Fluke 343A unstable voltage

[Fastah]

Hi

I'm lurkin this excellent forum for a long time... many times I found answers here and solve many tricky problems with electronic equipment but now it's time to ask for help.
I have Fluke 343A calibrator. I bought them on Ebay last year and seller write on description "Powers on. There is a note that says: When output is turned on it is unstable".
When I connect multimeter on the output and turn Fluke on, there is a voltage on every range, can change voltage with selectors - every switch working but voltage is totaly unstable on every range...
I've been monitoring the voltage for several hours, but do not stop... constantly goes down, up, down...
ex on 10v drift near 1v...

As I can see all electrolitic capacitors was changed by former user. I suspected electrolitic capacitors, desolder and check capacity and esr of all capacitors and all capacitory are good.

I don't know what the message writer meant "when output is turned on". I don't know how to output turn off and measure stability in this state. Where to disable output and check voltage stability in this state? Maybe desolder some link on board or what? I have no any idea. What do you think?
Please help.

Sorry on my bad English. I'm trying to be better :-)

Thank you

[2N3055]

Link to User /service manual

http://exodus.poly.edu/~kurt/manuals/manuals/Fluke/FLUKE%20341A,%20343A%20Instruction.pdf

Also FeedbackLoop video :



Start with measuring power supply buses.. Be VERY carefull !! Very lethal voltages inside, in excess of 1000V...

Good luck!

Regards,

Sinisa

[cellularmitosis]

defpom also did a multi-part series

[Fastah]

Hi

I've reviewed all the available videos and threads on various forums. Most of the problems are with electrolitic capacitors. In my case is not problem with the capacitors... obviously some more sophisticated failure... so I asked for help here...

[BNElecEng]

Found this  98 page manual, do you have it already?

http://exodus.poly.edu/~kurt/manuals/manuals/Fluke/FLUKE%20341A,%20343A%20Instruction.pdf

Edit: sorry, didn't see the post above where the manual was posted already.

[Dr. Frank]

Hi

I've reviewed all the available videos and threads on various forums. Most of the problems are with electrolitic capacitors. In my case is not problem with the capacitors... obviously some more sophisticated failure... so I asked for help here...

That's too few information you're giving, to help you.

Have you connected Sense and Output jacks?

Then, you need to measure systematically inside the instrument at some crucial points, like the supply voltages, the reference, at the output of the chopper amplifier, etc.

[Fastah]

Hi

Yes, Output and Sense jacks are connected with shorting links.
I will check stability and ripple of power supply voltages first... Can I test voltage reference directly on SZA263 or at some other test point?

Thank you

[Dr. Frank]

Use TP2 for the amplified +15V reference voltage.
Should be rock solid.
You can also test on the direct reference output, should read about 6.9V

Frank

[Fastah]

Hi

Today I tested some voltages inside... I made some short videos too...
+15V reference looks stabile at 14.9994V... direct reference voltage drift between 6.55355V and 655371V...

uploadad videos:

1000V unstable voltage example:
https://youtu.be/x3CB17hP2r4

1000 unstable voltage ripple:
https://youtu.be/zGm6lsPgCHM

reference direct voltage (1):
https://youtu.be/8Reavw5odLw

+15V reference voltage (1):
https://youtu.be/X6sR3ELYBV0

reference direct voltage (2):
https://youtu.be/qo-1YHlz0w8

reference direct voltage (3):
https://youtu.be/v46_DSmKdCA

+15V reference voltage (2):
https://youtu.be/Qmofv17TioA

Thank you

[Dr. Frank]

reference voltage (+15.000V) seems to  be ok. The variation on the raw reference might be due to the DMM, as it's a sensitive high impedance point .
34401A should be in the Hi Z mode, and in 6 digit, slow mode, when measuring precise voltages.

I would not test at all ACV ripple at 1kV dc!! Might damage your 34401A!
Maybe you test up to 500V only, because HV is switched from different transformer taps, so limiting the danger .
If you have set 1000.000V, and you measure 982V only, then there's a different bug than some kind of ripple.. the regulation fails.

What about the -15V auxiliary supply voltage?

If  10V range is already unstable, you should  concentrate on that range at first.
What's the output, when you dial 10.00000V?


You should check the chopper amplifier, if all voltages are present as expected, then the series regulator, and the protection circuits around.. the expected voltages should also be clear from the schematics, I hope.

Also check, if the current limit indicator (light bulb ok?) is on or off.. maybe this circuit is always limiting, which may lead to unstable output voltage.

Frank

[Fastah]

Hi

Yes, Fluke 343A drifts on all ranges...
-15V power supply is stabile at -14.96844V

videos:

10.000000V voltage drift:
https://youtu.be/ubQVbmMHWfo

-15V power supply:
https://youtu.be/eVU3THk9JC4

What do you think? I there problem with chopper amplifier?
What is the best procedure to test chopper amplifier?

Thank you

[Dr. Frank]

Again, the variation/drift of the 10V output is not your problem.
The output is way off already, so the regulation does not operate correctly.

I hope that you are able (understand) to test an OpAmp circuit, as the chopper amp is nothing else than that.

See also the chapter in the manual, Theory of Operation, you might at first measure through the crucial test points of the Block Diagram (pages 3-1, 3-2).

[artelse]

Am currently also repairing a Fluke 343A with more or less the same symptoms: i.e. slowly oscillating output voltage. This was solved by replacement of various gone bad caps. But the last digits of the voltage dials won't change the voltage. So am looking into the chopper amp and am seeing a high spike on the output TP10. It should be in the low voltage range with peaks of max 0.3V, but look at the scope picture, it's about 8V..!
So attempting to adjust the spikes by means of pot R160 and R162, I can only bring down the peak by fully turning chopper drive fully clockwise. Then R162 spike adjust is extremely sensitive. So according to the manual Q38, 39 and or IC1 (709 opamp) are at fault. How can I best test if one of those is out of spec?
Thanks.

[ClavinovaGuy]

I have also researched fixing the 3 Fluke 343A units that were in my loan pool. Found the parts I needed were obsolete. I found some new Tektronix programmable power supplies of various voltages that were extremely stable and had programmable ramps that satisfied most needs. To back them up, I just loaned a calibrated Fluke 8846A with them.