Fluke 5200A repair (?)

[Rax]

I'm the happy owner of this 5200A from a member of this community I have loads of thanks for (you know who you are!).

It's been measuring brilliantly (see attachment). I think the seller was very conservative in their assessment of the condition of the unit. I have a "calibrated" (long form Z540-1 available) Fluke 8502A I can compare readings with, and I think it's likely within cal most everywhere. BTW, the 8502A has a pretty amazing RMS module, being able to measure dead on to 1MHz to a certain level without a hitch. I am using it to measure the 5200A output, and the next column after that lists the values displayed by the 8502A for standard lab excitations.

Now, what's not so good is that sometimes it seems to go on a state where "OVERLOAD" light flickers on and the output starts fluctuating. This is rare (this far), and I'm still examining the mechanism by which OVERLOAD activates, but if anyone has experience with this unit and this looks familiar, I'd appreciate input.

Thank you.

[Rax]

My first assumption with this "flickering OVERLOAD" annunciation issue is a component becoming stressed by overheating.

When the light flickers, there's an audible noise from within the case - I don't think it's a relay (though I'm still looking over the OL circuitry, which I don't have a handle on yet) - I think it's rather a component exhibiting mechanical stress that becomes audible.

The instrument has been on for about 24hrs - not all of which supervised - but I feel it's been fine (not doing the above) for most of its current cycle.

[Ugur]

It could be an old capacitor. or an unstable op-amp...

[Rax]

The other condition that is occurring is that the OVERLOAD light never comes on during startup. Per the startup procedure, I believe it's supposed to be on at the beginning, the turn off after 5-10 seconds.

Instead, in my unit OL never turns on, but the STDBY/OPER lights are initially both off, then - just about when OL should probably go off - the STDBY turns on. Then, I'm able to turn that knob and put it into OPER to get signal out.

[Rax]

It could be an old capacitor. or an unstable op-amp...

Completely possible. I have not yet really opened the unit to look for any evident trouble - such as stressed or burned resistors, leaked caps, etc. - other than to see what's moving loose inside (I assume a screw...). This is because upon receiving it, I noticed/heard this and opened the case and internal shields. The unit had quite a lot of screws missing everywhere. It's a miracle it arrived in the shape it did, as structurally, the unit was essentially pretty "flexible." I have not yet found what's moving inside, but I rested the unit on its rear handles carefully and turned it on.

Other than this OL condition, the unit is in very good shape, operationally. I'm assuming the issue must be pretty localized to some specific condition or fault.

[tridac]

Still worklng to try and get just one 5200 fully working, Bought two other for parts donors and now have the basic system working on all except one voltage range. The power amplifier in particular, is fragile and there is no current limiting, so if it goes down, there are low wattage sacrificial resistors on the psu pcb that burn out to protect other areas. Rebuilt the psu, low value resitrs on pcb pins, fitted heat sinks on a couple of transistors and fitted a fan, cross flow across the psu compartment. Some of the psu parts run too hot to touch and discolour the boards over time, tracks lift and as a design, a bit on the edge. Psu is fine now, but mine has the o/l led flashing at times, so the next step is go through the cal procedure as far as possible. If still issue, then dig deeper. According to the the manual, just about all the basic functionality can be checked out, range by range, with the power amp board removed and a jumper fitted, which might save some trouble. Afaics, the overload circuit shorts the input to the power amp on an anomaly, so if that's happening, the issue might be earlier in the chain. Fully direct coupled signal chain as well, ouch.

Run the zero offest calibration procedure on the power amp at least, as it is direct coupled to the output ratio transformers, which can burn out, making the whole thing scrap. Also, replace the two power amp 68K 2w comp resistors, which run quite hot, as they will blow the power amp if they fail with age and go oc. (mine did) Replaced with 2w metal oxide. Old, but a really good calibrator, with accuracies of a handful of microvolts when right, but Fluke were stretching the art at that time. As shown by things like the power amp bootstapping technique, to enable the use of high frequency, but low voltage and power (TO5 can) transistors in the output stage. +/- about 300 volts on the power amp, ~600v dc total, so be carefull where you stick probes and for your own safety. Good news is that the power amp uses sockets for most transistors, so you have to ask, why they did that. The schematics are amazing and not too difficult to work out what each board does. Very clever design for it's time.

Definately a keeper, if you can keep it alive :-)...

[Rax]

replace the two power amp 68K 2w comp resistors, which run quite hot, as they will blow the power amp if they fail with age and go oc. (mine did) Replaced with 2w metal oxide.

Thank you for all that input - very appreciated.

I assume these are R40 and R46 on the A7 (Power Amplifier) module, right? For something needing high reliability and some muscle, I usually resort to Vishay Dale WW (such as RS, probably 5W here). Not non-inductive by default, unless needed (I don't think here that's the case?....).

[Rax]

a really good calibrator, with accuracies of a handful of microvolts when right, but Fluke were stretching the art at that time.

This thing's quite amazing, given the relatively wide bandwidth, keeping the levels so precise is pretty breathtaking. In the RF world, fractions of a...dB!! is an amazing level of performance, this is obviously in a different world of precision where one doesn't need to quantify in dB but %. Not quite ppm, but % is pretty dope.

I am currently trying to decipher any patterns in the occurrence of the OVERLOAD aberration - may be temperature related, maybe just something relatively random (= not yet determined) about the failing part?... Should be said that for all such work I always start from the power supplies, and in this case I haven't even yet started that examination. So maybe I just need to pull the sleeves on this thing. That said, today's been working without skipping a beat. I had nothing to go by, it's been flawless.

[tridac]

It is quite amazing, considering that the first edition manual is dated 1975, which means they must have started the design in 1972/73. Might look a bit clunky in places, but that was the tech of the day, what was available. The reference board is worth  a look. The usual Fluke dc reference amplifier chip design, the output of which is then pulse width modulated and filtered, to get  a variable reference, to compare against the output from the precision rectifier. Both the filter and galvanic isolation are interesting. If you take the cover off the reference board, there are wound ferrite toroids either side of a screen plate. a few turns on each toroid, then  a single turn loop across the screen, to couple the two together, pwm pulse on and off.

There is  an earlier note on some 5200 mods here:

https://www.eevblog.com/forum/metrology/fluke-5200-ac-calibrator-fan-notes/msg5001235/#msg5001235

The resistor mentioned are the right ones to change, bias current from the + / - 300 volt rails. They  can be unsoldered carefully from the board top side, then use a solder sucker to clean the vias. Don't need to take the board / heatsink assy apart, but do form the replacement leads to standoff from the board, to allow air flow. The two power amp adjustments are output offset and quiescent current adjust. All in the manual, but suggest power off, connect probes, then power on again procedure to adjust, to avoid possible smoke :-). You will need an card extender board for serious debugging, but found an Ebay seller in Korea making copies of the Fluke oriiginal. Seems unobtainium otherwise...

[Rax]

It is quite amazing, considering that the first edition manual is dated 1975, which means they must have started the design in 1972/73. Might look a bit clunky in places, but that was the tech of the day, what was available. The reference board is worth  a look. The usual Fluke dc reference amplifier chip design, the output of which is then pulse width modulated and filtered, to get  a variable reference, to compare against the output from the precision rectifier. Both the filter and galvanic isolation are interesting. If you take the cover off the reference board, there are wound ferrite toroids either side of a screen plate. a few turns on each toroid, then  a single turn loop across the screen, to couple the two together, pwm pulse on and off.

I think this was the only AC calibration fixture in the Fluke lineup for quite a while, before they started packing features into the multifunction calibrators. So a great unit to have and the only one at that... (for hobbyist-level metrology geeks like yours truly!).

The resistor mentioned are the right ones to change, bias current from the + / - 300 volt rails. They  can be unsoldered carefully from the board top side, then use a solder sucker to clean the vias. Don't need to take the board / heatsink assy apart, but do form the replacement leads to standoff from the board, to allow air flow. The two power amp adjustments are output offset and quiescent current adjust. All in the manual, but suggest power off, connect probes, then power on again procedure to adjust, to avoid possible smoke :-).

This is very thoughtful, thank you. I've been cruising it a little apprehensively, though keeping the space this is in a little cooler (SoCal is currently seeing some pretty serious heatwaves). But I'm an absolute believer in overdesign (and, consequently, huge fan of eliminating under-design wherever identified.... so many instances in the late '70s and throughout the '80s at the onset of solid state). I'll likely have appropriate resistors coming next week and will do the replacement. But it's been almost constantly on since I started the thread and no more OVERLOAD flickering. Hope for the best!

You will need an card extender board for serious debugging, but found an Ebay seller in Korea making copies of the Fluke original. Seems unobtainium otherwise...

I just got a couple of versions of extender boards for the 5440A calibrator, maybe I shouldn't stop at that... I'll look into how the corresponding extender cards looked like, but if you have some info, I'd appreciate it.

[bdunham7]

I think this was the only AC calibration fixture in the Fluke lineup for quite a while, before they started packing features into the multifunction calibrators. So a great unit to have and the only one at that... (for hobbyist-level metrology geeks like yours truly!).

Oh no, there's another that matches your 5440A.

https://download.flukecal.com/pub/literature/1260112E_w.pdf

I just got a couple of versions of extender boards for the 5440A calibrator, maybe I shouldn't stop at that... I'll look into how the corresponding extender cards looked like, but if you have some info, I'd appreciate it.

Having worked on one once without extenders, I'll say that it sucks but it can be done. 

[alm]

I think this was the only AC calibration fixture in the Fluke lineup for quite a while, before they started packing features into the multifunction calibrators. So a great unit to have and the only one at that... (for hobbyist-level metrology geeks like yours truly!).

Indeed, the next calibrator to offer AC was the 5100 series, but it had higher uncertainties and didn't have the frequency range or flexibility (only discrete frequencies). It did go to 1 kV. The unit that replaced the 5200 was the 5700 in the late eighties. The 5200 remained in the catalog for a few years after the 5700 was introduced: it's in the 1994 catalog, but not 1996. I'd say the 5200's biggest limitation is that to go beyond 120V you need an amplifier that is even bigger and heavier. Later units by Fluke and others incorporated this amplifier, although the Fluke 5700 still needs an amplifier to go beyond 1 kHz in the 1000V range.

I just got a couple of versions of extender boards for the 5440A calibrator, maybe I shouldn't stop at that... I'll look into how the corresponding extender cards looked like, but if you have some info, I'd appreciate it.

I bought an extender on eBay years ago. It was quite crude, just a piece of routed copper-clad with an edge connector but no plating or chamfering on the edge connector, or solder mask. I think it's the same seller that's still offering the partial kit on eBay.

Oh no, there's another that matches your 5440A.

https://download.flukecal.com/pub/literature/1260112E_w.pdf

No, but that one could be used to calibrate a 5200A in a slightly overkill way. That's just a very fancy AC voltmeter. The 5200A was the last AC-only calibrator from Fluke. Datron continued longer with the 4200(A) and later offered versions of the 4708 configured for just AC volt/amps (no DC or resistance). It wouldn't surprise me if some of those were used with the Fluke 5440 since Fluke had no AC calibrator with the same specifications until the 5700. The Datron calibrators all had the 1 kV amplifier built in, sometimes optional. They have a better frequency range at high voltage than the Fluke 5700 without amplifier.

[Rax]

The unit that replaced the 5200 was the 5700 in the late eighties.

Isn't the 5700 a multifunction calibrator, though? Not ACV only. 

[Rax]

Oh no, there's another that matches your 5440A.
https://download.flukecal.com/pub/literature/1260112E_w.pdf

I think the 5790A(B) is a current model, no? Or maybe the started making the "A" version in the '80s?

Now that you're mentioning this model, I think I recall an "AC-only" calibrator, but it can't be the 5790A, nor the 5700 (the latter being a multifunction). Am I correct in this?

So which exact model for AC exactly matches the 5440A?

[alm]

Isn't the 5700 a multifunction calibrator, though? Not ACV only.

Yes, but it's the first unit that surpassed the 5200 in uncertainty. Fluke hasn't made an ACV-only calibrator after the 5200, or a DCV-only calibrator after the 5440. They've only made multi-function ACV/DCV/ACI/DCI/R calibrators since.

[Rax]

Fluke hasn't made an ACV-only calibrator after the 5200, or a DCV-only calibrator after the 5440. They've only made multi-function ACV/DCV/ACI/DCI/R calibrators since.

What about bdunham7's example of the 5790A/B?

[alm]

What about bdunham7's example of the 5790A/B?

I already responded to that. The 5790 is basically a 540 AC-DC transfer standard replacement:

No, but that one could be used to calibrate a 5200A in a slightly overkill way. That's just a very fancy AC voltmeter. The 5200A was the last AC-only calibrator from Fluke. Datron continued longer with the 4200(A) and later offered versions of the 4708 configured for just AC volt/amps (no DC or resistance). It wouldn't surprise me if some of those were used with the Fluke 5440 since Fluke had no AC calibrator with the same specifications until the 5700. The Datron calibrators all had the 1 kV amplifier built in, sometimes optional. They have a better frequency range at high voltage than the Fluke 5700 without amplifier.

[Swainster]

Took me a while to realise that I have some experience of this model. Not the same symptoms but may be something to bear in mind. In my case, my 5200A blew out the magic smoke  a while back. The problem turned out to be a number of shorted tantalum caps, I think in the reference assembly. Anyway, it was the pcba with the small transformers linking the sections together. A number of tants had failed - enough that I wondered if they were a symptom of something else. However, once replaced everything seemed to work fine 

Anyway, the pics show the obviously popped caps, but there were a few more inside the assembly.

Rax's recent threads have inspired me to tackle my 5440B. I've noticed that it has suddenly drifted by 60 ppm. It's obviously not a happy bunny.

[bdunham7]

No, but that one could be used to calibrate a 5200A in a slightly overkill way. That's just a very fancy AC voltmeter.

I've not had one on the bench and I never realized that they didn't include a source.  For what those go for that would have been a pretty big disappointment!

[Rax]

I already responded to that. The 5790 is basically a 540 AC-DC transfer standard replacement:

I understand your point now. I was of bdunham7's mind, I thought the unit includes a source... That's a very odd unit, form factor is distinctively "calibrator-like," but it's essentially a very sophisticated ACV. Looks almost deliberately confusing.
Thank you for helping to clarify that!

[tridac]

Restored a 5100B a few years back. Power amp rebuilt, microprocessor, ram, the front panel push buttion switches and more. Like the hifi separates thing, all in one boxes always seem like a compromise, whereas dedicated function units like the  5200 and 335 seem far more complete. One tool to do one job really well...

[Rax]

Further observation and run time doesn't seem to reveal any of those OVERLOAD events. So it's been solidly working for a few days now.

But one thing that's off is that at 100V out and 1MHz setting, the OVERLOAD light may actually come on - the output is unstable.

I think it's just a matter of the relatively high frequency output, which both stretches the design, and makes things like cables a bigger factor (I just switched to 50 ohm BNC cables and things seem to be more behaved).

[Bill158]

But one thing that's off is that at 100V out and 1MHz setting, the OVERLOAD light may actually come on - the output is unstable.

I think it's just a matter of the relatively high frequency output, which both stretches the design, and makes things like cables a bigger factor (I just switched to 50 ohm BNC cables and things seem to be more behaved).

One point here.  The 5200A manual specs, pg. 1-4 under "MAXIMUM OUTPUT VOLTAGE".  120 volts @ 100 KHz is the maximum output spec.  From there increasing frequency requires lowering the output voltage to 8.33 volts at 1.2Mhz.  I have already PMed RAX with this information.  My 5200A is able to get to 200 KHz before going out of regulation.  RAX's seems to go higher.  I am surprised that FLUKE didn't implement some sort of logic to prevent this condition like turning on the "OVERLOAD" LED when in this area?  But in thinking about this it would be difficult without uP control of some sort.
Bill

[bdunham7]

120 volts @ 100 KHz is the maximum output spec.  From there increasing frequency requires lowering the output voltage to 8.33 volts at 1.2Mhz.  I have already PMed RAX with this information. 

Not to mention the input specs on most meters don't allow 10⁸ V-Hz.  Oops!

[Rax]

My 5200A is able to get to 200 KHz before going out of regulation.  RAX's seems to go higher.  I am surprised that FLUKE didn't implement some sort of logic to prevent this condition like turning on the "OVERLOAD" LED when in this area?  But in thinking about this it would be difficult without uP control of some sort.
Bill

I've read - and still do - that page spec differently - I assumed there's a built-in attenuation doing that linear decrease in output to the 8.33 at 1MHz. The OL actually kicks in most of the time in that part of the range, though not all of the time.

[bdunham7]

I've read - and still do - that page spec differently - I assumed there's a built-in attenuation doing that linear decrease in output to the 8.33 at 1MHz. The OL actually kicks in most of the time in that part of the range, though not all of the time.

I assume you mean this figure?  I'd think it's clear they don't specify it for over 10⁷ V-Hz.  Your seems to produce 10X that on demand, but that can't be good.  I didn't catch that before, but you have to watch the meter input rating as well as 100V_RMS @ 1MHz is beyond the limits of most meters.

[Rax]

I assume you mean this figure?  I'd think it's clear they don't specify it for over 10⁷ V-Hz. 

You're probably right, but that's not how I read it during the RTFM process. I think the protection circuit provisions for this, but not sufficiently. I think Bill is right, they should have provided something on the controls side of things. On his observation 200kHz is stable on his unit - note p. 4-11 (table 4-7) sets the voltage for 60V @ 200kHz for a verification step - so they factored in stability mid way through that slope by design. But admittedly, my unit is stable much above that (I am leaning on assuming my protection circuit in a bit out of whack, actually), though I will not rely on it at those levels. Nor do I need do.

Your seems to produce 10X that on demand, but that can't be good.  I didn't catch that before, but you have to watch the meter input rating as well as 100V_RMS @ 1MHz is beyond the limits of most meters.

I guess I was too absorbed by testing the unit in all ranges (and couldn't believe it can output 100V at 1MHz!... so I really had to test its limits and determine the causes of instability - I thought - to trigger the OL actuation; other than design!) to really be mindful of the meters I expose to this stimulus...
At any rate, no meter has been damaged in the making of this hectic story.

[TheDefpom]

I did a 19 part repair series on one of these, every single board had faults.

[tridac]

Still going through that saga on mine, though most functions and levels now work. What faults did you find on the boards, and did you document it as it progressed ?...

[Rax]

I did a 19 part repair series on one of these, every single board had faults.

Interestingly, the first thing showing up in your video is loose parts inside. When mine arrived, there was at least a screw bouncing around inside. Upon opening the case and shields, I noticed there were more screws missing than present. For this unit, I think that's a major structural issue. Luckily, the packing job was fantastic so it traveled safely and there was no damage.

This far, I've found no real issue with the unit as far as I could tell.  I still have to really dive inside, but basic functionality is OK. May need to do more detailed checks to determine all that may be going on.

Well, I put that there too fast. As soon as I did it, I walked to the unit and turned it on. Which reminded me that, as opposed to what should happen (I think), the OVERLOAD LED doesn't come on (mind you, it does in an actual OL condition) for a bunch of seconds. What does happen is the STDBY/OPER LEDs both stay off for about a matching length of time (meaning, what I'd expect the OVERLOAD light to be on during startup), then STDBY comes on and I need to switch it over to OPER to get signal out. Really, more of a nuisance than anything (it seems to have no impact on any relevant functions of the unit), and the OVERLOAD light does come on when actually needed.

Also, the actuation of OVERLOAD during regular operation has not occurred for many days now. I'll assume it was more of a case where the unit needed be run for a while after sitting mostly in storage, and/or some learning curve of its controls and regime by yours truly.

A couple of questions on the issue still outstanding:

• Can anyone confirm this is not expected behavior? The non-lit OVERLOAD at startup, that is.
• Any initial assumptions what could be causing it? If this is not expected behavior, what's puzzling me to is why it's working correctly in an actual overload condition but not at startup. I likely have to investigate how those two different conditions are designed to actuate the LED

Thanks!

[TheDefpom]

Here is the playlist:

Fluke 5200A AC Calibrator Repair
https://www.youtube.com/playlist?list=PL2z3V9RkHQE0pdgzjmzo3Oa11TdnvxtE5

[Rax]

Here is the playlist:

Fluke 5200A AC Calibrator Repair
https://www.youtube.com/playlist?list=PL2z3V9RkHQE0pdgzjmzo3Oa11TdnvxtE5

I've had the opportunity to scratch about .1% of the whole thing (awesome stuff, btw), and I wasn't able to identify a specific video where the sequence is being confirmed. I could use a confirmation if anyone has a chance.

[Bill158]

A couple of questions on the issue still outstanding:

• Can anyone confirm this is not expected behavior? The non-lit OVERLOAD at startup, that is.
• Any initial assumptions what could be causing it? If this is not expected behavior, what's puzzling me to is why it's working correctly in an actual overload condition but not at startup. I likely have to investigate how those two different conditions are designed to actuate the LED

Thanks!

First read page 2-9 of the manual right hand side of page if you haven't already done so.
Mine flashes the "overload" light on during "power on" for about 1 second, goes out briefly and then very briefly flashes again for maybe 0.1 second.  I see that the manual does say that this can occur, however in my case it is not the 5 - 10 seconds in the manual.  I would assume that this is mainly dependent upon how the various power supplies come up during power on.  Mine does take the required 30 seconds or so for the "Standby" light to come on as stated in the manual.
Bill

[tridac]

I had a look at some of videos, much later model, 1986,  than mine, which  has 1979 date codes internally. Thing is that the psu assembly can be first level verified with no other cards in the box. Verify the voltages are correct and even dummy load the rails to make sure. In particular, if the power amp is faulty, that can kill the psu board, so taking it an assembly at a time can save a lot of time and effort. Take all the mainbox boards out and check for shorted tants (yes, a lot of them :-). Get the psu right first, then put boards back one at a time, far left reference board first and verify the voltage looks ok. Then the rest, power cycle between each. Leave the power amp until last and set the slide switch to debug, as it can be checked for stability in that state. Also, the other parts of the system can be checked out by removing the power amp and fitting a jumper.  Buying a bad one can be a really bad idea, if you don't have at least one spares donor for card swaps and robbing for spare parts. Far more complex beast than it looks and there are loads of dependencies and gotchas for the unwary. Lot of volts as well...

[Rax]

First read page 2-9 of the manual right hand side of page if you haven't already done so.

Of course! That's how I had an inkling I should expect the "OVERLOAD LED on at startup" behavior in the first place. Not a case of "RTFM," quite the contrary. It's the first time I have one of these at my bench.

Mine flashes the "overload" light on during "power on" for about 1 second, goes out briefly and then very briefly flashes again for maybe 0.1 second.  I see that the manual does say that this can occur, however in my case it is not the 5 - 10 seconds in the manual.  I would assume that this is mainly dependent upon how the various power supplies come up during power on.  Mine does take the required 30 seconds or so for the "Standby" light to come on as stated in the manual.
Bill

Thank you, this is perfect. It confirms I don't need to expect the solid light on solidly for seconds at a time. I think my unit essentially behaves expectedly, at least it's definitely doing the 30s delay to STDBY.

[Martian Tech]

Mine flashes the "overload" light on during "power on" for about 1 second, goes out briefly and then very briefly flashes again for maybe 0.1 second.  I see that the manual does say that this can occur, however in my case it is not the 5 - 10 seconds in the manual.  I would assume that this is mainly dependent upon how the various power supplies come up during power on.  Mine does take the required 30 seconds or so for the "Standby" light to come on as stated in the manual.
Bill

Thank you, this is perfect. It confirms I don't need to expect the solid light on solidly for seconds at a time. I think my unit essentially behaves expectedly, at least it's definitely doing the 30s delay to STDBY.

FYI: Mine operates similarly, but the initial overload light is less than 1 second - probably about 1/10 second, and then I get 2 additional flashes (one at about the 1 second mark and the second around the 4 second mark).  But 30 seconds(-ish) to STDBY.

[Rax]

FYI: Mine operates similarly, but the initial overload light is less than 1 second - probably about 1/10 second, and then I get 2 additional flashes (one at about the 1 second mark and the second around the 4 second mark).  But 30 seconds(-ish) to STDBY.

Knowing what to watch for now, I'm noticing some very quick flickers (not occurring at every turn on), something similar to what you're describing (which I may have missed before). Like Bill, I am assuming this accounts for some transitory conditions triggered by the power supply rails while coming to stable operation.

I think all is well. Both issues that concerned me are clarified and not a concern... Thank you very much for your assistance with this.

My plan from this juncture is to continue observing its hopefully continued stable and - as far as I can tell, at least "loosely" - within calibration operation. If things stay fine, I am planning to check all boards for signs of stress such as leakages (caps), burned areas on the PCBs, etc. - maybe as soon as this weekend - likely recap what needs to be recapped, replace some/most power resistors that are carbon comp, then adjust the power supply rails. Then seek to adjust the bias/zero of some stages that seem to need it. It's very possible the unit may arrive closer to cal after that.

If still needed, I'll examine options for calibration at that juncture.

[Rax]

After a s#!%load of measurements with four meters - two of which having cal data attached - (I may try to attach that data here later) there's some indication the calibrator is pretty consistently high by about 400-600ppm or so.

I feel that it's entirely possible the PS adjustments and - more importantly - the zero and bias adjustments have a great chance to throw this back into specified tolerance.

[Rax]

Getting that second look inside underway, I looked again at this handle inside the PS cage that I am not sure what the purpose for is. Can anyone shed some light?

The only position I can have it in is diagonally as the picture shows it, but that's a little unsettling to me, as it's so close to the can of some caps in the PS. It doesn't seem to rotate enough to touch, but it doesn't seem to be particularly locked in place by anything (other than the U-shaped extrusion affixed to the top shield.

[Rax]

A couple of further work items:

• The loose parts sliding around inside the case were a couple of pieces of shattered plastic cable tie, and apparently a small screw... Still trying to figure where the latter belongs. Re-affixing the cable under the shield with new cable ties required detaching the bottom shield for access to the holes.
• The VOLTAGE RANGE switch's contact actuation was a bit "wobbly" and was triggering some flickering of the OVERLOAD light, and would sometimes jump ranges. Pretty perilous thing as far as the health of this thing goes (and instruments hooked up to it...). Some careful cleaning and application of Deoxit with a foam swab later, it's now sliding smoothly like a well oiled machine

[TheDefpom]

The handle in the PS cage holds the cards in, it presses against the end to push them into the slots, I had to cable tie mine in place so the cards didn't fall out, as mine is missing the internal cover.

[Rax]

The handle in the PS cage holds the cards in, it presses against the end to push them into the slots, I had to cable tie mine in place so the cards didn't fall out, as mine is missing the internal cover.

That makes perfect sense, thank you. Had I taken those cards out, this would have been apparent to me, but I had not done that this far (only the cards in the main compartment). A little clunky a system, isn't it?...

[Rax]

BTW, once I started populating all screws around the case, shields, and everything, it became kind of clear either why the screws were taken out, or that the screws not being in caused some pretty serious flexing and abuse to the unit (maybe during ground transit to me). The whole chassis is a bit wonky (not quite square everywhere), and some of the screws don't quite find their holes seamlessly, but after some whining and complaining and once in, it snaps.

It will feel good to have all this buttoned up and squared as it was meant to be.

[bdunham7]

BTW, once I started populating all screws around the case, shields, and everything, it became kind of clear either why the screws were taken out, or that the screws not being in caused some pretty serious flexing and abuse to the unit (maybe during ground transit to me). The whole chassis is a bit wonky (not quite square everywhere), and some of the screws don't quite find their holes seamlessly, but after some whining and complaining and once in, it snaps.

It will feel good to have all this buttoned up and squared as it was meant to be.

After working on a dozen or so of these various older Fluke monsters (and having a few to go) I feel that about half of the repair job is similar to doing auto body work.  Maybe someone could set up a mini frame and paint repair shop for electronic equipment.

[tridac]

That lever is to hold the psu boards in when the top lid is in place, stop them falling out with vibration. Also, use it to press the boards into their socket, which can be difficult otherwise. Might seem crude, but works..

Chris

[Rax]

After working on a dozen or so of these various older Fluke monsters (and having a few to go) I feel that about half of the repair job is similar to doing auto body work.  Maybe someone could set up a mini frame and paint repair shop for electronic equipment.

Seriously. In the case of this one here, after a handful of (different kinds of) screws (not kidding!) from a eclectic and unrestrained McMaster Carr order, it finally feels solid and chunky, like it could stand a... diagonal trip over the entire territory of the US! There were screws missing from more parts of it than I ever thought (before diving in and looking closer at some hidden parts...).

All in all, I'm am extremely happy and grateful camper (none of the above is a complaint to any stretch, quite the contrary). The unit works much better than I imagined, and the only thing I can fault it for is a slight out of cal output (about those 600ppm high). 

[tridac]

I know it's a heavy lump. but Fluke are good at lightweight aluminium consruction, with each chassis section a stressed member. They are actually quite rigid when all the panels are on and all the screws in and tight. Check by resting the unit on the back corners of the case, then lift one front corner and see how much it twists. As an engineer, quite respect that and the isolated internal chassis work...

[Rax]

I know it's a heavy lump. but Fluke are good at lightweight aluminium consruction, with each chassis section a stressed member. They are actually quite rigid when all the panels are on and all the screws in and tight. Check by resting the unit on the back corners of the case, then lift one front corner and see how much it twists. As an engineer, quite respect that and the isolated internal chassis work...

I agree. I think, if you compare it with the later units, it's a very eclectic mix of structural, functional, and esthetic solutions. But very solid once all together. And essentially all parts converge to the whole - meaning the shields fully screwed in will serve structural duty also.

As a matter of perspective, it's also quite interesting how rationalization occurred over time with the Fluke units - they became simpler, having fewer types of screws, etc. At least in my experience.

[Rax]

R40 and R46 replaced with Vishay Dale RS05 (5W WW) - solid as a granite slab. One of them was drifted by about 5%. Maybe not worrisome, except the other one was dead on and assuming same production batch, that's a bit off.

I also attempted to replace the electrolytic caps on A7, but hit the "desoldering brick wall of the massive ground plane" and decided to back off... I'll reassess later if they show serious signs of aging.

I also had a bit of a false flag (I think - after a good chunk of checking and double checking) where the third VOLTAGE switch would give me a persistent "4" on a "0" setting. Due to its intermittence, I assumed the switch may be faulty (not the logic circuitry), cleaned it up with Deoxit, and it seems to be behaving now. I'll keep an eye on it. I think all these will benefit from a serious switch reconditioning.

Third picture shows the resistors adjusted a bit to clear the "HV cage" - for heat and whatnot. Good sleep factor.

For now, I think this is possibly ready for adjustment...

[Rax]

I thought I'll be able to start adjustments tonight, but I am not sure I understand the Power Amplifier DC Zero Calibration (4-75, p.4-17). It mentions using an 887A etc., all sounding unnecessarily complicated - I assume the adjustment just makes sure there's zero DC on the output between OUTPUT HI and LO - is that right?

The instructions involve removing INT SENSE and whatnot and I'm not sure any of that is necessary.

[Rax]

I went ahead and did:

• PS adjustments - the 15V was at 15.045V (out of range), adjusted to 15.000V. TP3 on A7 showed 186.70V at that point, compliant.
• Power Amp bias was at .27V - adjusted it to .35V
• Power Amp zero DC was at 222uV to start with, adjusted to 0.000mV on my Fluke 189. I think floating this measurement is likely beneficial. Very finicky adjustment, having the shields on seems critical.

Tomorrow, I may endeavor on adjusting the actual gain and levels.

[Rax]

If anyone's wondering, doing these adjustments didn't change anything discernable in the levels. They still are just about 600ppm high.

There goes my theory!

[tridac]

Neat job on the two resistors, would not know you had been in there. Used metal oxide 1% on mine, good enough. Had considered replacing  the other 2 w comp resistors on the board, but they didn't look overheated, and values were about right. As for the psu electrolytics, replaced all the wire ended types with Vishay 105C as a precaution, but they tested ok as well, off the board. The screw teminal types checked out ok on the cap meter, and they are super expensive, even if you can find them. You can also scope the ripple at the electrolytics, as a check. Mine has to come out of the rack again soon, but too much other stuff on the bench at the mo.

One other thng to check is the reference board at the far left. There is an adjustment that sets the initial dc reference voltage and there are two test points for a voltmeter. Correct voltage value is in the manual, but that could be the cause of the output arror, as it's the dc reference for everything else that follows...

[Rax]

Neat job on the two resistors, would not know you had been in there.

Thank you, tridac!

One other thing to check is the reference board at the far left. There is an adjustment that sets the initial dc reference voltage and there are two test points for a voltmeter. Correct voltage value is in the manual, but that could be the cause of the output arror, as it's the dc reference for everything else that follows...

Will look into this.

I wonder if anyone would have some insight on performing the adjustments. I don't have a 540B - I'll have to rely on a couple of calibrated meters (meaning - long form calibration available at hand) - namely a 8502A and an 8846A. I'll definitely rely mostly on the 8846A, though I have a hunch the 8502A may be a bit more wide bandwidth.

I also have a Prema 6048, HP 34401A and a couple other meters, and the Prema seems very accurate and maybe a bit more digits.

I also have a 931B that passes its "operational tests" but I have no idea of its state of calibration. But maybe as a differential instrument, that's not so critical?...

I guess I'm looking for a sense of which adjustments may be more important (and what specifically they do, for those with good experience with the unit; maybe even recollection of a recent cal adjustment procedure?....). Otherwise, I may try them one at a time (section 4-76 in the Manual - if not available to most, I can snip and post here), and continuously check where the output lands on the characterized meters. Obviously, checking the output at higher frequency is nearly impossible with what I have.

[alm]

I wonder if anyone would have some insight on performing the adjustments. I don't have a 540B - I'll have to rely on a couple of calibrated meters (meaning - long form calibration available at hand) - namely a 8502A and an 8846A. I'll definitely rely mostly on the 8846A, though I have a hunch the 8502A may be a bit more wide bandwidth.

I would only perform adjustments if I think I can do better than its existing state. Depending on how much adjustments interact, it may well be that adjustments need to be made across the bandwidth. So if you could only do a reasonable job until 10 kHz, that might well mean the 10 kHz - 1 MHz part will get worse. Calculate for every instrument you have the absolute uncertainty to national standards based on their last calibration, the calibration uncertainty and the specifications for drift since then (e.g. 90 day or 1 year accuracy), and see how that compares to the specs for the 5200 and the current estimated deviation of 400-600ppm. If not, then how would you know you're not making it less accurate?

Unless your only goal is consistency, not absolute accuracy. Then by all means just twiddle trimmers until the display of whichever meter you like reads all nines or zeroes. Ideally also adjust all but that one meter to match the others.

Unfortunately AC calibration without spending a lot of money is hard. Pretty much everything, including a Fluke 540B, needs calibration across its bandwidth and ranges. There are ratio transformers that can help, but even those have a roll-off at higher frequencies. The traditional procedure involves a set of thermal voltage converters (TVCs). One of them is calibrated across its bandwidth, and that one is used to calibrate the next TVC value up or down, until you are at the highest and lowest values. A very tedious process even if you have all those TVCs and supporting equipment like switches. The modern way would be to throw money at something like a Fluke 792A or 5790A/B and spend a fortune in getting it calibrated. Getting a DMM with good ACV specs and BW is probably the most affordable option. The Fluke 8506 is known for its good AC performance, but rarely cheap. I didn't check how the P6048 compares.

[tridac]

You already said that the output is off by 600ppm, from memory, so what confidence do you have about the accuaracy of that ?  If you have several meters, different vendors,  that agree with each other within a few microvolts, it's reasonable to assume that they are all close to accurate, but leaving out any outliers, take an average of all those meters ot get a reasonable result. Not everyone has the luxury of a calibrated hp3458, so those without cal lab facilities have to rely on the best they can do at the time. As for the null voltmeter, that sort of kit was pre high input Z dvms, which make loading errors ignorable in most practical cases, especially when tuning  for a null. Just use a dvm as the null voltmeter. A good 7.5 digit dvm like the 3457 will resolve microvolts in 10 volts, much better than any of the early Fluke analog null voltmeters. Measure the reference anyway, as the output impedance at those pins is low and unlikey to be affected by a dvm load. At least you will have an idea if it's close to what it should be...

[alm]

Not everyone has the luxury of a calibrated hp3458, so those without cal lab facilities have to rely on the best they can do at the time.

That's very true, but sometimes the best you can do may be to not touch any adjustment.

As for the null voltmeter, that sort of kit was pre high input Z dvms, which make loading errors ignorable in most practical cases, especially when tuning  for a null. Just use a dvm as the null voltmeter. A good 7.5 digit dvm like the 3457 will resolve microvolts in 10 volts, much better than any of the early Fluke analog null voltmeters.

I'm not sure where this null voltmeter came from. I didn't see it in the F5200 suggested calibration equipment list. It used to be common to use a Kelvin-Varley divider + null meter for precise voltage measurements before long scale DMMs became available. You can substitute a good DMM here, though keep in mind that the F720A has a linearity specification of 0.1ppm of input, so while operating at ratios between say 0.1-1.0, it will likely be more linear than your DMM if that's important.

As for replacing a null meter by a DMM. Fluke has a white paper about it where they try to sell their DMM over null meters. There are two caveats to be aware of. One, adjusting a trimmer for null on a DMM is extremely tedious. I have done it trying to adjust a Fluke 720A, and because of the sign flip it's so much harder than with an analog meter movement. Instead of adjusting in a fluid movement, it's making small adjustments and waiting for the display to settle. Even 'analog' bar graphs are useless around 0, or if you're working on uV levels. The second consideration is bias current. Depending on the impedance of the two nodes the null meter is connected to, bias current may be a big problem. See for example this thread on replacing a null meter with a nanovoltmeter or DMM and the linked post of Dr. Frank describing his experiences with using a 3458A as null meter with a Fluke 752A Hamon divider and the errors this introduces.

[Rax]

I'm not sure where this null voltmeter came from. I didn't see it in the F5200 suggested calibration equipment list.

From the Service Manual, Section 4-30 Output Accuracy Test - Voltage Ranges, but it has nothing to do with DC nulling. The 931B (which is an exclusively AC differential voltmeter, and which I have, as I mentioned prior) is listed as an alternate instrument to the 540B.

DC nulling the power amp - I have no issues doing it with a regular DMM; this just balances the amp to have no DC out, as far as I can tell, so no reason to employ a KVD or anything anywhere near that level. Unless I'm missing something, but I've nulled many instruments before and it's a very cursory operation.

If you review my prior posts, the PS adjustments, power amp bias and power amp DC zero were all done a couple of days ago. And as I said after that, this regards Section 4-76. Let's please focus on that. And thank you very much tridac and alm for thinking on this with me.

[Rax]

To the inquiries on what I have available to dial this in - I attach a spreadsheet with the equipment at hand that I used to gauge how well the 5200A is doing, and some measurements.

Namely:

• An 8502A that was relatively recently adjusted and then "Z540-calibrated" (unfortunately, no uncertainties provided) in 02/2023.
• An 8846A that was calibrated by Fluke at the end of August. Unfortunately, they also replaced all the guts, so the internal reference is probably a little green for "certainty" (is this a pun?...). Not sure if it impacts the ACV accuracy, but it's likely my best reference to assess the accuracy of the 5200A. I enclose its 17025 long form cal data for ACV.
• A Prema 6048 which I have a fairly good level of trust for.
• A recently acquired 34401A, which I have no strong reasons to trust, but no reason to distrust either. I look at it as a fourth opinion that can reinforce the first three, and doesn't get disqualification rights over anything.

[Rax]

For the 8502A, the "calibration result" is what the cal house said it'd tell on standard levels the lab would apply to it.

[Rax]

On my measurements spreadsheet, I recommend focusing on the mid range of the Hz spectrum (100Hz to maybe 20kHz) as, obviously, that's where a meter like those used would have something useful to say.

I will have access to a Keysight-calibrated 3458A/002 for a weekend at some point in the next few weeks or months, but that's a thing I need to plan the heck out of, and am not sure it'll serve this purpose. I don't think the ACV performance of the 3458A is amazing, particularly at the ends of the frequency spectrum... or is it?

[alm]

So what you need to do to calculate what uncertainty relative to the national standards you could achieve, for the meters that were calibrated, is like described in the GUM sections 1-6, identify all the terms contributing to the uncertainty. Start with the value at the time of calibration:

• If you have data, then say at the time of calibration the value was the measured value plus the offset of their stimulus from nominal. It might be that the calibrator was not sourcing 1.0000 V but 1.0001V. See the first paragraph I wrote here for how to correct for that. The uncertainty at this time was the stated expanded uncertainty, which will likely be a 95% confidence interval, so divide my 1.96 to get the standard error.
• If you don't have the data, you should assume the meter was at that time at its nominal values but with the uncertainties it was adjusted to (1 year? 24h? it might say on the certificate). Ideally with some guard banding, like if they say they adjust if the value is beyond 80% of the tolerance band, then you can assume the value was within 80% of that value. Calculate the absolute value in Volt for a power of ten value near the top of the range (e.g. 1 V) reading by multiplying the value by the "of value" specification and adding the "of range" specification multiplied by the full range (check datasheet, could be 10V, or 12V or 20V for example). This should be treated as a rectangular distribution as described in section 4.3.7 of GUM, so the standard error is \$\sqrt{a^2/3}\$.

This calibration was some time ago, so apply the stability or accuracy specification (90 days, 180 days or 1 year depending on how long it's been any what kind of different specifications the instrument has). Convert to absolute volts (not percentages or ppms) as described above. This should be treated as a rectangular distribution as I described above.

The calibration was at a certain temperature that should be on the certificate, and the 90d etc stability / accuracy specifications will say something like within 5 degrees of the calibration temperature. If the ambient temperature at time of measurement is outside this range, apply the temperature coefficient from the specs multiplied by the number of degrees outside the range. This should again be assumed to be a rectangular distribution.

Then calculate the geometric sum (root of sum of squares) of all these components, and multiply by the coverage factor (k=2 is recommended by GUM). This is an estimate of what kind of uncertainty you might achieve, though the actual uncertainty will be slightly higher because you also need to add the standard error of the mean of the measurement as a factor, although this can be reduced by collecting more samples.

Do this for your 8502A and 8846A for every range and frequency. You could do this in Excel or Jupyter notebook with Python and Pandas. What I did for MM2022 was to make a table with all the calibration certificate data, one with the datasheet specs, and then had a function that looked up the correct value in each and did the calculation. You could do the same in Excel with VLOOKUP etc. Regarding the 8846A with replaced guts, it should still be within the manfucturer's specifications, but might drift more than a couple of years from now.

For every range / frequency I'd pick the meter with the lowest calculated uncertainty. This might well be the same meter for all. As a sanity check, the intervals of meter1_result +/- meter1_uncertainty and meter2_result +/- meter2_uncertainty should be overlapping for every value. The 34401A can vote but especially for the higher frequency AC ranges I wouldn't attribute too much importance, since I believe those are the most frequent to drift out of tolerance. Compare this expanded uncertainty to the accuracy and stability of the 5200A and to the offset that you measure now. The stability of the 5200A is like a lower limit: there's no point in setting the 5200A down to 1 uV/V if the 24h stability is 100 uV/V. The ~400-600 uV/V offset is the higher limit: If you measure the 5200A to be high by 400 uV/V, but the uncertainty of your measurement is 800 uV/V, then it might was well be 400 uV/V low and you wouldn't be able to tell from the measurement. Ideally you aim for the accuracy specifications in the data sheet, but otherwise you'll need to note that the uncertainty is higher than in the data sheet for those specific ranges.

You can do the same math I described above for the 3458A. The 3458A isn't really good enough above 100 kHz or 300 kHz to calibrate the 5200A, but it might be the best you have available. I would first focus on using it to measure the most stable standards you have, probably the 5440A. Depending on how long ago it was calibrated, the uncertainty might be about 4 uV/V for 10 VDC. If you plan any adjustments (like the 5200A) with the 3458A, I would rehease the adjustments with one of your other meters before you get the 3458A and investigate the best settings for the 3458A, since just punching the 'ACV' button may not give you the best results.

I see 0V measurements in your spreadsheet. I don't think measuring 0 VAC is of any use since multimeters are usually not specified below 1% or 3% of full scale. I don't think the 5200A performance verification procedure calls for a 0V AC measurement either.

[Rax]

So what you need to do to calculate what uncertainty relative to the national standards you could achieve, for the meters that were calibrated, is like described in the GUM sections 1-6, identify all the terms contributing to the uncertainty.
[...]
I see 0V measurements in your spreadsheet. I don't think measuring 0 VAC is of any use since multimeters are usually not specified below 1% or 3% of full scale. I don't think the 5200A performance verification procedure calls for a 0V AC measurement either.

Thank you very much, this is great, detailed, and very thoughtful information. Very much appreciated!

Not sure where are the 0V measurements. If you mean the value provided by the 5200A, I had it output .001V, .01V, .1V, 1V, 10V, and 10V. The low levels more of curiosity - I do think it makes sense to read useful, conclusive stuff only from the relative high levels 1V, 10V, maybe 100V), and the middle of the frequency spectrum 100Hz to, say, 20kHz). In a couple of cases, I had to output 3V or 7V, because that's what they did during calibration and I wanted to match their stimulus. Same for the frequency, at times I had to do other frequencies than what I started with.

Sometimes the reading would be zero - such as when maybe the frequency would be too high and the level too low etc., or it'd be "OL" if I was not careful enough to stay within the design limits.

But I don't think I applied "zero ACV" in any of the cases saved in the spreadsheet.

[alm]

But I don't think I applied "zero ACV" in any of the cases saved in the spreadsheet.

Ah, I see, due to the column width Excel was truncating 0.001 to "0" instead of "0.00" or some other clue that something is being truncated. Excel is such a wonderful product

[Rax]

But I don't think I applied "zero ACV" in any of the cases saved in the spreadsheet.

Ah, I see, due to the column width Excel was trunctaing 0.001 to "0" instead of "0.00" or some other clue that something is being truncated. Excel is such a wonderful product

Yup... I use it as a google sheet, where all that's better handled. But the columns may need to be expanded here and there to read the values listed. LibreOffice has a little red arrow, I think, where a value is being truncated.

[Rax]

Well, I guess I am finding an (intermittent!) issue with this.

Sometimes, when I switch to, say, a 1.000000V out, I'd get that with a jump up by four units on the second level control (namely, 1.040650V or so). Increasing that control higher by one unit would return as below (for clarity, I am listing all relevant values going up).

• 1.010000V -> 1.050650V
• 1.020000V -> 1.060650V
• 1.030000V -> 1.070650V
• 1.040000V -> 1.040650V
• 1.050000V -> 1.050650V
• 1.060000V -> 1.060650V

and so on.

So this "hopping by four" would stop at the "actual four" and it'd be correct from that point on. This gets replicated exactly on all ranges etc. It seems to be caused by the logic generating that specific digit.

Not sure what can cause this yet, but looking at the schematic next. I did clean the corresponding switch with Deoxit (not very hopeful it'd fix it, due to the symptoms), to no avail.

[tridac]

Ther reference voltage comes from a pwm signal generated by a programmable 1-2-4-8 input bcd counter chain, driven by a 10MHz clock. The pwm is low pass filtered to provide the dc reference voltage.  Sounds like one of the switch to counter chain inputs is at fault. You need the extender board, but just trace the logic levels from switch onward, for the offending digits. If you have good meter, can also measure the reference board outut dc for correctness, as you change the switch settings, which should be completely monotonic. If not that, then  it may be in the sine wave oscillator / rectfier board group...

[ken goodhew]

the purpose of the handle inside the power supply assembly is simply to push the cards into the slots on the motherboard,otherwise it is held out of the way by the bracket in the top cover

[Rax]

Ther reference voltage comes from a pwm signal generated by a programmable 1-2-4-8 input bcd counter chain, driven by a 10MHz clock. The pwm is low pass filtered to provide the dc reference voltage.  Sounds like one of the switch to counter chain inputs is at fault. You need the extender board, but just trace the logic levels from switch onward, for the offending digits. If you have good meter, can also measure the reference board outut dc for correctness, as you change the switch settings, which should be completely monotonic. If not that, then  it may be in the sine wave oscillator / rectfier board group...

Thank you for this input. I've been focusing on other projects, though the 5200A has been very useful to even one calibration check this far (with a calibrated meter characterization). The "+4 on second digit" persists, but is occasional. If I get a chance, though, I'll seek to further investigate. The other roadblock is I don't have an extender board for this, and I didn't look for them extensively, but I imagine they may be pretty hard to find.