[volt-nuts] Fluke 335A

[Bill Gold]

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