[volt-nuts] fluke 5200a repair

[Kgoodhew]

Hi Gerd and Charles,
	Thanks for the input it has given me other things to think about.
I checked the output of pin 6 of U1 and I get a voltage variation of 10.2 mv
from +15 to -15v adjustment on R6, so that appears to be working correctly.
Now the fun starts :- using my HP 3478A to measure the dc on the output
terminals I get  a reading of 0.9mv on the 1 volt range, 0.3 mv on the 10
volt range, and 1.8 mv on the 100volt range.
As a quick check of wether C8 may be leaky I pulled Q38 out of it's socket
and got a constant DC out on the output terminals of 0.22mv on all three
ranges, so maybe C8 is leaky?
Using a fluke 77 3.5 digit multimeter yields vastly different DC readings,
up to 5.5 v on the 100 volt range, so I suspect the Fluke 77 is reading some
of the AC component on the output terminals even though DC is selected.
Last night I did check the input and output on the amplifier, and on the 1 v
range I was getting an input of 1v and an output of 3 volts, so the gain was
correct, I did not check it on the 100v range though to see if the gain was
30.
The input and output was a good clean sine wave on the amplifier.
I have previously checked the 190 volt + and - supplies and they were spot
on, but I will check them again before I pull the amplifier board apart
again to remove C1 and C8 to test with a higher voltage other than the
battery voltage of the multimeter, to see if they may be leaking at a higher
voltage.
Thanks,
Ken

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Message: 2
Date: Wed, 09 Jul 2014 14:21:13 -0400
From: Charles Steinmetz <csteinmetz at yandex.com>
To: Discussion of precise voltage measurement <volt-nuts at febo.com>
Subject: Re: [volt-nuts] Fluke 5200A repair
Message-ID: <20140709222123.LMBGV5Je at smtp2o.mail.yandex.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed

Ken wrote:

>R6 slider to U1 varies from +15v to -15v but the output of U1 only 
>varies by mv's, I put this down to the loading of R9 to earth on the 
>input to pin3 of U1 (221 ohm),only allowing a small variation to the 
>input on pin
>3 of U1, maybe this needs further investigation!

Look at the circuit.  The 100k potentiometer (R6) feeds a voltage divider
composed of R8 and R9 (464k and 221 ohms, respectively).  The tap of this
divider feeds the reference input of U1.  R8 and R9 attenuate the +/- 15v
range of R6 by a factor of 210, so the divider tap voltage (i.e., the
reference voltage for U1) only varies +/- 7.14 mV from ground for the full
range of R6.

U1 is an integrator, so it has lots of gain at DC (Q38 switches the
integrating capacitor to give two time constants).  If one of these
capacitors is leaky or shorted (or there is another leakage path from Pin 6
to Pin 2 of U1), it would reduce the gain of the integrator and could
produce the symptom you are having.

Best regards,

Charles





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Message: 3
Date: Wed, 09 Jul 2014 14:29:40 -0400
From: Charles Steinmetz <csteinmetz at yandex.com>
To: Discussion of precise voltage measurement <volt-nuts at febo.com>
Subject: Re: [volt-nuts] Fluke 5200A repair
Message-ID: <20140709222949.TnBeU5B8 at smtp2o.mail.yandex.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed

Oops!  the divider ratio of R8/R9 is 2100, not 210.

Best regards,

Charles



>

Message: 4
Date: Thu, 10 Jul 2014 18:25:32 +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: <53BE4DFC.7090902 at controlelectronics.com.au>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Hello Ken,

The offset adjustment is about +-7mV to the input of U1. The amplifier has a
gain of 3 or 30 so adjustment is small. Is the problem the same on both the
10V and 100V ranges? Is the amplifier producing the full output voltage? Is
it distortion free?
Are both the 190V regulators working correctly?

Regards



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