[volt-nuts] Fluke 5200A repair
kgoodhew at iinet.net.au
Mon Jul 14 20:25:25 EDT 2014
Thanks for the comments.
I have already lifted one end of R3 to isolate the input to the power
amplifier and the fault is definitely in the power amplifier board.
I have done quite a bit of thinking about the problem with the circuit in
hand to try and make sense of the voltage reading that I have been able to
The lack of an extender card has certainly hampered my efforts to
troubleshoot the problem, hence I will probably follow Gerd's suggestion and
wire the unit up out of the chassis so that I can work on it whilst it is
As I said in one of my earlier messages it appears that the whole driver
amplifier assembly is not turned on hard enough as all the measured voltages
are higher than they should be as if the circuit is not drawing enough
That said the actual AC output at the output terminals is virtually spot on
in regard to amplitude and frequency.
This unit was purchased as a "not working" unit so I was unaware of
the previous history of repairs, but upon examination it was pretty obvious
that a lot of work had been done around the power supply regulator area, so
I concentrated on getting all the supplies correct, after this I then had to
correct an "overload" indication problem which when this was done the unit
functioned correctly other than for this DC offset problem.
As I did not know what the original problems were, I used the
transistor tester to check the transistors on the power amplifier board for
just a "good" or "bad" indication as a lot of the transistors are socketed
so it is relatively easy to test them.
Using this method I did find a number of failed transistors on the
board, the reason they failed was unknown, but may have been due to power
supply problems from the former owner.
Unfortunately due to home problems I have not been able to devote a
lot of time in one go to try and solve the problems, so the troubleshooting
has been a bit disjointed as you tend to forget were you were up to after a
But I will plod on and all suggestion are welcomed as I am sure there are
lots of people out there that have more experience than me.
>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
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
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