[volt-nuts] Fluke 5200A repair

[Kgoodhew]

Hi Gerd.
             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.
While disconnecting things to try and isolate where the 400 mv was coming
from the resistors in series with the +15 on the p/s regulator board decided
to smoke!
The excess load was on the A7 power amplifier board, so something appears to
have died on this board whilst I was testing, wether I caused it or it was
just a coincidence I do not know.
Using the transistor tester I first tested all the socketed transistors and
found the n channel fet Q1 that I had previously replaced had failed,
however this was not causing the overload condition on the +15 rail, nothing
shows up with an ohmmeter test on this rail by the way.
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.
Your suggestion to run extension cables is a good idea as continually
removing the board to solder wires on to the points I want to measure is
slowly damaging the socket the board plugs into, as well as making it
difficult to do measurements.
So when I have the board disassembled I will run some cables to allow me to
operate the board out of the chassis.
Ken.

Date: Sun, 13 Jul 2014 21:42:23 +1000
From: Gerd <admin at controlelectronics.com.au>
To: Discussion of precise voltage measurement <volt-nuts at febo.com>
Subject: [volt-nuts]  Fluke 5200A repair.
Message-ID: <53C2709F.50406 at controlelectronics.com.au>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Hello Ken,

If I understand this correctly, R6 adjustment produces a few millivolts
change at U1 output but no change at all at the power amp output. That seems
to point to the input differential stage or the current to voltage converter
stage following that. This will be difficult to find without removing the
board from the machine. Given the amount of time you have spent so far it
would be quicker to rig up enough extension leads to operate the board
outside on the bench. I've done this many times when making or obtaining
extender boards was impossible.

Regards



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