Another Fluke 5440B/AF Repair

[essele]

After a break from all of this volt-nuttery I got a bit carried away on eBay and got myself a really beaten up 5440B ... it's definitely time for a project!

It's in a bit of a state ... it looks like it may have some water damage (or at least corrosion due to high humidity), plus there's a huge amount of grey dust in there, not sure if this is just dust or whether it's the result of some burning -- I haven't inspected many of the boards yet.

I've only done an initial power on test and it powers up, there was something on the display, but the fan wasn't running and there was a noticeable whine which sounded like it was coming from the main transformer (is that normal?) ... so I switched it off again pretty quickly and decided I should clean it up and at least replace the fan before going much further.

To get at the fan (and mains filter) I've removed the back panel and the A19 Outside Guard Terminator PCA (which seems to be just a set of unregulated power supplies) ... on the board there was an area which was quite darkened, mostly around CR2, CR3, and F2. Some tracks on both sides were missing their solder mask and I think the plastic coating that would have been on CR2/CR3 appears also to have burnt off! F3 had some damage to the sheath underneath where it was close to a track which appears to have overheated.

All of the fuses are intact and don't look like they have been replaced (they do now because I messed with them to apply some new solder mask underneath -- and looking at the photo I need to redo my soldering!)

So I am a little confused ... CR2 and CR3 feed the unregulated +5V supply, both are fused with 5A fuses, both diodes appear to be ok. However something has caused them and the PCB tracks to get very hot but it hasn't blown the fuses! R2 measures fine, and C2 seems to be ok from an external timed charging test.

I'll obviously check the voltages once I get it all back into the chassis.

Any thoughts?  Is this just to be expected with age? Should I replace CR2/3 and the fuses?

This is probably the simplest board in there --- this is going to be a long journey ;-)

Thanks,

Lee.

[dietert1]

That design is from a past when engineers thought human ingeniuty could surpass physical laws. Those diodes have no other cooling than the board. They last but the board won't, pretty obvious. I would remove the diodes and replace them by shottkys in TO220 with a clip-on cooler. Temperature will be a lot lower.

Regards, Dieter

[essele]

Thanks Dieter. I was thinking about excess current heating the traces rather than simple heat transfer from the diodes -- doh!

I've done a quick test, at 3A those diodes have a Vf of just over 1V, some others (schottky) I have to hand are down at 0.8V, and I've ordered a couple that should be under 0.6V .. they should bring the heat generated down fairly significantly without me trying to bend TO220 leads and fit heatsinks in.

I've cleaned up the the boards in the rear section as well as the two filter boards. I've fitted a new fan (along with a 3d printed fixing bracket) and mains filter. I'm waiting for an inline connector before I can connect the fan.

I have powered it up again ... the front display just says "Fluke DV Calibrator" and "Front Panel Running", the cursors seem to work but no other key does anything. There also seems to be a somewhat inconsistent set of red LEDs lighting up on the various boards.

A first batch of caps ordered, and on to check voltages...

[MegaVolt]

You have to make sure that all 4 diodes on the A16 board go out one by one. If this is not the case there is an error on board A16. Until you fix this there will be no communication with the front panel.

[essele]

Thanks MegaVolt ... that's helpful.

I've checked the outside and inside power supplies and most are ok. The outside +28V is actually +24V but I'm going to hold off adjusting anything until I've recapped that PCB.

The inside regulator has two failed rails TP7/8 +30V (measured -0.9) and TP11/13 -15V (measured +0.9) ... a quick look makes me suspect the LM317 and 7915 respectively (although I'm not sure I understand how the polarity could be wrong), the caps look ok, although I'll replace all of them once new ones arrive. I doubt any of that is related to the A16 issues you mention.

Quite worryingly I get different behaviours each time I turn it on ... sometimes all 4 LEDs go out and stay that way (still with the same front panel behaviour), sometimes it does that but then seems to get into a reboot cycle every 5 seconds or so, other times different LEDs don't go out.

I'll get all the power rails sorted and some of the recapping done and see where that gets me.

[MegaVolt]

Quite worryingly I get different behaviours each time I turn it on ... sometimes all 4 LEDs go out and stay that way (still with the same front panel behaviour), sometimes it does that but then seems to get into a reboot cycle every 5 seconds or so, other times different LEDs don't go out.

The A16 board has a watchdog that receives reset signals from the A2 board. If for some reason these signals do not come, the board A16 resets again.

Also pay attention to the LED on the A2 board it goes out after the internal checks of this board are passed.

For a successful operation of the digital part you may remove A4...A10 and A14. Everything should work and show a communication error with board A14.

[essele]

Thanks again MegaVolt -- really helpful, and I now have some good progress!

I re-examined each of the digital boards and found quite a crusty almost calcium like build-up on many of the fingers on the edge connectors, so a careful scraping and cleaning of each of them and I now have some progress ... it seems consistent, the LEDs go off, the front panel now says "power supply fault", "check guarded power" ... and I already know I have two rails on that with problems ... so this is great!

Thanks!

[MegaVolt]

"power supply fault", "check guarded power" ... and I already know I have two rails on that with problems ... so this is great!

This error is for the A10 board. It must be inserted back into the unit.

[essele]

This error is for the A10 board. It must be inserted back into the unit.

Yes, thanks, it is back in ... and it's the one with the two failed rails, I'm just waiting for an LM317 as I don't seem to have any.

[MegaVolt]

Yes, thanks, it is back in ... and it's the one with the two failed rails, I'm just waiting for an LM317 as I don't seem to have any.

That's all that could be done so far. We have to wait

[essele]

OK .. some great progress.

I replaced the LM317 and associated caps, however the behaviour seemed the same ... more investigation revealed that with no other cards in both "failed" rails seemed to be ok, as soon as you put any other card in they dropped to around 1V. I couldn't find any obvious shorts.

It turned out to be related to the crap on the card edges again ... a good clean of the inside regulator and the +30V rail was all ok, not sure I actually needed to replace the LM317, but no harm done. After cleaning the -15V rail was now at -20V and causing the error LED to remain lit (I did a load of comparator and logic probing before I went back and remeasured the voltages) ... so I replaced the 7915 and 22uF cap and it came good, the LED went out, and front panel was giving board errors (because they weren't in there!) ... so power problems seemingly solved. (Still a load of re-capping to do!)

The next error was "board ack fault", "check output board" ... so I cleaned the edge connector for A4 which (a) needed it, but (b) was the wrong board ... should have RTFM and cleaned A5. Once A5 was cleaned up I get the "VOLTAGE MODE" message and it seems to be getting through the power on sequence ok!

I tried a 1V output and got 34V (I think from memory) quickly followed by an over-voltage warning. But I still have 4 cards to properly edge-clean so I'll wait to do that before going much further.

There were a few relay clicks, which from how they look is a miracle ... I think they will need quite a bit of attention.

Thanks for the help so far ... assuming I can get this back to a working state does anyone have any ideas about replacement key caps?

[essele]

I think I'm going to have a major problem with the relays ... they all look badly rusted.

I've just opened one and cleaned off the worst of it with IPA, it all moves ok, but there's a lot of corrosion I just can't see these all working to a satisfactory level and I know proper replacements are prohibitively expensive.

Is there a reason that immersing in IPA is not recommended? I think that doing that followed by a careful contact cleaning is my best bet at this point?

Otherwise I'm going to be looking at replacing these with custom PCBs and lower cost alternatives (I certainly don't have the patience to do all the wiring detailed in the link below!)

I have seen both of these posts which are helpful...

https://www.eevblog.com/forum/metrology/replacing-relays-in-a-5440b-or-5440baf/
https://www.eevblog.com/forum/metrology/fluke-5440b-_repair_/

[essele]

When I said "it all moves ok" that was from manually prodding, when powered it was only moving a small distance and not properly making contact. I've had to scrape a load of crap out from between the magnet and the plate that it attracts (not sure of the proper terms) and now it seems to be properly making contact.

Everything is covered with that brown dust so I really feel like I need to soak these in IPA to have any chance of getting rid of it.

Should I just give up and find another solution or are these salvageable?

[dietert1]

I suspect the dirt you see is from the previous use of some kind of deoxit.
Don't get desperate, i've seen worse. You can certainly salvage the relays, but i would try to keep the coils dry, as the wire is really thin and you would not like a solvent cause shorts.
As far as i remember the contacts can be unscrewed (after taking some pictures!!). Then use a toothbrush with soap. When everything is clean and dry again, it should work.
In the future, don't use deoxit, but some software to exercise the relays regularly. Same when beginning measurements on the Advantest R6581T. For 10 V DC measurements, as far as i understand it has three relays that need exercise: One for front/rear, one for high voltage ranges and one for ohms ranges.

Regards, Dieter

[MadTux]

Is that the one that was on ebay for $699??
Kinda lucky I didn't pull the trigger on that one, corrosion/water/fire/smoke damage are the worst.

I'd fully disassemble that instrument and put all the parts into the dishwasher, except the transformer and maybe the reference board, the LM399 inside my Keithley2010 didn't care about the dishwasher though, still agrees to my other K2015/HP3456As within a few ppm.
Then use compressed air to blast away all moisture directly after dishwashering, so a minimum of salts remain on the boards, at least that's what I usually do in such cases.

Relays actually look like they've spent some time partially filled with water, looks a bit like first smoked by fire, finished with water from firefighting, discarded, salvaged/dumpster dived and put back on ebay :-)

[essele]

Hi MadTux ... yes, although I got it a bit cheaper ... he was willing to accept a fairly low offer, makes sense now as I think your summary of fire/firefighting/resale is about right!

I've managed to completely wreck that first relay ... got it working reasonably well with one pole higher resistance than I'd like and one not working, but then tried the "removing the screw" option after which I found it completely impossible to reassemble in a working way and eventually broke the coil terminations.

That said, even with a missing relay, it's partially working ... output above 22V seems to be fine ... checked a few spots between 25V and 400V and all seem ok.

I can't believe many of those relays are going to be functioning properly (although some of them must be at least partially working!) ... so I think I'm going to have to do something with them first before trying to diagnose much further. I've designed a PCB that should allow me to mount three DS2E-S-5VDC's (or similar) in the space of the normal 6PDT relay (using the same approach as @wn1fju) but without the painful wiring. If all goes to plan these should fit into the existing sockets. If that works I can also look at the other configurations. I'll slowly work through recapping while I wait for the PCBs.

I'm quite nervous about the dishwasher option, any gotcha's I should be worried about (apart from the wife moaning)

[Kleinstein]

The normal detergent used in the dishwashers is quite aggressive to aluminum. So it would not be a good idea to use it with the AL electrolytic caps.
It also takes some time to get the dishwasher clean afterwards - there often remains some of the dirty water in the sump. So do some rinse with extra water and pump empty from the machine.

One may as well to a manual wash in the sink with warm water,  more normal detergent and a brush. Then follow with quite some clean water and air to blow dry and maybe than some IPA. After a water damage cleaning with clean water is definitely a good ideal, as some of the dirt can be water soluble and not alcohol soluble. Some parts also may need just the mechanical action.

[TheSteve]

Nothing causes more grief in a 5440B than those relays, even when they look pristine. I would plan to replace them all.

[essele]

Nothing causes more grief in a 5440B than those relays, even when they look pristine. I would plan to replace them all.

I have seen quite a few painful stories about the relays ... if my PCB/off-the-shelf relay approach works then I'll plan on doing a complete replacement ala @wn1fju.

In the meantime to allow me to get a bit further with fault diagnosis I've figured out a reasonable routine to get the existing relays cleaned up a bit, I'm breaking all the rules, but it's a choice between just throwing them away or having a go!

Bear in mind that almost all of the relays I've looked at so far are not even able to move enough to make physical contact when applying 5V, let alone low resistance electrical contact.

So I give them a 2 minute clean in my ultrasonic cleaner but in a small container of IPA ... this is generally good enough to start them properly moving again. At this point there are usually some good contacts and some bad ones.

I gently scrape (caress?) the relevant contacts with a scalpel (although I seem to spend more time cutting myself at the moment) and repeat until I get sub 1R resistance on everything.

Then I setup my PSU to do 3 second on/off cycles and monitor the resistance as I've noticed a few creep back up again if they aren't properly clean.

If all else fails then a squirt of contact cleaner and another 2 mins in the ultrasonic/IPA seems to do the job. It's a bit painstaking but it seems to be getting there.

Probably not a good thing to do to ensure long life, but as per the above these things are a nightmare anyway!

Photo attached ... three done so far!

[Kleinstein]

With the relays it is not just about getting good contract, but also possible problems with leakage currents. So a cleaning step with clean / demineralized water at the start may be a good idea to remove salts.

To protect the iron parts one may consider wetting the iron part at the magnet with some oil. A fully degreesed iron core may start to corrode again rather quick.

[wn1fju]

I am the poor soul who replaced all of the relays a few years ago in my 5440B with a set of 62 DS2E miniature sealed relays.  I am happy to report that the 5440B is still working just fine.  And the best part is that I don't hear big "clunks" when changing 5440B settings.  All I hear are very tiny muffled "clicks."

With the original relays, I tried all kinds of cleaning fluids.  I also tried a "mechanical" cleaning with a burnishing file.  I also tried "electrical" cleaning with a few hundred mA across the contacts.  Although things would improve for a few days, the problems would always come back.

I also have a Rotek 320 multi-function calibrator.  Same problem, same attempt at repair, and finally, the same solution - replace all (21) relays.

So it has been my experience that once calibrator relays start to degrade, they really can't be brought back to acceptable functionality.  Perhaps others have been successful, but I haven't.

[essele]

One step forward and one step back .. I ran the self tests, the digital one passed, but the analog and HV ones both failed, however I think the HV one has done some further damage as I now can't get any good output on any ranges.

I've finished cleaning the relays and think they are working ok for now, however I'm seeing consistent problems even in standby so I don't think that's relay related.

The DAC output is fine, however the pre-amp output isn't, it's pegged at the negative rail (and then clamps by the zeners on the output PCA) which then also causes U7 to get very hot.

I've managed to get the preamp up an running on the bench and I see similar behaviour (I've added my own feedback path from U7 back into a dummy PREAMP-SJ) and I'm just working through comparing results to a falstad simulation.

I am totally confused by the different reference points (A and R on this board) and how they relate. They are tied together by CR19 and CR18 which I assume allows for some variations without letting them get too far apart (not sure of the reasoning behind this!)

One thing I'm noticing which I find very strange is that if I measure the voltage drop across CR18 or CR19 I see different results depending which way around I use the test meter leads ... for example with a 25mV test input signal (to PREAMP-SJ) I see 115mV drop in one direction and 9mV drop in the other ... how can that be?

(Also, BTW the schematic is wrong ... CR18 and CR19 do NOT connect to the +17 rail, they actually connect to S COM which seems more sensible and wasted many hours trying to get the simulation to work!)

Any help appreciated ... I'm well out of my depth on this!

Lee.

[dietert1]

The diodes are a protection in case of missing ground connection. If all ground connections are normal, the voltage difference should be some mV at most, ideally below 0.1 mV. The connection should be near the output terminals, maybe there is another relay or a jumper for 2-wire/4-wire mode.

Regards, Dieter

[essele]

Thanks Dieter, that helped simplify my testing.

So good progress now ... discovered that the UA714's were not a suitable replacement for the LF356's (U6 and U7) ... with proper parts in place I'm now getting good results on everything up to 22V, after that it goes mad and I see some quite alarming voltages on the output.

So it looks like an HV issue ... onwards and upward!

[essele]

So it was an HV loop problem ... caused by K1 on the pre-amp PCA which although working fine a couple of days ago again wasn't playing ball ... so I completely agree with wn1fju that these relays really do need replacing! (My first batch of PCB's should arrive tomorrow for the 6 pole variety, if they seem ok then I'll get the other variations designed and ordered.)

Once that was 'fixed' I now get output above 22V, however it's low ... a quick run of the analog self test and you can see the issue, test M34 should be +20V (on the 250V range) and I get 12.625V...

Code: [Select]

[...]
     M31: -.093960V
     M32: +19.8853V
     M33: -.000760V
     M34: +12.6588V

END OF ANALOG DIAGNOSTICS
I'm assuming there is another relay somewhere messing up the loop path ... I seem to be getting around 30% of the requested value on the 275V range and 60% of the value on the 1100V range...

100V => 34V
200V => 67V
300V => 180V
400V => 290V
600V => 360V

... so I'm going to go through and check all the relays that should be on for both ranges ... there is the possibility that another relay is stuck on when it shouldn't be, but I've not seen that at all as I've gone through testing them, so seems a much less likely scenario.

[essele]

I'm a bit of a muppet ... the relay that I destroyed I put back together as best I can and actually installed it in the unit (sample string K9) because I wanted the "off state" to be properly reflected.

Firstly the off-state of K9 is basically unconnected to anything, so having it in there was pointless ... and more importantly my reassembly of it actually caused some of the contacts to make when they shouldn't have, therefore messing up the 2M resistor string and therefore impacting the feedback look when on 275V and 1100V ranges. With that relay removed all output looks good.

The analog test now fails at M33 presumably (hopefully) because of the missing K9 relay.

I'm assuming it got past M33 last time because the broken relay was incorrectly making contact.

[e61_phil]

Hi,

very interesting thread!

My 5440B has a quite high TC ~0.1ppm/K. That is the specified value and I'm sure it was better some years ago, but in that region it is hard to find the problem. Maybe also a relay problem?

Another question I always had: Does anyone know if there is something harmful in the foam used in the 5440B for thermal insulation? Something like hazardous flame retardents or asbestos?

Btw: All the relays in my 5450A got quite high in resistance (some up to 100R), but I was able to cure them by cleaning up the contacts with a business card and some IPA. They are still fine since at least 4 years.

Best regards
Philipp

[essele]

So first attempt with a 3 x DPDT relay PCB seems to be successful.

I used some round headers that are thinner on one side and they go into sockets reasonably well. The socket pitch isn't actually 0.1" but it's close enough. I've also cut off the top of the pins so that they don't stick through too far and then covered them in solder mask.

This 6-Pole PCB is jumperable to cope with either side being positive (thanks for the heads up wn1fju) although they all seem to be the same on the sample string PCA.)

This PCB can also be used for the 4PDT versions which fit in the 6-Pole socket just by omitting the middle relay, and I have two other PCB's ready for the 4PDT and DPDT versions.

I am slightly worried about isolation although I've used a 4-Layer board to make sure I have as much space as possible ... and even with this version the smallest clearance is quite a bit bigger than the actual gap between the old relay contacts. There are a few changes I'm going to make though ... the holes for the pins could be slightly smaller and have a smaller pad and then I'll go from 50mil tracks to 40mil to increase clearance at the cost of a few milliohms of extra resistance.

I was also hoping to use the existing relays springs to keep these in place along with a 3d printed "cover" (hence the cuts in the ends) however that doesn't really work so I think I'll just resort to cable-ties around the socket.

Interested in any feedback or suggestions before I cut the next version ... also very happy to share the designs and/or get more boards made.

Back to the repair .... well all was going well, after some warm up it passed all the analog test.

The HV tests were going well, but then there was an arcing sound (I think at the 1100V test) and now I'm back to no sensible output on any range again! This is the gift that keeps giving (but I'm enjoying it!)

Code: [Select]

JOHN FLUKE MFG. CO., INC.                          5440  ANALOG DIAGNOSTICS

---------------------------------------------------------------------------

     ADM1: +.000000V
     ADM2: -.001698V
     ADM3: +2.52722V
     M41: -273.483V
     M42: +273.483V
     M43: +549.544V
     M44: +870.112V
     M45: +115.711V

END OF ANALOG DIAGNOSTICS

[MegaVolt]

Beautiful board. Do you plan to share the files for making it?

[essele]

Beautiful board. Do you plan to share the files for making it?

Thanks MegaVolt -- Absolutely ... I'm just going to make the clearance adjustments I mentioned, then very happy to share all three sizes.

I've just confirmed that the arcing isn't actually caused by my board ... I've found that U7 on the preamp board has blown again, and replacing it (the ua714 does seem to work in this one) I can reproduce some arcing at 500V and turn it off again without re-destroying it.

It sounds like its coming from a relay on the output board, although I'm not quite sure how a fault there would lead to HV at U7 without several other things being destroyed along the way .. but this has happened twice now. Not sure whether to revisit all the relays again or just focus on getting the boards right and everything replaced.

[MegaVolt]

Interesting. Is it possible to use contacts from the original relays for the output contacts. They can be taken out of faulty relays and soldered onto the board if you change the holes on the board a bit.

[essele]

I thought about taking the contacts, but each one is a single piece that runs all the way to the movable contact, so you could do it, but it would take some cutting!

It also means destroying the relays completely, so I gave up on that idea.

[essele]

I've gone through the PCB's again and have achieved a 40mil separation at all points, and generally I've managed with 50mil tracks with just a couple of small areas where I've needed to shrink slightly to get through a gap.

40mil (1mm) separation seems to match exactly the gap on the existing relay socket where it's soldered onto the PCB so anything more is unlikely to be beneficial.

However ... the pins I'm using are 0.5mm diameter for the side that goes into the socket, on the other side they are 0.65mm diameter but have a thicker support which is 1.8mm in diameter, on a 2.54mm pitch that gives only a 0.74mm clearance which is only 30mils. I'm going to find an insulation tester to do some tests, however it doesn't feel right.



Normal square pin headers don't have the extra support and so have plenty of clearance, however the pins are 0.65mm thick and just won't go into the socket.

So ... I need to find some 0.5mm thick (or diameter) headers that don't have extra meta supporting bits!

[dietert1]

For our oximeter modules we use 0.5 mm pins that come in a black plastic strip similar to the one you show. But the pins are flat /like wire, so we pull the plastic carrier after soldering through-hole. Looks like a hybrid.

http://cadt.de/spo/Views_3D.jpg

Regards, DIeter

[essele]

Hi Dieter ... do you have any details? that sounds/looks like it might be perfect.

I've also found these https://www.farnell.com/datasheets/2340109.pdf which might work if cut off, but they're quite expensive.

[dietert1]

The 40x strip had the product code 952-08-040-1-00 and we got them from W+P PRODUCTS GmbH in 32257 Buende/Germany. Certainly made in China..

Regards, Dieter

[essele]

Thanks Dieter ... I've found a few length variations on Conrad, so I've ordered a selection to see how they work out.

[essele]

Here are the KiCad files...

There are three variations....

6 Pole ... this can be used for full 6-Pole (602953) as well as the 4-Pole-in-6P-Socket variant (519405)
4 Pole ... this only seems to be used on my A7 Preamp PCA despite not being called out on in the service manual??
2 Pole ... this is a normal 2P version (514240) -- (EDIT: see post below, may not be suitable)

The 2 Pole variant is just double sided, and I've moved the power selection jumpers to the other side so they are clear of the socket.

CAVEATS: I have not size tested the 4 Pole or 2 Pole variants yet, and I haven't insulation tested anything (but do intend to)

In my 5440B I need 20 of the 6-Pole board (15x6P and 5x4P), 1 of the 4-Pole board, and 4 of the 2-Pole. If my numbers are right, that's 446 pins and 61 relays!

I'm just about to place an order at JLCPCB and follow it up with some HFD27/005-S from LCSC since they seem to be good value and specs look comparable to the data I can find about the American Zettler or Potter and Brumfield relays.

EDIT: updated with the correct relays after I ordered the wrong ones. Also worth noting that these relays are not polarised, so no need to have the polarity switching jumpers, but will leave them in to allow more flexibility.

[essele]

My insulation tester just arrived ... I've been waiting for a good excuse to get one for a while 

A very rudimentary test of the original header pins I was using show's that they are *probably* ok ... on the first pass through the voltage ranges I didn't see any arcing and resistance off the scale until 5kV. At 5kV there was considerable arcing which started at the narrow point but then travelled along the length of the pins (and it didn't start at my shoddy solder peak!)

Interestingly on a second run through there was also some arcing at 2.5kV, much less, and short lived. Not sure if this is to do with surface damage or something in the air post the previous 5k test.

The same test with some standard 0.1" 0.65mm square headers showed considerable arcing at 5kV but I was unable to get anything at 2.5kV with resistance off the scale consistently.

The 0.5mm pins will have a greater clearance still, so they are unlikely to be the weak point .... once the new boards arrive I will do an insulation test to make sure that they don't negatively impact the relay dielectric strength.

BTW ... I have dishwashered the first couple of boards, and apart from having to spend a fair amount of time getting the water out of the relay sockets, they look 100 times better. It's just a shame I can't fit the whole chassis in there!

[essele]

I've just been staring at the output board after the cleaning, re-capping, and sorting out a couple of rusted screws in the shields ... and I've noticed that the sockets for K9 and K10 are very close together and because of the orientation of the relay on the 2-Pole board I don't think it's going to fit properly!

Also K5 (6-Pole) is right up against one of the shields and the current design has an overhang on that side which I also don't think will fit ... and even if it did I suspect the relay pins would be too close to the shield.

I've done another 2P board design with the relay oriented differently and the power selection jumpers removed ... it's much simpler, and only has a very minor overhang at the end rather than the side, so this should be no problem on this PCA.

K5 is going to be a pain ... anyway, I'll work on that now, just wanted to highlight.

[essele]

New PCB designs attached ... there was another problem in addition to the above, the 4-Pole in a 6-Pole socket is actually not as simple as just removing a relay as the pin configuration is different to the normal 4-Pole design. So quite a few errors overall, thankfully JLCPCB let me ditch the boards that were in progress and refund the shipping (which is the bulk of it!), which means not an overly expensive mistake.

So the new designs are as follows:

1. 6-Pole -- this is as before, just with power choices removed and some minor tidying.
2. 6-Pole-K5 -- this is a new design specifically for K5 on the output board.
3. 4-Pole-in-6 -- this is a new design for the 4-in-6 configuration. This is just double sided.
4. 2-Pole -- this is a new 2-Pole design which avoids the "too close together" problem.

I've also left the old 2-Pole and the 4-Pole designs in the zip file but I don't intend to use them. For the single actual 4-Pole relay I'm just going to use the 6P board without the last relay fitted, there is plenty of space.

I've checked all the designs to ensure I maintain 40mil clearance on all non-power traces, and they should all have at least 20mil clearance to the edge of the board.

Fingers crossed these ones are right ... waiting game now ... 8 days or so.

BTW -- I will have loads of spares of the 6-Pole board, I need 15 but there was very little difference in price between 20 and 50, so if they work ok I'm happy to send some out at postage cost if anyone wants any.

[essele]

So the various size pin headers have arrived from Conrad (thanks for the pointer Dieter) ... as always not a clear cut decision.

In all cases the plastic part of the header is 1.7mm thick with the various size pins protruding different amounts from either side. Unfortunately none of them are "perfect" in an out-of-the-box way.

The -08 variant protrudes 2.3mm and 2.5mm and they really don't feel like they get much bite at all into the socket when the plastic bit is still in place, they are fine with the plastic removed, however this then brings the solder joints for the relays very close to the metal receptacles in the socket which I wasn't comfortable with, so I really need to keep the plastic bit in place.

The -09 variant protrudes 3.3mm and 3.4mm. The 3.4mm side does a reasonable job of engaging with the socket, but it just doesn't feel quite far enough.

The -12 variant protrudes 3.3mm and 6.3mm. These give a really secure mating with the socket, however I don't know what the mechanical arrangement is internally and I'm worried that the excessive length may bring pins too close to other bits of the socket internally.

The -13 and -15 are even longer so not viable.

So I've opted for a modified approach to the -09 variant. I've printed a small jig that allows me to setup and hold all the pins before starting soldering, and it allows the pins to sit 1.3mm further down than they normally would so there is a 1.3mm gap between the plastic and the PCB. Once they are soldered the plastic bit can be pushed up to effectively extend the pins by 1.3mm bringing them to pretty much the same length as the original relay pins. This then gives a really nice mating with the socket and has the added advantage that there is less pin exposed on the other side of the PCB to interfere with the relays.

The relays also arrived today and look good. Just waiting for the PCB's now ... they're due to be finished tomorrow, so hopefully I'll have them mid next week.

[essele]

Some progress on the front panel keyboard ... I managed to get my hands on a Fluke Y1700 keyboard which uses the same (almost) switches as the 5440B front panel.

So the keyboard PCA is now cleaned up (as best I can) and I've replaced the two broken switches, I've cleaned the key caps that I have and I have a masterplan to re-create the missing ones (just waiting for a few things to arrive.)

The keyboard switches are Datanetics DC-60 series, you can see some of the history here: https://deskthority.net/wiki/Datanetics_DC-60_series

There is a very slight variation between the switches from the Y1700 and the original ones in the 5440B, both seem to the right hand variant from the "evolution" pictures in the above URL, but the 5440B ones have a much shorter tab, the long tab gets in the way slightly, but it's very easily removed by 10 seconds with a file.

I have quite a few spares now, along with all of the Y1700 key caps, chassis, and PCB if anyone has a need. I'd like to recoup a little of the cost from the switches if possible, but the other Y1700 bits I'm happy to see go to a good home for postage costs. I'll post on the selling forum when I get a chance.

The PCB's have also arrived .. super impressed with JLCPBC turnaround and delivery times, the order included 4-layer PCBs, was placed on the evening of 18th and arrived this morning.

[essele]

Ok, all relays replaced and it was looking really good. The arcing stopped once I'd done the pre-amp, so it was presumably a problem with a relay on there.

The output all looks good, and I've tested up to 900V so far.

The analog tests fail on a DAC problem which only started since I had the arcing issue before, so presumably that's damaged something.

However, I've just powered it up again and I'm getting a front panel comms error! Everything was completely fine just a few seconds ago, so I'm hoping that this is a dodgy connection somewhere.

[dietert1]

Your relay replacement work looks great.
If the contacts inside the relays were unreliable, the contacts of the backplane are at least as unreliable. Also it looks like those gold plated edge connectors of the boards may need attention. Yesterday i read about using vinegar with salt to remove oxide layers from copper parts - as a preparation for gold plating. But i don't know whether the 5440 backplane has copper or brass springs. Probably they are gold plated anyway, so it's just removing dirt.

Regards, Dieter

[essele]

Thanks Dieter ... I'm very pleased with the relay replacements, just a bit annoyed I can't continue testing it because of this comms problem.

It actually looks like it's the UART on the front panel PCA ... I couldn't see any output signals at all, the pin is just sat at about 2V with a small amount of ripple that looks like the CLK. The TBRL (transmit buffer register load) line was going low which should then load the data and cause it to go out of the output .. but no output.

I tried cleaning and reseating it, to no avail.

So I've temporarily replaced it with the one from the A16 Controller and I can see some data on the output ... so looking fairly conclusive.

I can't find a sensible place to get a replacement other than eBay ... hopefully should arrive by the end of next week! More waiting!

[essele]

OK ... UART replaced and we're back in business ... not sure what caused that to fail.

The unit now seems to be functioning all normally, however I'm getting an error during the analog self tests that's to do with the zero amp. The zero amp is only used during internal calibration and helps to figure out the offset of the output.

The test that's failing is a "shorted input" test (M23) that should see a result between -0.25mV and +0.25mV and I'm seeing results that range from 1.5mV to 1.7mV.

The zero amp is a fairly simple OP07 configuration with a gain of either 100 or 1000, with the feedback resistor being switched by a JFET when needed, x1000 is the default, but x100 is used for the particular test.

I've tried swapping the OP07D with an OP07CPZ (which should be lower offset) and actually saw the numbers get worse ... I'm hoping this is because the OP07D is performing better than spec.

I've tried swapping the analog mux just in case there was some leakage there, but it made no difference.

So I'm left with two theories:

1. The JFET Q23 is leaking Gate/Source (do they do that?) which I think would then pull the inverting input slightly lower, thus increasing the output.

Or hopefully much more likely....

2. The transition from x1000 to x100 isn't working and the result is 10 times bigger than it should be. This could be an issue with the switching signal (ZCA) or Q21, Q22, or Q23?

Does that sound feasible? Or is there anything else that could be doing this?

I've attached the schematic ... the zero amp is towards the bottom-centre surrounded by the dotted shield line.

This did only start happening after I had the issues with arcing, so I was really expecting something to be fried by excessing voltages caused by faulty relays, but everything seems ok, at least at very initial testing.

[Kleinstein]

Gate leakage is possible with a broken JFET.

A not working JFET and thus always the x 1000 setting makes sense.  The JFET should not be something special - likely some 2N4393 or J113.

[essele]

Thanks Kleinstein ... I explored this idea a bit.

I managed to power up the board on the bench with +/- 17V and the +5V relay power, and it behaved pretty much exactly as expected, I saw 4mV at x100, and 40mV at x1000 which all seemed nicely within tolerance and ruled out any issues with the jfet.

However, once back in the 5440B, at standby (which should be x100 shorted inputs) I was getting 250mV ... took me ages to figure it out [note to self: use a scope as well as a dmm], but it was ultimately a couple of electrolytics I hadn't yet changed on the PreAmp board, whenever that board was plugged in I saw over 500mV of noise/ripple on the 17V supplies.

There were three caps I changed, one for the oven circuit which was definitely faulty, but didn't improve things, but then when I did the two on the 17V supplies it all cleaned up nicely.

The analog tests still fail, but further on now at M33, I'm getting -7.7mV instead of something +/- 1.6mV, this is to do with the HIGHV INTCAL and seems to be pointing to a problem with the sample string PCA.... job for tomorrow!

EDIT:  Problem solved ... M33 got me thinking back to the similar problem I had when I had one of the sample string relays making contact incorrectly ... a quick inspection of that replacement relay PCB for that relay and I found three pins on one of the relays that I hadn't soldered (rushing too much obviously!). With this fixed, the analog checks all pass ok, other than a sample string oven issue which I need to look into, and I'll wait until tomorrow to run the HV ones, end the day on a high.

[baof]

Is the output voltage stable? I replaced the relay with Panasonic. It was found that the output voltage was affected by the heating of the relay coil. The thermoelectric potential was very obvious. After 10V voltage was output, it would drop by more than 1.5ppm after the coil was heated

[essele]

Ok ... all tests now pass, both analog and high voltage. I still need to deal with the missing buttons on the front and a bit of tidy up (and a few more caps to do), but I think it's almost there as far as basic functionality goes!

Is the output voltage stable?

I've only just started to look at the output and there are lots of caveats to this ... the room is a bit chilly and I can feel cold draughts (although can't find a source) and I'm stringing two connected sets of cheap Chinese test leads across the room, neither the meter (DMM7510) or the 5440B are properly warmed up (probably about 90 mins at this point) ... ignore the absolute numbers, the 5440B isn't calibrated and the DMM7510 isn't self-cal'd ... so there is a huge set of error sources in this, however....

From the attached graphs you can see the output, this is from about 1 second after hitting OPER ... each time I run it the ultimate output seems very consistent and settles at 10.000417 (averaging over different time periods, using 5NPLC and no filter) although has been slightly rising in more recent attempts to 10.000419, I suspect this may be to do with things still warming up.

(Later note -- it's now at 10.000425 and has been consistently increasing, does look like a warm-up process)

The first graph (102055) is from a run where it wasn't at standby for very long, probably no more than 3 minutes ... it looks absolutely fine to me, the specs say it can take 10.5 seconds to settle to within 0.5ppm from STBY to OPER, and it's probably within 0.2ppm at that point.

The second graph (103148) is after a much longer STBY period, probably 10 minutes or so, and you can see a lower dip (from stable state), although still only about 0.6ppm, but it does then take a bit longer to stabilise which may well be to do with relay heating/equalising.

Either way I'm not sure I'm seeing a particular problem -- although I will look again once I've found less error-prone ways to measure this. To be honest I'm absolutely stunned how good this seems to be ... this thing has clearly been in a room with a fire, filled with smoke, filled with water, left to soak for a while, had all the really expensive relays replaced with different configuration low cost Chinese ones, and been messed with by a fairly incompetent idiot that only half knows what he's doing ... it's a miracle it's working at all!

[baof]

At the beginning of output, the curve is straight. In less than 1 minute, the coil heating begins to affect. The curve continues to decline until the coil heating temperature reaches a stable level

[essele]

I'm certainly not an expert but 30 minutes seems like quite a long time for small relays to heat up, what makes you think it's the relay?

Which model relays did you use?  I used relatively low power ones (< 200mW) ... if you used the standard power Panasonic ones (400mW) then I suppose that could account for some differences, with three relays in place of one original one, that's 1.2W, which is significantly higher than the original relays which I *think* are 450mW.

Have you changed all of the relays? I'm wondering if having a mix might introduce inconsistencies between output and sense perhaps? I'm not seeing this at all, but I've changed everything.

Have you also tried running the same experiment with different cooling-off times (i.e. time in STBY before switching to OPER) that may help to confirm it's heat related.

If you're concerned about a specific relay, and it's a 6-Pole one, then I can probably knock you up one of my PCB's with the LCSC relays on it (for cost + postage) if that's useful?

[Kleinstein]

There are several parts that can show a slow response. Thermal effects at the relays can be one - some 10 minutes may be about the right time scale, at least if there is no strong fan. There may be other parts that are effected from heat flows changing too, like some OPs and maybe the DAC part, though ideally the effect should not be large.
Another point is dielectric absorbtion in the capacitors at the filter of the PWM DAC.  This can also take quite some time to stabilize.
One would see the difference by comparing the turn on with a large step (e.g. 0.1 V to 10 V). The relays would be the same for 0.1 and 10 V. For the DA the start from 0.1 V should not make much difference from the off state.

[baof]

Thank you for your reply
I use Panasonic ds2y-s with a working current of less than 40mA. I replace all relays on A7. I tried to turn off the output for a short time, but the coil did not cool down. When I turn it on again, the output will not be affected. If the output is turned off for a long time, the coil will cool down, and the output will be affected. It is confirmed that it is the effect of relay heating. Maybe it is a perfect replacement. I really need to use magnetic holding relay.
I'm in China. I have two free opportunities to do PCB at JLC every month (even postage is not needed)

[Kleinstein]

Other effect like the fitler cap DA can have an effect on comparable time scale as the relay heating. So I don't think the simple test with a short and long turn off does proof that the drift is due to relay heating - it is well possible, but not for sure. At least the filter caps could be excluded, by comparing to steps in voltage, turning down the voltage to near zero only with the DAC.

It looks like they sometimes use multiple contacts in parallel. So there may no actually be a need to have so many contacts and in some places one could replace a 6 pole relay with just one 2 pole relay , e.g. leave some small relays on the adapter board unpolulated.

Magnertic latching relays are a bit tricky:  they need additional control circuit and more important the state after turn on is not so well defined. A similar problem can happen on power down if only some relays are replaced.  Wrong set relays may do damage to the circuit or give out dangerrous voltages.
Latching relays are more a thing to consider in a design from the start, nothing to change easy later.

[essele]

First attempt at a keycap from the front panel ... not bad for a first ever resin 3d print!

I still have a little tweaking to do on the shape ... I was convinced the front was a "squircle" but perhaps I'm over complicating things and it's just a rounded corner square. General dimensions are pretty close.

And obviously the legend isn't quite working yet, I have a few different ideas to try ... and I need to see if I can find a better font.

The next one will be in a CMYK pigment mixed resin, so I can start to see how close to the colours I can get.

[MegaVolt]

Here's an interesting video on how to make buttons. Maybe there will be something interesting there. The quality is great

[essele]

Wow .. they are stunning, lots of work and lots of jigs/tools needed. A very interesting video, but I'm not sure there's anything specific that would help my approach. I love the use of lego though!

Mine are getting better ... the shape is much closer now, but still needs a couple of tweaks, and the deeper legend cutout means more depth of colour on the legend which seems like it will work out, I get some discolouration caused by seepage into the tiny ridges on the print surface, I'm hoping they will polish out.

The pigments are a challenge ... one drop is not granular enough, but I'm reasonably close, I'm hoping adding a bit more white base resin will lighten this up sufficiently. (Although the resin doesn't seem to be 100% opaque so I don't think it will ever completely match.)

I found the right font for the smaller text (I'm using Blogger Sans Medium which seems to match pretty well), however the bigger font (i.e. the digits) seems to be a wider variety ... "Kingthings Clarity" looks reasonable, although I haven't tried it yet.

[essele]

Ok ... getting better, but still a few problems.

For the larger symbols I went with the same font but just expanded it horizontally, seems to look ok.

The colour doesn't quite match yet, it's a bit challenging because it darkens slightly during UV curing, but I've got some scales coming today which will hopefully help with more accurate mixing, and solve the one-drop is not granular enough problem.

The biggest issue is the lighter colours are not particularly opaque and when combined with a need to have a deep trough for the legend it means that the black is visible through the lighter colour making it look like it's smudged. I don't think this will be an issue on the darker ones. The solution may be a much shallower trough and using a different solution for the legend (i.e. not black UV resin.)

I've even managed to get a bit of a gloss finish going!

This is a bit silly really, but I'm enjoying it (apart from the resin fumes, which are evil!)

[MegaVolt]

It looks really good!

[ch_scr]

Agreed on it looking really good! How long do you take to do one cap? Because that's the easy way out on 'identical looking keycaps' - "just" manufacture them all...

[essele]

Thanks ch_scr.

They take just under an hour to print, although I could do several (in the same colour) at one time ... at the moment I'm still trying to get the colours right so it doesn't make sense to do more than one.

The "post processing" probably takes an hour as well ... clean, UV cure, sand a bit, do the legend, cure again, then lots of sanding/polishing.

My challenge is that pretty much all the keys I need are different ... only one light one (the number 7), a few brown ones all with different legends (well, 2 of them are blank) and then 1 greyish one. So there isn't really a mass-production option.

I'm really struggling with the opacity challenge ... I don't think they are going to be quite right, but hopefully they will be good enough. I've still got a few more things to try, and it's all good learning experience (apart from the fumes ... definitely need a better mask!)

[essele]

I think I've got this figured out now... the solution is acrylic paint markers!

For the legend, on the lighter colour I've gone for a slightly shallower recess, but then by using a light colour acrylic paint marker to start with (I had a very light pink which was the closest to the light resin) it puts a fully opaque coating in the recess, but it dries thin so doesn't fill the recess. Then you can cover with black and it doesn't have the smudgy appearance that was previously caused by the black being visible through the lighter colour.

Also, by polishing a bit more the appearance is more opaque ... so I'm going to go with the number 7 as shown ... just a little more polishing on the edges.

The acrylic paint markers also work brilliantly on the small text as you can see on the "limit" cap ... I have a few more experiments to hopefully improve the colour, and then I still need to do the grey/beige one.

See the photo of the front panel with the "7" ... it's not fully pressed in yet as it's pretty tight and I don't want to keep taking it off, but otherwise I don't think it looks too bad.

[MegaVolt]

If you hadn't told me I wouldn't have been able to find a different one

[essele]

I think I'm done ...

I've given up trying to match the brown key caps and created a nice dark grey which I think looks great. I'm not entirely happy with the "div" key, but I'm not doing it again, and one of the three menu buttons is slightly sticky so I should have replaced the switch for that ... I'll do it next time I open it up.

Other than that I'm pretty pleased with the front panel now -- before and after photos attached.

Everything is working nicely, on to some testing!

[rigrunner]

That's an excellent job. My hat is off to you. 

[MadTux]

Just milled a new display window for another cheap 5440B from ebay on my FP4NC.
Used dark grey 3mm plexiglass window from ebay ($20), looks better than new, IMO.

Unfortunately also missing the EMI wire screen, no idea if I should replace it with stainless/copper wire mesh from ebay or just glue it in place without EMI wire mesh??
Got a quote from some China company for new real EMI wire mesh, $280. Total ripoff!

Apart from that, that's my 4th fully functional Fluke 5440B 

[essele]

Very nice!

What was the reason for milling? Just a thickness difference? I don’t recall there being any indents on there.

[MadTux]

Reason for milling/routing: much quicker, easier and much more accurate than by hand with hacksaw and files ;-)

Somehow you can only get 3mm Acrylic, think the original window is 2mm, but not sure.
First plan was to mill the window down to 2mm, where it contracts the front frame, so the window would sit at a millimeter deeper than with 3mm Acrylic.
But when I tried it, the 3mm version was flush with the rest of the front panel, probably because I left out the EMI screen, so I just glued it in place.
No EMI screen shouldn't matter that much, Keithley 2000 and friends don't have them either...

[leighcorrigall]

While I was searching for other things, this came up and might be useful to people.


Datanetics DC-60 series:

https://deskthority.net/wiki/Datanetics_DC-60_series

[leighcorrigall]

...

To get at the fan (and mains filter) I've removed the back panel and the A19 Outside Guard Terminator PCA (which seems to be just a set of unregulated power supplies) ... on the board there was an area which was quite darkened, mostly around CR2, CR3, and F2. Some tracks on both sides were missing their solder mask and I think the plastic coating that would have been on CR2/CR3 appears also to have burnt off! F3 had some damage to the sheath underneath where it was close to a track which appears to have overheated.

All of the fuses are intact and don't look like they have been replaced (they do now because I messed with them to apply some new solder mask underneath -- and looking at the photo I need to redo my soldering!)

So I am a little confused ... CR2 and CR3 feed the unregulated +5V supply, both are fused with 5A fuses, both diodes appear to be ok. However something has caused them and the PCB tracks to get very hot but it hasn't blown the fuses! R2 measures fine, and C2 seems to be ok from an external timed charging test.

I'll obviously check the voltages once I get it all back into the chassis.

Any thoughts?  Is this just to be expected with age? Should I replace CR2/3 and the fuses?

This is probably the simplest board in there --- this is going to be a long journey ;-)

Thanks,

Lee.

Hi Lee,

I have the exact same issue with my board. The diodes seem to be the problem given the localization of the burn marks. On further inspection, no other boards on my unit appear damaged. The instrument passed its diagnostics and it can output 0 to 100 V without any noticeable problems so far. I have yet to test above 100 V because I have trust issues with antique equipment that has not been fully inspected. The unit was calibrated in 2018, so I imagine that it has passed some quality checks and this problem is nothing new.  My VAC was set to 115 at the factory and may cause some additional warming because my line power is 120 VAC. I'd say this instrument has been around the block.

Can you please tell me how you removed the outside guard terminator PCB? It looks complicated and I must be missing something so far.

Thank you.

EDIT: Solution found. See attached pictures of the removed outside guard terminator PCB.

[essele]

Hi Leigh,

It's been a while and I can't easily open it open again to refresh my memory, but it absolutely was a pain.

I think it's the screws on the back panel (can't remember if this is two or four) that hold the transformer in place, and I think you need to remove the rectangular metallic capacitor as well as it gets in the way. The transformer stays attached to the board and it all comes out ... there's an edge connector that it plugs into, so once you've unscrewed everything you need to it should just pull out (remembering to remove the connections to the other large cap.)

Mine looked very similar to yours ... just worse!

Lee.

[leighcorrigall]

Hi Leigh,

It's been a while and I can't easily open it open again to refresh my memory, but it absolutely was a pain.

I think it's the screws on the back panel (can't remember if this is two or four) that hold the transformer in place, and I think you need to remove the rectangular metallic capacitor as well as it gets in the way. The transformer stays attached to the board and it all comes out ... there's an edge connector that it plugs into, so once you've unscrewed everything you need to it should just pull out (remembering to remove the connections to the other large cap.)

Mine looked very similar to yours ... just worse!

Lee.

I figured it out based on what you said. The process is simple:

1) Remove the top cover of the instrument that is held in place with 8 flat head screws.
2) Discharge the two large capacitors that are near the outside guard terminator. I used a voltmeter set to the highest range. Disconnect them and cut the cable tie. Remove the capacitor connected to the rear panel with two screws.
3) Remove the two top screws on the rear axial fan after removing the fan filter. The bottom of the fan is held in place with a bracket. You will need to use a bit of force to remove the fan because it is tangentially spring-loaded on the screws.
4) Unfasten the outside guard terminator transformer mounted to the rear panel with four screws.
5) While making sure that the board doesn't short on the chassis, lift the outside guard terminator by the transformer. If you raise the card, the weight of the transformer will cause the card to unintentionally contact the metal chassis after it is removed from the card socket.

I do not have any image editing software installed on my computer. Sorry about only providing 3 images. These should be sufficient as references.

EDIT: See images of PCB in the previous reply.

[leighcorrigall]

There seems to be more thermal damage at the outside guard regulator (A17) on my unit.

Damage is related to R1 (0.15Ω, 2W, WWR?), Q1 (40251) and U5 (LM317HVK). R1 appears to have been replaced at some point but other components may have as well.

Edit: Someone else has the same issue with R1. URL: https://dabbledoo.weebly.com/fluke-5440b.html

[essele]

Yes, I also replaced R1 … if I remember correctly it was crumbling. I used the following by part from Farnell … I don’t recall replacing anything else other than caps.

https://uk.farnell.com/tt-electronics-welwyn/wp2s-r15ja25/res-0r15-5-2w-axial-wirewound/dp/1219188?CMP=e-email-sys-orderack-GLB

[essele]

But … the “darkness” on the PCB isn’t necessarily damage … loads of the PCB’s In my system have similar heat, er, signatures ;-)

I think it’s a function of age and somewhat questionably design … I think someone said something appropriate in one of the first messages in this thread … something about thinking human ingenuity can outsmart physics!

[leighcorrigall]

Thanks Dieter. I was thinking about excess current heating the traces rather than simple heat transfer from the diodes -- doh!

I've done a quick test, at 3A those diodes have a Vf of just over 1V, some others (schottky) I have to hand are down at 0.8V, and I've ordered a couple that should be under 0.6V .. they should bring the heat generated down fairly significantly without me trying to bend TO220 leads and fit heatsinks in.

...

What Schottky diodes part number did you end up using in the end or did you simply install 1N5550 again?

[essele]

Looking back through my orders I ordered two different ones at the same time … I tested them for forward voltage drop at 2A and used the lowest one, but can’t remember which one it was.

So one of these two….

http://uk.farnell.com/jsp/search/productdetail.jsp?sku=2750946&CMP=i-bf9f-00001000

http://www.mouser.com/ProductDetail/panjit/sb5100l%5fay%5f10001/?qs=sPbYRqrBIVkRScF5BGm%2fDQ%3d%3d&countrycode=GB&currencycode=GBP

Lee.

[leighcorrigall]

Looking back through my orders I ordered two different ones at the same time … I tested them for forward voltage drop at 2A and used the lowest one, but can’t remember which one it was.

So one of these two...

Lee.

Hi Lee,

Unfortunately, neither of those can be ordered from DigiKey where I am.

The MBR15200 (https://www.digikey.ca/en/products/detail/smc-diode-solutions/MBR15200/6022118), with a TO-220-2 package accompanied by a 6025DG (https://www.digikey.ca/en/products/detail/aavid-thermal-division-of-boyd-corporation/6025DG/4974546), seems to be a decent substitute for the 1N5550 (https://www.digikey.ca/en/products/detail/microchip-technology/1N5550/4378601). The 'Typical Forward Voltage vs Forward Current' characterization chart places the MBR15200 voltage between 0.53 V to 0.68 V at 2 Amperes which is far less than the 1N5550 voltage (0.72 V to 0.83 V) under the same current and temperature conditions (i.e., 125 °C to 25 °C). n.b, these figures were read off the datasheets. I assume that the higher forward voltage will cause a higher temperature response which should be compensated by the TO-220 package and selected heatsink. There should be plenty of headspace above the outside guard terminator board for the 6025DG.

The MBR15200 also shares the same Reverse Leakage Current (i.e., 1 µA @ 200 V) as the 1N5550. Are there any other parameters to consider for a replacement candidate? The only characteristic that is dissimilar that I can make out is the 'Average Rectified Current'. The MBR15200 is 15 A vs the 1N5550 being 5 A. Perhaps not as is not important.

Regards,

Leigh (the other Lee) 

[leighcorrigall]

...

To get at the fan (and mains filter) I've removed the back panel and the A19 Outside Guard Terminator PCA (which seems to be just a set of unregulated power supplies) ... on the board there was an area which was quite darkened, mostly around CR2, CR3, and F2. Some tracks on both sides were missing their solder mask and I think the plastic coating that would have been on CR2/CR3 appears also to have burnt off! F3 had some damage to the sheath underneath where it was close to a track which appears to have overheated.

All of the fuses are intact and don't look like they have been replaced (they do now because I messed with them to apply some new solder mask underneath -- and looking at the photo I need to redo my soldering!)

So I am a little confused ... CR2 and CR3 feed the unregulated +5V supply, both are fused with 5A fuses, both diodes appear to be ok. However something has caused them and the PCB tracks to get very hot but it hasn't blown the fuses! R2 measures fine, and C2 seems to be ok from an external timed charging test.

I'll obviously check the voltages once I get it all back into the chassis.

Any thoughts?  Is this just to be expected with age? Should I replace CR2/3 and the fuses?

This is probably the simplest board in there --- this is going to be a long journey ;-)

Thanks,

Lee.

Hi Lee,

I have the exact same issue with my board.

...

It appears that the Filter A board (REV E) also has the same problem. The 1N550 diodes are not good for this design. Luckily, there is plenty of space for a heatsink and a TO-200-2 diode as suggested by dietert1.