[volt-nuts] Fluke 335A

[gbusg]

I had similar, severe repeatability problems with the rotary switches in a 
very old (blue front panel) Fluke 335A.

By comparison I later acquired a much newer (late serial number with beige 
front panel) 335D having much more stable and repeatable output as related 
to its rotary switch settings.

Joe and Bill, did you happen to notice the date codes on the capacitors (and 
other parts) in your old 335As? (I no longer have my 335A, but I suspect it 
was made in the 1970 timeframe?  By contrast, IIRC, my 335D was made in the 
1990 timeframe.)

Greg


----- Original Message ----- 
From: "Bill Gold" <wpgold3637 at att.net>
To: <volt-nuts at febo.com>
Sent: Tuesday, September 13, 2011 1:08 AM
Subject: Re: [volt-nuts] Fluke 335A


Joe:

    Your experience with the electrolytics in your 335A matches what I have 
found in four 332Bs and one 335A.  Except for the null meter in the 335A the 
boards are exactly the same.  I found that the best way to insure everything 
was OK was to check every electrolytic cap on every board in the instrument. 
And like you I found that most of the bad ones were "open" with no 
measurable capacitance.  And the majority were the 50uf/50v caps.  Just a 
poor choice of manufacturer and I am sure that Fluke didn't realize what 
they had done at the time.

    I also found that the feedback string rotary switches, the seven rotary 
switches that set up the voltage for the range selected, were all unreliable 
as far as constant contact resistance each time they are set to a different 
digit.  So what you will probably find is that when you rotate the most 
significant digit switch, i.e. 0 to 10 volts on the 10 volt range, you will 
have a slightly different voltage every time, if you have a meter with 
enough digits to show this.  Also this makes it difficult to adjust the 
first three sets of decade resistors because you will have varying contact 
resistance every time you change the switch settings.  The 0 to 10 volt 
switch is selecting 100k resistors so the contact resistance effect is 
minimal.  I could see this on a 6 1/2 digit meter (HP 3456A).  But where you 
are going to see this problem manifest the problem the worst is the lower 
digit switches where the resistors in the string are the smallest.  Here any 
unstable contact resistance is going to be worse because it is a larger part 
of the resistance being switched.  I believe these are 0.1 ohms each on the 
1 uv switch.  While this switch is only 1 uv per step and is insignificant 
in the overall accuracy of the instrument, it does affect the zero setting 
of each range.  The cal procedure wants each range to be set to +/- 2 uv on 
each range ( 10v, 100v, 1000v ).  While the 1 uv switch on the 10 volt range 
will affect the zero very little, the same switch becomes the 100 uv switch 
when the range is set to the 1000 volt range.  Now the smallest change in 
contact resistance will affect the zero on the 1000 volt range and you will 
not be able to keep the zero anywhere near ideal.  Again this may not be 
important to you if you are simply trying to cal a 8050 meter.

    My solution was to remove the feedback string board from the instrument, 
clean all of the switches, and then apply a thin film of Caig Labs DeOxit 
and then a thin film of Presevit.  Be careful as you may also have another 
problem depending upon when your instrument was manufactured.  The insulator 
standoffs for this board can simply crumble because the standoff material 
has degenerated over time.  I had to manufacture new standoffs for my 335A. 
Also the range switch which selects the 10 v, 100v and 1000v range can also 
suffer from the same contact problems.  This is much more difficult to 
service, but I used the same chemicals on this one also, with fairly good 
results.

Good luck

Bill
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