation.

author: bitseeker
author_rank: Super Contributor
timestamp: March 21, 2019, 05:57:59 pm
content: The E3640A has a history of its EPROM going bad, resulting in powering on, but then locking up or otherwise having no display. This could be a first sign of some corruption, but a peek with a protocol analyzer (or a scope with decoder if you can trigger where you need it) seems like a good next step to confirm what the DAC is getting.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 23, 2019, 03:20:34 am
content: Quote from: bitseeker on March 21, 2019, 05:57:59 pmYou must be talking about the permanent ROM with firmware, right?I happen to have a working unit with firmware revision 1.8, but the faulty unit has revision 1.5. I tried putting the 1.8 revision into the faulty unit. No change at all.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 23, 2019, 03:23:41 am
content: And by the way, here are the files just in case

author: bitseeker
author_rank: Super Contributor
timestamp: March 23, 2019, 05:59:24 pm
content: Quote from: feedback.loop on March 23, 2019, 03:20:34 amYes, the firmware one. Well, since that didn't fix it, there must still be a hardware issue somewhere. I'm very curious what a protocol analyzer will reveal.

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 24, 2019, 05:19:42 am
content: I checked the output from the DAC. Three pictures are attached: power on state, output enabled, output disabled. By manipulating the voltage, current limit and over voltage protection settings I figured out which portion of the waveform corresponds to which setting. I annotated that on the first picture of the power on state. As you can see, the voltage level is slightly higher on the first picture compared to the second and the third.

author: bitseeker
author_rank: Super Contributor
timestamp: March 24, 2019, 05:11:15 pm
content: Yep, I see. That is weird. I wish I had one of that model for comparison. It seems odd that if it was a software bug, it would've stuck around so long unnoticed (or at least not publicly documented in a service note). I guess it is rather subtle.

author: wictor
author_rank: Regular Contributor
timestamp: March 24, 2019, 06:35:58 pm
content: I think OP said he has working unit also, so it could be used for comparison.

author: wictor
author_rank: Regular Contributor
timestamp: March 25, 2019, 04:52:13 am
content: Hi,Does the unit ADC part work? So is it displaying correct output voltage and current?Wictor

author: feedback.loop
author_rank: Frequent Contributor
timestamp: March 26, 2019, 04:35:58 am
content: Quote from: wictor on March 25, 2019, 04:52:13 amYes, everything looks fine except the 1V after the power up and until the output is enabled or reset is performed.