Author Topic: Troubleshooting a Fluke 87 iii (87-3) with low Vdd-Vss and high current draw  (Read 625 times)

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Offline ogdento

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Hi,
I'm trying to troubleshoot a Fluke 87 iii I obtained in a non-working state.  I'm starting from scratch since I have no history with the unit...

On initial powerup all segments were very dim, so after checking the battery I measured the current... it draws 15mA (way too much!) when ON, and 9.2uA when OFF which seems about right.  Vdd and Vss measure to be 1.69 and -2.01 which is not good (should be 3.0 and -3.2).  I checked the rotor traces (they were not fouled but I thoroughly cleaned them with IPA anyway) and examined the board for any burned/broken/missing traces or components, and looked for bad solder joints.

I know the 87-1 has two interconnected regulators - the Vdd regulator and the common/shunt regulator (thanks to mrmodemhead for the great writeup)...

The VR1 regulator on the 87-3 is still an LT1034 like the 87-1 but it's now an so8 package... when the meter is ON it only shows 1.03v on pin-8 instead of 1.23.  With the power off, an in-circuit diode check shows forward voltage as 0.56 which seems fine but reverse voltage shows 1v which is bad (a working 83-3 shows 0L when reverse biased... so I think something in this path is cooked).  With the 87-3's VR1 anode disconnected (pin lifted off board), a diode check shows the expected 0L when reversed biased.

I "think" the common/shunt regulator is OK... if I drive Vdd-Vss with a 6.2 volt supply, I get Vdgnd-Vss = -3.04 which is in spec per the manual (3.2 +/- 0.3), so I believe the problem is in the Vdd regulator.  This is where the 87-1 and 87-3 start to have some bigger differences... Q8 is now an NPN, the C35/CR4/CR5 connections are different, the U6 selector circuit has changed - Q12 seems to be gone and an extra inverter (pins 1-2) is now used with the PDS inverter (pins 8-9) to drive a whole bunch of new logic gates on the back, etc.

The only other strange thing I've noticed was with C14 (a 47uF Vdd regulator bypass cap).  In-circuit, I couldn't measure it on the 87-3, but I got 47uf on the 83-3 and an 87-1... I pulled it off and it then measured fine, so something smells a little fishy there.

Has anybody traced out the Vdd regulator and/or selection circuitry on the 87-3?  I've started tracing and sketching it myself but I figured I'd ask before I re-invent the wheel.

Thanks,
Tom
 

Offline mzacharias

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Do you have an actual schematic (and maybe full parts list?) for the 87III?

If so, I'd sure like a copy. I have a "service manual" on PDF which includes calibration but only a partial parts list, and no schematic.

I'm pretty sure if Fluke can't fix your 87III (and they can't/won't seriously try) they will replace it with an 87V at no charge.

They don't enforce "the original purchaser" thing for the warranty as I understand it.
 

Offline ogdento

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Hey thanks for the reply... I don't have an actual 87-3 schematic, only the same service manual you have.  I've been going by the 87-1 (v1000/rev AG) service manual schematics and then trying to trace the differences.  All of the vias and the multiple layers (3, I think) on this board make it a real pain!  At least if I hold the board up to the light I can see through it.

I hadn't heard that Fluke would honor the warranty on an 87-3 for the non-original purchaser.  While it would be tempting, it kind of feels like cheating since I never paid Fluke for the device... stupid conscience.  I guess half the fun is fixing the darned thing anyway  :-//

I'll wait a few more days to see if anybody else has sketched anything for the 87-3, but there are 6 new 14-16 pin 74HC logic chips (HC11, HC14, HC112, HC4016, 2xHC4538) on the back that are of course not on the 87-1 but need to be mapped out and understood.

 

Offline retiredcaps

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Just a quick question, other than the obvious problems you have reported, does the meter actually work when you measure DCV and resistance? 

For example, if you measure a 1k ohm resistor, do you get 1k?

PS.  Getting a free warranty replacement from Fluke varies.  The manual clearly states you have to be the original owner with proof of purchase, but some have reported otherwise.
 

Offline retiredcaps

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If I remember correctly, normal current draw should be around 2 to 3mA depending on the function.  So 15mA is high.

If you have thermal camera, try putting on the backlight on the 87-3.  Obviously this will cause even higher current consumption, but maybe something will stand out in the thermal camera?

As for the schematic, there is a schematic for the Fluke 87V which might help posted somewhere here on eevblog.  Obviously, the 87-3 and 87V are different, but it might yield clues?

Fluke 87V schematic courtesy of mzacharias

https://www.eevblog.com/forum/testgear/87v-schematics!/
« Last Edit: August 08, 2019, 03:43:53 pm by retiredcaps »
 

Offline ogdento

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Hey retiredcaps,
Good question about DCV and resistance... I couldn't actually tell if it worked because the LCD either doesn't turn on or is too dim to read.  When I get home I'll try your test with a light and a magnifier.

And the thermal camera didn't even occur to me... :palm:  If it's drawing 15mA then something's probably getting at least a little warm.  Comparing the thermal image to my other 83-3 might yield some clues.

I had no idea there was an 87-V schematic, I'll check it out.

Thanks!
Tom
 

Offline mzacharias

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Hey retiredcaps,
Good question about DCV and resistance... I couldn't actually tell if it worked because the LCD either doesn't turn on or is too dim to read.  When I get home I'll try your test with a light and a magnifier.

And the thermal camera didn't even occur to me... :palm:  If it's drawing 15mA then something's probably getting at least a little warm.  Comparing the thermal image to my other 83-3 might yield some clues.

I had no idea there was an 87-V schematic, I'll check it out.

Thanks!
Tom

If there's a constant 15mA current draw, it's quite possible something is getting warm to the touch.

No thermal camera required?
 

Offline ogdento

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mzacharias - very good point.  I did touch a couple of parts and chips to see if anything was hot to the touch, but something's got to be warmer than normal.  I had to leave town for a few days but when I'm back I'll try again with the backlight on, AND I'll take some thermal pictures.

and well done finding that 87-V schematic.  I checked it out, but save for maybe the input protection it seems vastly different... I didn't know they used an msp430 in those things!  If there's a military version of the 87-3 there's got to be a similar manual out there somewhere.  In any case, I'll post whatever I can sketch up - might not be complete but it'll be a start.
 

Offline ogdento

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yeah, somehow this didn't occur to me either, but 15mA at 9 volts is 135mW!!  whereas the normal consumption of 1mA is only 9mW.  that's a pretty big swing.
 

Offline ogdento

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I took a couple of interesting photos with a thermal camera.  I've got two 87-3 boards side-by-side, bad one on the left. 

When both are powered on (to ohms), after 10 seconds or so the bad board clearly shows heat in Q8 and in U4, but they were not warm to the touch.  I was alarmed to see U4 warming, hopefully I'm not doing more damage running it that long.

Q8 tested OK in-circuit, so to find the cause of the current flow I think I've got to follow "3-36 Vdd Regulator Troubleshooting" and start removing some components!  Of course, I still have to sketch out the differences in the power up circuitry because all of the components in 3-36 don't map to the 87-3
 

Offline ogdento

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And I tried retiredcaps' first test tonight... I put a 2.4k resistor in the V/com jack, set it to ohms, and the meter does not measure it - it turned on, did it's startup test (all segments on but dim), then it showed ACV, beeped a couple of times, restarted, etc.  I tried it 3 or 4 times and got similar results, it behaves erratically.
 

Offline ogdento

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I sketched up some differences around the turn-on circuitry and power supply... here are the basics:

- Q12 was moved and is now between U2 (ad737) and U6 (hex inverter)
- there's a new 47k resistor next to Q12 that is tied to the backlight switch and all the new logic crap on the back
- there's a new 2k resistor on the bottom between U6's pin8 and Q12's base
- U6's first inverter (pins 1-2) has been stolen from the piezo and is now used with the PDS inverter (pins 8-9) to drive Q8
- the base of Q8 is connected to U6 pin2 via a new 100k resistor on the bottom
- Q8 still drives Q3 which is now a BJT (it was a FET)

The rest of the power supply looks the same, so I didn't sketch it all out.  It's a terrible drawing and I need to clean it up, but here it is anyway.

(I also started a schematic with all the new logic gates on the back... it's a mess!  there's an inverter (HC14), a 3-input AND (HC11), a dual J-K flip-flop (HC112), two dual monostable multivibrators (HC4538), and an SPST analog switch (HC4016).  For some reason I started drawing it up using digi-key's online tool... I "think" I can share it so other people can edit it)
 

Offline ogdento

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Re: Troubleshooting a Fluke 87 iii (87-3) with low Vdd-Vss and high current draw
« Reply #12 on: September 03, 2019, 12:35:07 am »
I finally got around to creating a "new parts" list, and have also updated my 87-3 parts layout with the numbers (originally posted in another thread).  I'm almost done with a schematic showing all the new 74HCxx logic, so that should follow shortly.

Here's the parts list (the parts marked with an * are on the 87-1 list but the values have changed for the 87-3):

*C5 226 16K   22uF, 16 volt, 10%
*C6 106A M500U   10uF
C50 ?
C51 10-25   10uF, 25 volt
C52 475J M510C  4.7uF, 5%
C53 ?
C54 ?
C55 E105      1uF
C56 ?
C57 ?
C58 E105      1uF

CR20 A7      Dual Diode 70V 50mA SOT-23

F10         Fusible link?

R80 104      100k
R81 473      47k
R82 104      100k
R83 104      100k
R84 5363      536k
R85 3243      324k
R86 473      47k
R87 202      2k
R88 1503      150k
R89 104      100k
R90 104       100k
R91 104       100k
R92 104       100k
R93 104       100k (left this off the original layout)

*Q8 1AM      now an NPN, 60V, 350MW (was a FET)
Q20 1AM      NPN, 60V, 350MW
Q21 702H      N-Channel MOSFET

U7 74HC11   Triple 3-Input AND
U8 74HC4016   Quad SPST Analog Switch
U9 74HC14   Hex Inverter
U10 74HC4538A   Dual Monostable Multivibrator
U11 74HC112M   JK Flip-Flop
U12 74HC4538A   Dual Monostable Multivibrator
« Last Edit: September 17, 2019, 07:27:25 pm by ogdento »
 

Offline AngraMelo

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Re: Troubleshooting a Fluke 87 iii (87-3) with low Vdd-Vss and high current draw
« Reply #13 on: September 17, 2019, 03:15:11 am »
Im loving this thread so much.
Ive got a 87-V that never turns off. When the wheel is on "off" it goes to uA mode and stays there until the auto turn off kicks in.
Ive tried all my tricks with no success.
Hopefully this new 87-3 schematic will help me! thank you!
 

Offline retiredcaps

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Re: Troubleshooting a Fluke 87 iii (87-3) with low Vdd-Vss and high current draw
« Reply #14 on: September 17, 2019, 04:21:12 am »
Ive got a 87-V that never turns off.
I suggest you start your own thread for your problem.  There is a 87V schematic.
 
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Offline AngraMelo

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Re: Troubleshooting a Fluke 87 iii (87-3) with low Vdd-Vss and high current draw
« Reply #15 on: September 17, 2019, 05:18:27 am »
I just found the schematic for the 87-V, I remember a couple of years ago looking for it with no success.
It was not the intention to disrupt the thread
 

Offline ogdento

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Re: Troubleshooting a Fluke 87 iii (87-3) with low Vdd-Vss and high current draw
« Reply #16 on: September 17, 2019, 07:12:53 pm »
i have to get off my butt and finish the 87-3 schematic.  i'm only sketching the new/modified bits and am maybe 80% done, but there are some 74xxx chip pins that i haven't traced out and I need to add my hand-drawn changes to the power circuitry.

I previously said Q3 changed from a FET to a BJT... but it was Q8 that changed from a FET to an NPN BJT. 

I also missed a part in my layout... there's a new 100k resistor on the back that goes from pin 2 of the 4069 to Q8.  It's toward the upper left, above C30 and I've called it R93.  I've updated my parts layout and list above...

« Last Edit: September 17, 2019, 07:29:36 pm by ogdento »
 

Offline tooki

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Re: Troubleshooting a Fluke 87 iii (87-3) with low Vdd-Vss and high current draw
« Reply #17 on: September 17, 2019, 09:13:11 pm »
I just found the schematic for the 87-V, I remember a couple of years ago looking for it with no success.
It was not the intention to disrupt the thread
:O

Everyone has been looking for the 87V service manual for years, without luck. But you're right, it seems to be online now!! (the folder listing of the first link seems to indicate it was only uploaded last December.)
http://doc.xdevs.com/doc/Fluke/87V/Fluke_87V_Multimeter_Service_Manual-Fluke87servicemanual.pdf
https://www.manualslib.com/manual/1505585/Fluke-87-V-An.html

THIS detail actually probably is worthy of its own new thread!
 
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Offline ogdento

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Re: Troubleshooting a Fluke 87 iii (87-3) with low Vdd-Vss and high current draw
« Reply #18 on: September 18, 2019, 04:42:27 am »
Quote
THIS detail actually probably is worthy of its own new thread!

It already has one... mzacharias, the first responder to this 87-3 thread, is the guy who found the 87-V schematic to begin with and started a thread on it... and in an effort to help me out, retiredcaps shared the link to that thread in his second response above!!  :-DD

Now somebody please go find me a schematic for the 87-3 so I can stop holding this 4-layer board up to the light trying to see where the vias and buried inner traces go  |O
 
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