Fluke 8506A damaged ohms convert and rms converter

[giacomo]

Hi everyone, i got a fluke 8506A sold as not working, the seller has described it as "it blows the fuse when power on".

I received the unit, the fuse was intact and configured correctly for 240V. It powered on with error 9.

There is a very nice article about the 8505A on xdevs, the 8505A is the same unit apart for the thermal rms converter module, the error rapresents a faulty module.

After removing the top cover it was apparent that something has released the magic smoke, so i removed the modules one by one to have a look.

The fuses were at least 3 transistor in the "ohm converters", two of them Q31 and Q29 Fluke specials fets no. 393314. the third Q17 is a 2N3906, Q30 2N3904 looks it has overheat the pcb under it

Without the "ohm converters" installed the meter seems working just fine, respond to the commands, measure dcv correctly but nothing on ac, this time it does not yield any errors.

I will take some time for inspetting the rest of the unit and taking some pictures to share.

I do not think that the ohms converter is savable, but i would like to hear your opinions.

Soon i will update about the rms converter.

[Kleinstein]

As far as the schematics is in the manual, it look like Q29, Q31 and also Q35 may be easy to blow. They are used as switches and likely no really special fets needed. The main point would be looking for not to high leakage and a threshold that may need to be reasonable. So if Q29 / Q31 are bad, I would also check Q35 and order afew more replacements if not too expensive.

The typical replaement would be something like PN4393 , 2N4393 or as a less tested part maybe J113. With added leads one may be able to bodge in a SOT23 version too.
Chances are the Fluke Fets may be additionally tested for low leakage, but likely no need for better than typical.

Besides the FETs the driver circuit part (often LM339 / LM393) may also be damaged.

[bdunham7]

I do not think that the ohms converter is savable, but i would like to hear your opinions.

I'm sure it is savable somehow. 

The 393314 FET is a Siliconix J2086, which doesn't help much because that p/n is pretty much unobtanium as well.  Central Semiconductor PN4392 or PN4393 will probably work, but you'll have to do some datasheet research and perhaps give it a try.  You may have other components damaged, it looks like this was caused by a backfeed of high voltage through RT1?  You probably need to look at Q33 and Q35 as well, which should be evident as you look at the schematic.

I have an 8505A that was obviously damaged by overvoltage through the rear connector and this caused some additional damage on the input board.  I've managed to completely repair mine and you probably can too.  You may need to do some further disassembly to examine that input board.  Also, you may want to pull the Active Filter board and inspect that closely.

There are several EEBbloggers that have these meters and have dug into them pretty deep.  If you haven't already, look for those threads with users SilverSolder, joeqsmith, dietert1, kinklesstetrode (and others).  I don't have time to look at the details right now, but hopefully you have the manual by now.  I've attached a schematic of the ohms converter so everyone can see it.

Edit:  This schematic may not be accurate for your board due to Fluke having many revisions.  Specifically, U1 appears to be incorrect.

[giacomo]

Thank you very much guys.

I checked the transistors around and marked in the schematic.

Yes, my board is different from the schematic, it does not have U1 but two CD4042 and one MC14023

I also found that Q8 has a slightly burned pcb under it and it is dead.

I am going to looking for those threads and probably start ordering these replacements.

The last two photos are the rms thermal converter, Q22 FET and Q38 2N3904 next to it are dead.

[SilverSolder]

The A/C board looks like there has been arcing between Q38 and Q22... which are on the output side of the board, not the input...  i.e. the over-voltage appears to have made it through the Ohms unit and into the analog bus, and from there into the A/C board,  which fried the A/C board.  The arcing between those two transistors might be what saved the entire instrument from being electrocuted! 

[giacomo]

The pcb has some burned spot around some faulty transistors.

Should i do something about it ?

[SilverSolder]

All the places that have suffered flash-over burns need to be restored to working condition - the carbonised material acts like a resistor between two parts of the circuit that should not be there!   Depending on where it flashed, it could affect the accuracy / reliablity of the instrument.

Badly burned areas may require surgery, e.g.:

[bdunham7]

I would try to clean the board with a stiff fiberglass pencil brush and alcohol until all of the charred stuff has been removed.  These boards will also slightly brown with age in the warm areas, so don't carve away the board trying to eliminate all discoloration.  If any significant material has been removed, clear fingernail polish may be easier than epoxy.  It looks like the electrical damage is pretty severe here, I would recommend disassembly and inspection of all modules, the main board and the difficult-to-see input board.

[giacomo]

I never done one of these surgery, but ok. I do not have a fiber glass pencil, but i will get one.

I made mistakes in the drawing, LM301 U4 is ok

I removed the active filter, it seems ok

[SilverSolder]

The fact that it starts and works, and shows a reasonable reading, is very good news - it obviously isn't a total meltdown, it looks completely fixable...

Sometimes units turn up on eBay that are cheap enough to buy for parts.  Sometimes spare modules turn up too.  That's the easy way to fix things... 

[giacomo]

Dremel might do the job.

I have some of these accessories that come from some dollar store.

I have tried on some scrap board.

[SilverSolder]

Looks like you are a budding professional electronics archaeologist! 

Did you notice in the video, how he had some piece of plastic on the tool bit to act as a fan to keep the work area clean?  - personally, I would prefer to do this work outside in the fresh air to make sure none of that dust ends up in someone's lungs...

[giacomo]

Here is my first attempt, probably i digged too much..  I am almost on the other side, the surface is pretty even and i guess should be easier to apply the epoxy

I end up by drilling completly in between the pin because in those spots i saw a little burn from the other side

Before and after

[SilverSolder]

That looks pretty good!  You can just put a piece of electrical tape on the back to cover the holes when applying epoxy, the epoxy won't stick to the tape.  (Make sure this is true with the particular type of epoxy you are using...  but it has always worked for me!).

[giacomo]

That looks pretty good!  You can just put a piece of electrical tape on the back to cover the holes when applying epoxy, the epoxy won't stick to the tape.  (Make sure this is true with the particular type of epoxy you are using...  but it has always worked for me!).

I have a type from pattex, they give you a smooth piece of plastic for mixing the two components and when it dryies it cleans easily.

I thought the same about the pvc tape.

I will leave to rest for 24h then i might sand a little bit the execcess.

[giacomo]

Corrections..

I measured the resistance between the pins after the surgery but there was a 100k resistor in between that i did not see So the photo was meaningless

Before proceding with the ohm board i removed RN2 and desolder one pin from R55 and R57 respectively so i can at least measure the resistance between the pads of Q29, Q30, Q31, Q33, Q35 without problems. As far as i can measure there is nothing, so also the second photo was meaningless. The ohm converters board does not have a large brown spot like the rms board, it might be just slightly discolored from the top by the heat of the exploded transistors

[SilverSolder]

Looks like a good fix...  now, the rest is "normal" electronics...

[bdunham7]

Here is my first attempt, probably i digged too much..  I am almost on the other side, the surface is pretty even and i guess should be easier to apply the epoxy

I end up by drilling completly in between the pin because in those spots i saw a little burn from the other side

 

I admire your dedication.  I think the old girl has fallen into the right hands.

Since this appears to have had a multi-circuit input accident, don't overlook the hidden input board on the right side of the meter.  It's a lot of work to get out, but the crossover of the various circuits likely happened there and there is likely some damage. 

[giacomo]

Here it is, fuses and thermistor are ok, fuses all measure around 3.7ohm, thermistor 1.25k

I will continue tomorrow with the rest of the boards and providing some photos

[bdunham7]

That layout looks a little different than mine.  I don't recall seeing a thermistor thingie, my fuses were zip-tied on thru the holes in the board and the layout was just a bit different.  I see the schematic doesn't exactly match either and I looked at a few 8506A manuals and found them to be lacking schematics.  Is there an 8506A-specific schematic available?

Can you check the x-ref resistors (30.9K)?  Also, the J2 AC Input connector--does that physically exist somewhere?  I don't recall seeing this and the motherboard diagram doesn't show a separate J2 in that area--it's on the left under the front display board.  If not, I'm wondering how the AC input to the RMS converter is actually wired.

[SilverSolder]

It looks like the same schematic is used for both 8506 and 8505.

[bdunham7]

OK, nevermind.  I see the note about J2 being 8506A only and there it is right in front of me.  Is there a schematic for the Thermal RMS converter?

[giacomo]

I found a manual with the schematic for the thermal rms converter, quality is not good as the one from KO4BB.

https://elektrotanya.com/fluke_8506a_service_manual.pdf/download.html

Schematics start at page 408

Faulty components Q22 and Q38 at page 417, amplifier pcb, bottom board in the photos

I will check the resistor connected to x-ref later, i had just put back the pcb when i saw the message