Electronics > Metrology

Another Fluke 5440B/AF Repair

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leighcorrigall:

--- Quote from: leighcorrigall on June 12, 2022, 12:34:30 pm ---
--- Quote from: essele on September 29, 2021, 09:15:19 am ---
...

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.

--- End quote ---

Hi Lee,

I have the exact same issue with my board.

...


--- End quote ---

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.

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