Electronics > Metrology

Another Fluke 5440B/AF Repair

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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.

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