Another Fluke 5440B/AF Repair

[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.