Fluke 8000A Help Needed

[m k]

Charge indicator?

Seems that the level is more critical.
Try 0.1s/div.

My 1st bet is R21.

[bdunham7]

What’s the purpose of the incandescent lamp DS5?

It limits battery charging current if they are discharged.  High-tech stuff, hey?

I'm not sure what to advise you to do next.  Those PSU rails look pretty similar in terms of ripple at both 60Hz and the ~4kHz of the inverter.  Perhaps we shouldn't ignore the very obvious fact that the voltages on the bad meter are much higher, although I don't know what that would cause in terms of the instability.  Can you do the same scope capture using points 1 and 5 from the diagram I posted earlier?  And then do a second capture zooming all the way out to 100ms/div (1 second total capture)?  Earlier you said those points were unstable, I'd like to see what that looks like.  If there's any instability at points 1,2 or 5 that isn't reflected in the power rails, then there's an issue elsewhere.

I can't say that this is a PSU problem at this point.  If I had it I would connect a bench PSU to it for further testing, but since that's not currently an option you'll have to keep poking.

[Goatropercu]

A bench power supply is also now on my must get list.

The incandescent DS5 bulb is faintly pulsing/flickering like something is drawing down the current. Just an observation.

[Goatropercu]

Can you do the same scope capture using points 1 and 5 from the diagram I posted earlier?  And then do a second capture zooming all the way out to 100ms/div (1 second total capture)?  Earlier you said those points were unstable, I'd like to see what that looks like.  If there's any instability at points 1,2 or 5 that isn't reflected in the power rails, then there's an issue elsewhere.

Images attached. The + and - rail seem to behave the same from the CR15-CR18 rectifier onward. They both bounce around between 19 and 20vdc (+ and - of course).

Voltages at Q22 and Q23 below. These are not too far off from the working meter, other than the base of both measure -.2vdc.

Q22:
E: 8mVdc
B: -.4vdc
C: 5.3vdc (+/- 0.2v)

Q23:
E: 8mVdc
B: -.4vdc
C: 5.3vdc (+/- 0.2v)

Thank you.

[bdunham7]

Alright, I think we're onto something here, although it still isn't clear at those scales.

If you can make this work, set your scope scale to 2V/div and position your CH1 off the bottom of the screen (-14V if you can) and the CH2 off the top (+14V) so that the traces appear on the screen.  This should work if you scope has sufficient offset capability.  If not use 5V/div and just put CH1 at the second line from the bottom and CH2 second from the bottom.  Now set the scope for 500ms/div and post the results.  Do the same for the working meter and post those results too.  If I'm seeing what I think, the difference should be obvious.  Unfortunately the fix won't be...yet!

The incandescent DS5 bulb is faintly pulsing/flickering like something is drawing down the current. Just an observation.

Yeah, it may turn out that as crude as it is, missing battery and all, the PSU may still be good enough for the meter to work more or less properly.  And the fluctuations may be due to some other part of the meter drawing it down erratically.

Edit:  Just out of curiosity, what happens if you take the light bulb out entirely?

[m k]

The incandescent DS5 bulb is faintly pulsing/flickering like something is drawing down the current. Just an observation.

Dang, I didn't check the manual and thought the switch is either or.
Surely the lamp is then the major part compared to R21, maybe it's too bright.

[Goatropercu]

…set your scope scale to 2V/div and position your CH1 off the bottom of the screen (-14V if you can) and the CH2 off the top (+14V) so that the traces appear on the screen.  This should work if you scope has sufficient offset capability….Now set the scope for 500ms/div and post the results.  Do the same for the working meter and post those results too.

Images attached. Not exactly sure what the others indicated and/or if these are useful in making a determination. That being said if I need to change something up let me know.

Just out of curiosity, what happens if you take the light bulb out entirely?

With the incandescent bulb DS5 removed the first thing I noticed is that the display is significantly dimmer. Rail voltages have slouched to positive and negative 11vdc.

[m k]

So to operate correctly the machine needs a correct lamp, it seems.

I had a car once that had an ashtray lamp as a headlight level motor fuse.

[bdunham7]

Images attached. Not exactly sure what the others indicated and/or if these are useful in making a determination. That being said if I need to change something up let me know.

OK, even though there is quite a ripple envelope, you can clearly see the whole envelope going up and down over the timeframe of ~100-200ms.  In addition, the two power rails are moving with each other in magnitude, or opposite each other in absolute voltage if you like.  That indicates that the two likely causes are either the 5V input to the inverter is varying or there is some variable but matching load on the power rails.  Since the AC input buffer stack can't be it (the CL1 keeps the current to 1mA), I'm thinking it is probably the 5V.  One possible cause is the digital display which almost certainly draws some varying current.  The light bulb flickering seems to indicate the same thing--varying current draw on the 5V supply.  None of this would be surprising without batteries, the true mystery for me is how the other meter manages to work properly!  Are you sure that no modification of any kind has been done to the good meter or that there aren't some little batteries hiding in there? 

A couple of random things to look at:  What happens to the power supply voltages, the light bulb flickering, etc, if you put the meter in the DCV range?  Is either U3 or U4 getting excessively warm?  Are all of the display segments working?

It also has occurred to me that the milliamp-second function is a signficant revision of the meter overall, including the AC input circuitry (which seems fine at this point).  Without a diagram of that board and how it connects to the rest of the meter, it is pretty hard to know if that might be involved.  However, right now I'd try to force the PSU to be stable and quiet even if it doesn't want to and even if the other meter functions with fairly noisy power rails.

[Goatropercu]

At this point I like the crowbar via zeners idea.

Now that you mention it the connector at the mA/sec board is wired differently on both devices. EDIT: and now that I look even more closely the big 5uf/50v white cap is not present on the working device. The additional fuse tacked on to the one above the shield has stood out, but i just assumed this makes no difference in what I am experiencing since I have continuity there.

[Goatropercu]

For some reason I couldn’t upload this pic in the previous post.

Can’t thank yall enough for your time. 95% of what I measure is dc and it’s ok if the acv function on this meter just wasn’t meant to be due to design, non documented modification, and/or remote troubleshooting w/ the likes of myself. I’m not throwing in the towel, just don’t want to be a burden. This has spurred me to really start looking into purchasing a dedicated bench meter and power supply.

All of the display segments are working.

I will compare readings w/ the dcv function selected and check U3 and U4 temps and report back.

Just out of curiosity, how would I go about connecting an external 5vdc power supply to serve as the unregulated ps? Would I just disconnect the cathodes of CR13 and CR14 and feed 5vdc to those two points? Ground?

Thank you.

[m k]

No, unreg. is not used, so not needed.
Connection is of course correct but voltage you would need is that earlier measured 8V, but then you also need a correct lamp, diodes can also stay.

For 5V you use the other side of the lamp.
Treat it as a normal battery operation.

[Goatropercu]

Ah, thank you.

[m k]

The real power route is supposed to go through the lamp, problem is that its resistance comes along, so no very easy way out.
Now R22/R24 is 330/82, so around 10mA and a quarter of 5V to Q22 and Q23 bases.
R24 voltage is what it is, so that the lower part of T2 can switch those transistors, and R22 is just its feeder.
So changing R22, or R24 to a potentiometer is not going to do anything to the actual feeder voltage.
Maybe the easiest permanent way is to remove R21 and replace the lamp with adjustable mini regulator board from China and add a LED to its output side, if needed.
Since C25 is the only reservoir and then missing you may need a new one, how big depends how low input your regulator accepts and how steady output is needed.

For a curiosity you can also put the regulator before the lamp and make the whole thing adjustable.
Lift CR13 and CR14 and connect them to regulator's input as a source, for output you can then use those new freed holes.
Then adjust the output so that the other side of the lamp is 5V, after that you can compare how the output compares to that earlier 8V.

For a curiosity you can also just lift R21 and see what it does to +/-15V.
The problem with a lamp as a voltage divider is that its resistance is temperature dependant.
So lifting R21 will change the resistance over the lamp as high as it can be and so drop the 8V as much as it can.

You can also find a low value potentiometer and try that in place of the lamp, but few Watts may not be enough, so better use a heavier one.
Then you can also test how the machine reacts when 5V changes.

BTW,
I'm still wondering how unregulated is changing to not unregulated over a lamp.

[Goatropercu]

I measured much of the same with the meter set to the DCV function and received the same results.

The mA/sec boards are different revisions; one is Rev C and the other is Rev D.

On a side note, early on in the discussion I noticed that the display on the "functioning" meter is not flashing in an over limit situation when the "2" and "20" ranges are selected. I didn't want to derail my own thread, but figure it worth mentioning at this point.

I may continue to fiddle around with it if anyone has any further suggestions (m k and I posted at the same time) or after I up my bench game to include a power supply

I suppose adding the zeners won't help if the pulsing stems from upstream.

Thank you for your time.

[bdunham7]

The problem with a lamp as a voltage divider is that its resistance is temperature dependant.

Maybe he should swap the lamps between the two meters!

[bdunham7]

I suppose adding the zeners won't help if the pulsing stems from upstream.

It probably would help anyway because you are well over the nominal voltages for the power rails.  The current through the zeners would be varying, but the voltage across them would vary less.  The dynamic impedance of a 15V 5W zener under those circumstances would be just a few ohms, so probably pretty decent control over a moderate range.

https://www.mouser.com/datasheet/2/68/CSEMS05385_1-2539170.pdf

But this all sounds like a great excuse to acquire a bench PSU.

[m k]

The problem with a lamp as a voltage divider is that its resistance is temperature dependant.

Maybe he should swap the lamps between the two meters!

Yes, and even more if they seem to be equals.

E,
'' <-> 's'

[Goatropercu]

I will swap lamps and see what happens.

If I were to add 15V 5W zeners across C17 and C18 would it go like this:

C17+ to zener 1 cathode
C17- to zener 1 anode

C18+ to zener 2 anode
C18- to zener 2 cathode

Don’t grill me if that is incorrect.

[bdunham7]

C17+ to zener 1 cathode
C17- to zener 1 anode

C18+ to zener 2 anode
C18- to zener 2 cathode

Not grilling, but why would you connect the zeners differently on the two rails?  For a diode, the pointy end (on a diagram) is the cathode and the "positrons" (current) flow in the direction of the arrow.  A zener has a regular diode function as well, but you want it not to conduct in that manner so you'd always put the cathode to the more positive side.  Then when the zener voltage is reached, the current (conventional current being the opposite of electron flow, thus the positron joke) flows against the arrow.  So cathodes go to the positive sides of both capacitors.

[Goatropercu]

Thank you for being easy in your explanation. I’m learning, everyday.

I don’t have any 15v 5w zeners in my stash, so will have to include them in my next order.

Thank you both for your time and patience.