[volt-nuts] Fluke 5200A repair

Charles Steinmetz csteinmetz at yandex.com
Mon Jul 14 04:24:31 EDT 2014

Ken wrote:

>Yes R6 changes the output of the op amp by about 15 mv..
>During further investigation I found the input to Q1 was sitting at -400 mv
>when it should be at 0 +/- 100uv.

Suggest lifting one end of R3 to isolate U1/Q1 from the oscillator.

>Using the transistor tester I first tested all the socketed transistors

>I then tested all the soldered in transistors and found a number that give
>incorrect readings, so I will have to dismantle the board again and remove
>these transistors to properly test them.

You need to apply more analytical thinking and less doing.  A very 
valuable technique I often use to teach troubleshooting is to have 
someone troubleshoot a device that they can't put their hands 
on.  The student has a schematic and an initial description of 
symptoms (and usually a few measurements and observations of mine) 
that I give him/her.  The student asks me for measurements by phone 
or e-mail.  I will measure anything the student want measured (I'll 
generally ask why they want it measured, to keep them focused, but 
I'll always measure it if they ask).  This reduces troubleshooting to 
its essence -- thinking and making logical deductions -- and tends to 
prevent the bad habits we so often fall into, to tear into the 
equipment before we have fully digested the information we already 
collected.  Anyone can duplicate this method without actually being 
separated from the equipment -- it just takes discipline.

I strongly recommend against using any sort of transistor 
tester.  The best transistor tester is the circuit you are 
troubleshooting.  Think about the circuit, then measure DC voltages, 
then think about what those voltages mean.  Think until you can 
explain why ALL of the voltages are as you measured, looking only at 
the schematic.  When you can, you will generally have a few potential 
culprits.  Sometimes your thinking will lead to other measurements 
you need to make.  Make them, then continue thinking, looking only at 
the schematic, until you can explain why ALL of the voltages are as 
you measured.  Don't remove any parts if you don't know why you're 
removing them.  ("To check them on the transistor tester" does not 
count as knowing why.)

I understand that you are limited by not having a card extender.  But 
that is no excuse for doing ineffectual things just because it seems 
that's all you can do, like the drunk looking for his lost keys under 
a streetlight -- "Did you lose them here?"  "No, I lost them over 
there, but it's dark over there and here I can see where I'm 
looking."  Make an extender, or bring out pigtails from the important 
nodes until you have the measurements you need.

You may have finally put your finger on the fundamental problem -- 
leakage current into the Q1 gate node, possibly through C24 or CR29 
(and possibly due to excess DC offset in the oscillator amplifier -- 
see schematics p. 8-27/8-28 and 8-33).  If so, lifting one end of R3 
should give you a good clue.

Best regards,


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