Fluke 5440B repair - Page 1
Forum section: electronics / repair

author: Rax
author_rank: Super Contributor
timestamp: July 09, 2024, 12:49:27 am
content: I landed a 5440B at my bench that's exhibiting some confusing symptoms.The unit came to me in pristine form - Fluke original stickers everywhere, original manuals and cables. Even the registration form was enclosed.The unit exhibits no signs of life, other than the green power LED lighting (next to the turn on knob). Nothing else seems to be alive, including the fan. I opened the unit (tearing all those cal stickers...) and checked the rails, both unreg and reg on A10 and A12, and they seem reasonably close. Definitely "alive."Upon checking A19 though, there's no DCV present on it. Which makes some sense, given the fan is also off (they feed off of the same 120V VAC line). Upon checking for continuity between J2 (13/26; 14/28) and J8 (12/24; 11/23), I see that the lines on the motherboard seem to be OK.Other checks:I see 116VAC between 9F and 9S on A11The line switches (S1, S2, S3) seem to show correct continuity between their polesI measured all "full winding" taps of the GUARDED XFMR and they all show reasonable ohmic valuesI'm not sure how to explain the presence of the line VAC between 9F and 9S, and no VAC at the fan and UNGUARDED XFMR... Particularly as CR2 is lighting. Not sure if I should suspect the E1 spark arrester or what else could be going on here.Thanks for all input.

author: picburner
author_rank: Frequent Contributor
timestamp: July 09, 2024, 05:52:50 am
content: with the 120V mains one reason for the failure of the instrument and the fan to turn on could be one of the switches s1-s3 which has/have false contacts. Although you have measured them (in circuit) the presence of the transformer windings may have distorted the measurement. Try exercising the three switches several times.The green LED CR2, connected to a secondary winding of the transformer (by cr1/r1), could light up dimly even with a small flow of current and this contributes to confusion.Check that s4 is also working properly of course.Also, to be safe, I would also check the fan itself by unplugging it from the instrument.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 09, 2024, 05:51:28 pm
content: You probably want to start by reviewing the following documentation from the service manual:Overview:Figure 3-1 module locationsFan-related diagrams:Figure 2-7 power supply simplified schematicFigure 7-6 A3 (motherboard)Figure 7-14 A11 (guarded transformer termination)Specific to the outside regulation that is connected with the fan:Figure 7-20 A17 (outside guard regulator)Figure 7-22 A19 (outside guard terminator)Assuming the fan is working and unable to receive power, you should look at the switch (Figure 5-25), the mains input and guarded transformer (Figures 7-6 and -14), and all of the fuses (Figure 7-14).EDIT:Believe it or not, you should also remove the fan filter. Sometimes it gets stuck in the fan and prevents free movement. I have seen a video somewhere on YouTube with this problem. Might as well rule out the really simple things first.EDIT:If you are to remove the front panel to access the switch, pay extra attention to the DIP plug. It is extremely fragile and should be handled with great care. The front panel can be removed after both clam-shell covers are removed and the rack handles.EDIT:To remove A19, the fan must be removed and both of the large mounted capacitors. The large filter capacitor should eventually be replaced as it can cause overheating of A17 and A19 boards, resulting in a 'check guarded power supplies' fault. C7 36DX343G015AF2B (CAP, AL, 34000uF, +/-10%, 15V) should be replaced by CGO343M020L or MAL210116333E3. I use the Vishay capacitor even though it is much shorter than the original.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 09, 2024, 05:53:06 pm
content: Attached is a picture of the guarded transformer (A11) with a lot of the fuses.EDIT:This module is connected to the frame from the adjacent side with a set of screws. You will have to remove the clam-shell covers to access them.

author: Rax
author_rank: Super Contributor
timestamp: July 10, 2024, 04:08:18 am
content: Quote from: leighcorrigall on July 09, 2024, 05:51:28 pmThank you, Leigh. Very thoughtful, insightful input.I am pretty intimately knowledgeable of the 5440 series:https://www.eevblog.com/forum/metrology/fluke-5440a-datasheet/,which I think you've seen.I am currently suspicious of the card edge connectors (bad contact?...) and maybe the E1 spark arrestor. Also, picburner's recommendation to closer examine the line switches. I may either condition them and try again on board, or pull them off the board and properly test them.I suspect once I figure out why the 120VAC line going to the outboard supply and fan is fixed, the unit may just go back to operation. But why that's offline is currently a mystery to me.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 11, 2024, 05:13:31 pm
content: Hi Rax,I just thought I would give you some ideas to rule out the trivial solutions and perhaps provide advice for others who have not had much experience tackling the Fluke 5440B before. Your introduction post sounded like you just opened one of these devices for the first time. Sorry about that.If you suspect that the card slots are dirty, then try the following:- Use a paintbrush to loosen the dirt, then apply a blower (not compressed air as it has oils)- Find a stick of the same width of the card+ I have heard people using erasers, but I have not tried this approach before+ If a stick is not easy to find, perhaps just fold a piece of paper until it becomes the correct thickness- Fold some printer paper around it- Apply a solvent like isopropyl alcohol or a contact cleaner+ I use a spray bottle to reduce waste and contamination- Run it in and out of the affected slots until no more dirt can be seen, then keep going for a while just for reassurance- Make sure to clean every contact on the cards with a Q-tip or paper towel before reinsertion+ The fibres and hairs that may get caught along the trace corners can be removed with a brush or tweezersI know that the instrument will not respond or display anything without A11, A17 and A19, which seems to be your problem. I would start there for the slot cleaning. I do not recall if the green light turns on when the cards are pulled. My instrument was really dusty when I first got it, especially on the bottom where the motherboard is mounted. While you are there, there are some shielded cables that you could examine. I think one is for the power button, but I don't remember for sure. Perhaps there is a bad solder joint, or something got torn -- hard to believe, though, given that no one ever wants to go back there.The green light being on seems odd to me. Maybe there is something to that.If you have another 5440B that you do not mind opening, I would suggest swapping those cards, including the transformer, to see if you can isolate the problem.In any case, I would pull every single card from the 5440B to give the mainboard a thorough examination. I know that some people with 5700As have had mainboard burnout traces before. That would be an ugly repair, but it is possible.Regards.

author: Rax
author_rank: Super Contributor
timestamp: July 11, 2024, 05:56:29 pm
content: Quote from: leighcorrigall on July 11, 2024, 05:13:31 pmAbsolutely no problem, Leigh! I meant to manifest appreciation for your very detailed, 100% comprehensive, and extremely thoughtful post, while stating my prior exposure to this unit group. I am also always striving to close the loop on any one of my repairs, so anyone coming behind me would have a clear depiction of the symptoms, repair endeavor and hopefully solution.Quote from: leighcorrigall on July 11, 2024, 05:13:31 pmThank you for the above also (me and all others taking on a similar project!).I'm actually done with the project... (or 99%! More on this below).I think the issue was the line voltage switches - which needed a bit of a wash with Deoxit (D5). I also cleaned the card edges of A11 and A19 (only IPA, didn't condition those edge pads). I've inserted an IPA soaked swab (small one) also on the last couple of connectors of A11, where the 120VAC is sent to A19. So any one of these may have corrected the issue. So just a tiny bit anticlimactic, but the satisfaction is there in strides. I have two 5440s operating at my bench!The 99% statement above comes from some residual flakiness of the fix. The unit came with A4 fit for rear output, and after seeing this works, I turned it off and switched A4 to the other position. Upon turning it on, it flaked on me again, but reseating the cards a bit and a bit more fiddling around to make sure everything sits well into their slots the unit came back online without aggravation. I probably have to sit everything - and while at that, clean all card edge connectors, like you're suggesting, Leigh, and like I did with my 5440A - and close the shield and case and it'll work reliably.Thank you very much, Leigh and picburner! Happy camper here.

author: Rax
author_rank: Super Contributor
timestamp: July 11, 2024, 10:06:56 pm
content: I may get a lot of flack for this, but upon warming up the unit up the about 4hrs it typically needs to be in spec, I am seeing it around 2.2ppm low of 10V, and about as much away from 1V too, as per my Prema 6048's 10V/1V, so to speak. I have some level of trust on the Prema's volt, so I take this as good news.The only downside is that, as much as the unit is just about pristine, the display is really dim. A bit of a disappointment, as one thing I was looking forward relative to my 5440A is a legible display, as the 5440A's is very inconvenient to use in practical terms. The 5440B was probably on for years and years in a system environment where it was probably never turned off.

author: Rax
author_rank: Super Contributor
timestamp: July 11, 2024, 11:00:27 pm
content: Stop press!! It looks like there's a bunch of work ahead.As soon as I try to go over about 40V (>= 50V), the unit fires an "OUTPUT LIMIT FAULT. OUTPUT UNDER VOLTAGE" and resets itself. Per SM (p. 4-14), I'll need to do some A8 (possibly A9 and A4) investigations... Back to pulling up the sleeves!

author: TheDefpom
author_rank: Frequent Contributor
timestamp: July 11, 2024, 11:38:48 pm
content: hmmm, just did a quick search and found this....https://www.adret-electronique.fr/fluke_5440B/fluke_5440B_display.html

author: Rax
author_rank: Super Contributor
timestamp: July 11, 2024, 11:52:02 pm
content: Quote from: TheDefpom on July 11, 2024, 11:38:48 pmVery interesting, but I'm not sure I get it - is there a board and BOM available to do this upgrade? They say this much, but I couldn't find a link to a storefront. I'm reading this with google translate, my French speaking days in school are long gone.The other thing interesting to find out is whether this upgrade can be applied to the 5440A. I know the display module is completely different between the two.

author: picburner
author_rank: Frequent Contributor
timestamp: July 12, 2024, 04:33:46 am
content: The person who made that board, if I remember correctly, is a user of this forum: jfphpThe board in the link is a prototype, a long time ago he sold the assembled board in his eBay shop but now I can't find it.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 12, 2024, 01:15:02 pm
content: Quote from: Rax on July 11, 2024, 10:06:56 pmYou reminded me of something very important. TheOutside Guard Regulator (A17) should be operating TP4 at 24.0 V, not the specified value in Tables 3-4. This supply is for the VFD. R18 on A17 can be adjusted to lower this voltage.In your case, the VFD is dimming because of age, or, if you are really lucky, the voltage is too low.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 12, 2024, 01:17:57 pm
content: Quote from: picburner on July 12, 2024, 04:33:46 amI remember this being true. The user might be willing to help out.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 12, 2024, 01:31:31 pm
content: Quote from: Rax on July 11, 2024, 11:00:27 pmGlad that you have fixed the line voltage issue. Now, it is onto more problems!Although I have not experienced this fault before, I highly recommend that while you are removing boards, you replace all of the thermal paste on each of the heat sinks. A common problem with overheated regulators is that they tend to produce undervoltage. These calibrators run very hot, and this type of maintenance will greatly improve the performance and lifespan of the unit.You should also continue to clean the card slots as well. A4, A8, and A9, according to Table 4-1.

author: Rax
author_rank: Super Contributor
timestamp: July 12, 2024, 01:59:43 pm
content: These will come in handy (see below) - extender boards I designed and fabricate for the 5440A/B. They were instrumental in fixing my 5440A, and they helped some others that used them in their repairs.They're keyed for the compounded combination of slots for all modules in the main compartments.

author: Rax
author_rank: Super Contributor
timestamp: July 14, 2024, 12:13:52 am
content: Some progress today with this.First thing I had to acknowledge today was that my board needs an accessory little pad to serve A4. I enclose pics on how I built it. Having the custom EDAC card edge connector handy helped. I also had to trim a "two contact" segment, and the same for the PCB part. It works perfectly as built.I'm still digesting through the data I collected, but here's what I'm seeing this far:setting a non-faulty condition output (say, 40V) sets K1 off and K2 on (about 4.5V over it)going into a fault-triggering condition (say, 100V), outputs a non-compliant output (about 60V for a setting of 100V), which, upon the unit actualizing the fault, turns K2 off (0V over it from the 4.5V just prior to that) and resets the unit and puts the output in STBYthe other thing the manual says pay attention to is the reading over C9. I am seeing about 26.6V over the output over it unless the fault occurs and then the drop over it goes to zero.

author: Rax
author_rank: Super Contributor
timestamp: July 14, 2024, 12:20:19 am
content: Pics

author: TheDefpom
author_rank: Frequent Contributor
timestamp: July 14, 2024, 12:44:38 am
content: Something to consider is that those old relays often give trouble due to oxidisation on the contacts, I have come across this issue in different pieces of equiment.it is probably worth opening each up and running a piece of paper soaked with deoxit through each one to clean them up.

author: Rax
author_rank: Super Contributor
timestamp: July 14, 2024, 05:48:57 pm
content: I've collected more info per SM troubleshooting steps, none of which makes a ton of sense yet to me in terms of origin of the fault(s).For instance, 4-23 at p.4-29. I'm measuring the below with a DC voltmeter, though maybe I should look at these with a scope.TP2-TP120V out: 0V (expected, this doesn't engage yet the 23-275V HV path). 25V out: 14.5V (ramping up slowly, which doesn't look right; but the output is solid and correct). 35V out: 13.4V (sloping down now). 60V: 7.3V. 100V: 2.25V, which goes to 0V upon fault condition actuating the monitoring system. 300V: 2.5V (same as 100V)TP2-TP320V out: 0V. 25V out: -16V (slowly ramping up in absolute value, as above). 35V out: -14.35V (same fluctuation as above). 60V: -7.9V. 100V: -2.4V (zeroes at fault). 300V: -2.65V (zeroes at fault)Noone of the above is anywhere near the +-40V indicated by Table 4-7. Additionally, they vary widely with the set voltage, which I'd have expected to rather stay close to those +40V or -40V turning the state of the triacs on or off (if I understand how they're supposed to work).Generally speaking, setting 100V out outputs about 57V (though sometimes seems to go to 100V, then dropping at this 57V), which triggers the fault condition, fires the error code and puts the unit in STBY. If I request 300V out, the same happens, though I don't this I've ever seen it outputting more than the 65V it settles at, right before the unit gets switched to STBY.Further on the same procedure (step 5), when I try to measure state at pin 8 of P21 (TS1+) relative to the PCOM (23/13), for:20V out: 4.46VDC30V out: 3.8V70V out: 3.8V which goes to 4.46V at fault triggering

author: TheDefpom
author_rank: Frequent Contributor
timestamp: July 14, 2024, 05:59:10 pm
content: Sounds similar to an issue I had on my 5200a, one of the output transistors were breaking down under high voltage, causing it to short out and drag the output down.

author: Rax
author_rank: Super Contributor
timestamp: July 14, 2024, 06:29:08 pm
content: Quote from: TheDefpom on July 14, 2024, 05:59:10 pmTrying to see trough the weeds of the troubleshooting steps and the eclectic results they return, one thing stands out from my symptoms, namely the fact that the output plateaus at about 60V regardless of setting, which I think seems to point to a hard level-related failure like I think you're describing, Scott. I don't think the issue is the "level-composing" logic, or even the monitoring/protection systems, but a simple hard stop at 60V out.

author: Rax
author_rank: Super Contributor
timestamp: July 17, 2024, 04:32:37 am
content: Quote from: picburner on July 12, 2024, 04:33:46 amI tried to contact him to no avail. Having the possibility to use such a board would be fantastic. I'm sure there's others like me that would appreciate such a board. Though I can't imagine this was/is a large market for their board.

author: Rax
author_rank: Super Contributor
timestamp: July 17, 2024, 04:45:13 am
content: Quote from: leighcorrigall on July 12, 2024, 01:15:02 pmThank you for this pointer. I have not run into this with the 5440A, I assume because of the entirely different readout.In my unit this seems to be adjusted to 24.1V, so I assume this was the Fluke techs lowering it to what it should actually be. As a reminder, this unit came to me with all original Fluke stickers everywhere (and in pristine condition at that).

author: Rax
author_rank: Super Contributor
timestamp: July 17, 2024, 04:57:07 am
content: Quote from: leighcorrigall on July 11, 2024, 05:13:31 pmThis may be my shortest path to success, as I do have my 5440A, and they're very very similar, if not outright identical (other than the display board). In fact, I seem to recall I identified a couple other less know differences going by just the SMs, which may be documented in my 5440A thread.In any case, I need to collect some guts to begin such an endeavor, as maneuvering two behemoths like these guys are are not for the faint of heart (or people liking theirs backs working OK). I need to wrestle them in a way where I could have both open side by side, and this in my cramped workshop will be tough.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 17, 2024, 01:38:35 pm
content: Quote from: Rax on July 17, 2024, 04:57:07 amI have reviewed the manual and seen enough teardowns to agree with you that there are very few differences between the A and B versions. The rear interface panels are different, too.Be careful. The plastic handles are prone to breaking and should not be used for moving the unit. I move the instrument by sitting it on an office chair and moving it around my office when I need to work on it. One of my long-term goals is to replace these handles with steel equivalents so that I don't have to worry about breaking something off. My 5440B will be with me for a long time because it is tremendously reliable and has excellent specifications.

author: Rax
author_rank: Super Contributor
timestamp: July 18, 2024, 12:12:33 am
content: Quote from: leighcorrigall on July 17, 2024, 01:38:35 pmIf you ever get to, I dunno, generate some cnc files (or other machining) you can share, or maybe have a few more made, I'd gladly get that if you're willing to share. Cover costs, etc, of course.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: July 18, 2024, 02:54:31 pm
content: Quote from: Rax on July 18, 2024, 12:12:33 amIf you are suggesting that I make a CAD model, I have already done so. Merry Christmas.I have not formally shared them on EEVBlog because I still have to confirm the dimensions. It should be a near-identical copy.Please let me know if I should pursue this. The material selection should also be correct. We will have to think about that.EDIT:According to the service manual, the handles are as follows:Table 5-1 (5440B Final Assembly) and Table 5-2 (5440A Final Assembly)Reference Designator, Description, Stock No., Supply Code, Part No., QuantityMP 2, Corner Handle 8.75", 656199, 89536, 656199, 2MP13, Corner Handle 8.75", 656199, 89536, 656199, 2A private conversation with Rax confirms that these handles are interchangeable between the Fluke 5440A and the 5440B.A single part (656199) can be used on all four corners because of the symmetry.

author: Rax
author_rank: Super Contributor
timestamp: July 25, 2024, 06:35:35 pm
content: The other things I'd like to see as a 3D print file is the spacers used inside these 5440s (see pic). I'll dig up the part number and real name, but anyone having opened one of these has stumbled upon these little knoblets. They become very brittle with time, and I have not run into the situation where I'd pull them with "no victims" in a new unit having landed at my desk.I'll give a shot to making some 3D files for this and see what comes out.

author: Tony_G
author_rank: Super Contributor
timestamp: July 25, 2024, 08:13:15 pm
content: Random question Rax, did you replace/check the relays? One of my 5440s Fluke rejected for cal due to relays and I've never got around to addressing.Just wondering,TonyG

author: Rax
author_rank: Super Contributor
timestamp: July 25, 2024, 08:18:41 pm
content: Quote from: Tony_G on July 25, 2024, 08:13:15 pmTony,Not really, no. I've just ran some of the corresponding diagnostics and I think my next step will be to swap modules to localize the issue (as I have this other 5440A that works correctly).On checking the relays as its own thing, I am not entirely sure how I'd ago about that, need to think about it and figure it out. It's a good recommendation though.

author: Tony_G
author_rank: Super Contributor
timestamp: July 25, 2024, 11:49:41 pm
content: I wouldn't have bothered getting the unit calibrated but I live down the road a bit from Fluke so I could drive the unit to their location rather than shipping it - I'm not sure how they determined that the relays were an issue (metrology, volt-nut noob...) but I haven't tried some of the suggestions of cleaning the relays.Interested to see what you discover.TonyG

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: August 02, 2024, 06:03:12 pm
content: Quote from: Rax on July 25, 2024, 06:35:35 pmIf you keep the parts, they can be glued back together with epoxy and care. Once dry, you can use a razor blade to clean off the excess. I think I have a CAD file for them somewhere, as I was also concerned about them breaking. Unfortunately, I have no time to print them right now, but if you remind me after August 12th, I can see if I can fabricate them with a high-quality printer. You have my email.I think I also saw a talk about calibrating this unit. I highly recommend that you do not calibrate a unit until you have had plenty of experience with it (> 6 months of operation and use). If there is a subtle fault, you may be wasting your money. Believe me, I made the mistake of being hasty a while ago with an Advantest R6581T.

author: Rax
author_rank: Super Contributor
timestamp: August 03, 2024, 10:21:24 pm
content: Quote from: Rax on July 25, 2024, 06:35:35 pmI think these are Fluke 421776 ("DORCAS?..."). So I made this in Fusion.

author: Rax
author_rank: Super Contributor
timestamp: August 04, 2024, 12:38:46 am
content: This afternoon I finally got the earthmover (...) to carve out some room at the bench, enough to have two of these behemoths open side by side. Not sure I'll be able to get the car we usually garage inside the garage tonight...So, I was able to swap modules from the known good unit to the issue unit such that I'd isolate the issue. And this far tests are a bit confusing. Per SM (p.4-14), given the error message, A8, A9, and A4 are prime candidates for issues. A4 andA8A9 seem to change nothing, essentially, so I'm deeming them good. SwappingA9A8 doesn't fix the issue, but instead returns a different error. Actually, errorS! Initially, there was something about, I think, "inside guard" (it was one time and I didn't write it down immediately), and now I'm consistently getting "OUTPUT LIMIT FAULT. OUTPUT OVER VOLTAGE." But, this was upon me asking it to output 10V, and I didn't see it outputting more, so that's a bit weird. It seems there's some false flag errors triggering or something.I'll dig down deeper, but if anyone sees any sense in the above, I'm all ears. All I can conclude this far is that the issue may have to do with A9, though a known good module didn't fix it. For anyone asking, the module in both units is a "5440A" module, so I don't suspect any updates to the "B" unit modules make the swapped "A" unit unfit.

author: Rax
author_rank: Super Contributor
timestamp: August 04, 2024, 12:42:41 am
content: With the replacementA9A8 board, if I try to output a "previously error condition triggering output" (say, 100V), I'm still getting the "UNDER VOLTAGE" error. If I try to output, say, 10V, I'm getting an "OVERVOLTAGE" error. Outputting 10V was successful with the original A9 board. So there seem to be new errors of sorts with the replacement board.I think I'm just going to have to take a very close look at the trouble unit's A8.

author: Rax
author_rank: Super Contributor
timestamp: December 03, 2024, 10:10:27 am
content: This project's still active. I've not had a ton of time to spend on it, but during the winter holidays there may be some additional time so I am getting this back to the front of the docket. I am hoping for some input on what's going on here and where to head next.A quick and probably partial recap and status report.1. The unit initially was completely dead, which I tracked to A19 not being supplied with 120VAC. Which meant all outguard modules were essentially off.2. Once the above was corrected, the unit started largely working. Except that, when I attempt to output more than about 60V, the unit initially successfully outputs the requested voltage (say, 70V), but just for a few moments, then it drops to about 57V, which triggers the fault mode "OUTPUT LIMIT FAULT. OUTPUT UNDER VOLTAGE."3. My next step at this point was to swap modules (A4, A5, A13/12, and I may have tried others) from my fully working (successfully repaired) 5440A with inconclusive results.Then I went ahead and tracked the symptoms above (the lower than set output voltage) backwards through the output board and back to A13 FILTER B board. Technically, I think the issue resides on what Fluke calls the "HV Loop," and they have some pretty contorted and not illuminating (at all, to me) steps in section 4-23.I've done this by going by section 4-19, where I've applied a DCV proportional to the set output vs. full scale between A4 (OUPUT board, NOT A5, as per Service Manual, so pleasenote the error in the manual) A4TP5 and A4TP13. For instance, a set 70V out would want this applied DC to be about 112mV. I did this with my HP8904A, which has the great convenience of being able to key-input voltage settings directly. This step is in order to disable the monitoring system, so an out-of-bounds condition (more than 5% difference between the output and the set voltage) would not reset the unit and further troubleshooting would be possible to make at length.Under these conditions, the events at #2 above occur and stay active past the few seconds it'd take the unit to actual its reset. To me, this confirms the monitoring is operating correctly and the issue originates with the generation of the HV.4. I looked at +TSV and -TSV by probing A13TP1 to A13TP2, and I am seeing the voltage go up to about 5V (as Vout stabilizes to 70V), then dropping to about 2.15V (as the voltage out is falling from the 70V set to the 57V is settles for), and then going back up a bit to something like 2.5V or so. I do see some ripple riding on top of that (per my Fluke 189), though it varies pretty wildly (it starts low, maybe mV, but as the fault occurs, it jumps to maybe 250-300mV, then goes down in the tens of mV and stays there). I am not 100% sure what I should be seeing or looking for at A13TP1 and A13TP3, so I don't have a satisfying analysis of my own of what this means.5. I am not seeing anything significant happening over R20 (the +TSV .33 ohm) when the fault occurs. Barely in the mV range if even that. So there seems to be no protection actuation on the HV that could be causing this.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: December 09, 2024, 02:09:14 pm
content: As I understand it, the problem is with the voltage drop at the output in the range of 275V.First, take a multi-channel oscilloscope (4 channels) and study the picture "Figure 2-8. Sample String PCA, Simplified Circuit Diagram"Put the oscilloscope probes in many points of the device at the same time and see in which node the first voltage drop occurs.1. DAC output.2. PREAMP SUMMING JUNCTION point3. PREAMP PCA OUT4. OUTPUTLooking at these graphs, you can understand which of the nodes breaks through first. This will greatly narrow the search area.

author: Rax
author_rank: Super Contributor
timestamp: December 17, 2024, 05:05:50 am
content: Thank you for thinking on this with me, MegaVolt.Yes, the 275V, but also the 1100V. So everything over about 60V seems to trigger this fault state. Apparently the unit is unable to output more than the about 57V + 5% for fault to be triggered, but that seems to be the case.Right now, I'm trying to get a four channel digital scope at my bench (it just so weirdly happens I don't have a fully working one right now, just one that's a project...). Before I do that, I'll be trying a few other things, but if they go anywhere, I'll post here with results.

author: Rax
author_rank: Super Contributor
timestamp: January 06, 2025, 01:01:52 am
content: Well, project's done... Hugely gratifying and a gianormous project in a few different ways for sure.I hope I'll not miss anything, but this unit was inflicted with more separate and possibly disconnected issues than anything I've seen before. Attempt at a summary:The HV issue tracked back to A13, where HV gets cooked. That's because the TS +/- lines coming from the digital drives in seemed correct. Almost accidentally, I discovered that voltages over 876V were output successfully, so that put the TS1, TS2, and TS3 triac drives under suspicion (TS4 deemed good). Replacing the corresponding triacs (Q2-4, 5-7) restored operation of the corresponding range for TS2. No luck on TS1 and TS3.Given A13 seemed to be the origin of all evils, I went ahead and replaced the optocouplers on the faulty paths. Also, all bridges (including TS4, while I had my hands dirty). This way, those paths had essentially all new parts in them. No luck.Further investigation revealed the TS AC inputs into those triac drives were present at A11 and missing at A13. Stripping the unit bare revealed that A11/J3 and A13/J6 were not talking to each other. Taking a magnifier to A3 (MB) revealed the MB card edge connectors had broken pins. This included TS5 (magnifier was a literal must to figure this one out) which wasn't a suspect before, but a perfect tracking of both + and - sides may have been explained by this third AC path being broken. IMHO, the mechanics of this is a design flaw - the massive transformer attached to A11 should be fastened to the A11 board with plastic washers, or something else that would provide a tiny bit of leeway. The massive transformer "manhandles" A11 during insertion into the MB connectors during servicing; or any shipping event, for that matter (even if the transformer is fastened to the chassis, ultimately). Not very smart of you, Fluke.Once the edge connector was fixed, the unit had one issue left, namely an "INSIDE GUARD FAULT. CHECK GUARD CTRL BUS" error, which became more and more persistent.This was a tricky one, and close examination of the GPOP line revealed some odd handling of the +5 TS AC signal by A10/U13. Replacing C31 fixed this in a climax I have not frankly anticipated.Exceedingly happy camper. Thanks again, MegaVolt, for your kind pitching of thoughts on here. Effusive kudos for your availability.

author: Rax
author_rank: Super Contributor
timestamp: January 06, 2025, 01:22:02 am
content: Lessons learned on the 5440 series?A11 has a slight design issue, where, imho, the transformer needed a different type of fastening to the board. Take a magnifier at the edge connector if symptoms point to issues there, especially if some HV ranges work and others don't.Recap. Not indiscriminately (though, really, up to you), but suspect caps being guilty of symptoms you observe.Triacs on A13 are suspect. To my surprise, more suspect than the optocouplers (which I've seen go bad before), so examine (both of) them closely.Dallas SRAMs tend to go bad. Check the data on them and replace with known good chips with correct bits on them. Data is available from different sources, here and elsewhere