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Fluke 97: Auto Set not working, just cycles down to the maximum voltage

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I picked up a Fluke 97 this weekend, which was a great deal and a very welcome find.  I'm looking forward to being able to use it as a portable scope for my hobby functions.  The scope did not come with original probes, but the knurled BNC are insulated, so I should be in good shape with regard to 'hot' grounds (thank you EEVBlog postings for keying me in on that).  And, the EL backlight is a bit dim, but I will deal with that later.  The bigger issue for me is that AutoSet does not work properly.

I was looking at the YouTube "how to use" Fluke video on the 97 and I quickly noticed a problem.  When I inject a (F97 internally generated) square wave, and press AutoSet, it works great.  However, that is not always the behavior.  Very often (and I haven't quite figured out the reproducible sequence), when I press AutoSet, I'll get more relay clicking than when things are working properly.  From a display perspective, the scaling starts out nicely and then gets progressively smaller as the v/div increases. The process stops, of course, when the scope has reached is maximum deflection setting (or minimum, depending on how you look at it).  It is as though there is a stuck button, which *may* be the case (see further in this posting).  Once in this state, I get the same behavior each time I press AutoSet... Starts off looking great, more relay clicking, and the deflection gets smaller and smaller until I have basically a flatline (with some noise).  It is worth noting that the increase/decrease deflection buttons seem to have no effect.

There are only two ways that I've noticed that seem to reliably get me out of this state.  The first is to cycle the power.  The second is to go to 'Meter' mode and then back to 'Scope' mode.  I have further noticed that as soon as I press either the increase or decrease buttons for the deflection, I trigger the scenario (assuming that AutoSet is in the working state at that point).  I've mulled over whether or not this means there could be a stuck button, but my sense is "no" because the pressing of the button does trigger some action, namely the undesired behavior.  I have only experimented with the A channel, so I don't know if AutoSet works on the B channel yet.

I do have a little more information.  Although this behavior existed prior to doing so, I am fully aware that the units are 30 years old and that electrolytic caps could be faulty; as such, I opened the unit up.  Sure enough, one of the surface mount caps is leaking a little (not too bad).  I'll be replacing all of the SMT caps.  Hopefully this will make the EL a little brighter (?) and decrease the whine.  Interestingly, one of the FOUR caps is missing... completely MIA.  Based on the look of the solder on the pad, it looks as though the cap was rather rudely yanked from the board.  It was not in the case floating around.  I don't know the story on this, but I am wondering if it could be part of the problem.  I don't have the scope open right now, but it was the cap that was on the far right of the "four" when looking at the scope in the normal position (top away from you).  Although this might cause more noise, though, I can't really see how it could cause the problem I'm seeing, but I'm curious if someone has thoughts on the matter.

I have also wondered if I could have a corrupted OS.  I have either 4.02 or 4.03 (again, don't have the scope in front of me right this second) as my firmware.  I have read elsewhere on EEVBlog that it is possible to reload the OS, but I'm not sure if this will eliminate the calibration values which I'm reticent to do, even though the values probably are not all that good anyhow, given the age of the scope.  I'm more worried about bricking the whole scope.  Further, I don't presently have an RS232 IR interface for the scope.  If there is a way to get a checksum on the firmware, however, that I can verify against someone else's values, that would be a good way of figuring out if corrupted code is the problem (unless the checksum included the calibration data).

Well, that is about it... I'm hoping that someone out there has an idea as to what can be causing the problem.  If you do, I am all ears!  I'd love to get this working again as I have lots of places I can use it.


P.S.  Outside of the electrolytic SMT caps, the others looked good.  Has anyone had a need to replace these or is the general rule to just replace all of them?

Does anyone have ideas, seen the behavior before, etc?  The whole thing is odd to me.  It doesn't feel like a debounce problem and, frankly, debouncing is likely to be done in software anyhow.  But if there were a corrupted OS, then....

I'd make my first point of call to clean the keypad membrane and contacts, and check/clean the board for SMD cap puke. If you've had a confirmed cap leak, it probably needs cleaning thoroughly, either in an ultrasonic tank or several passes by hand with the IPA.
Electrolyte in the wrong places can cause all kinds of "fun".

The missing cap would want replacing too; debouncing less likely an issue than pure instability from an improperly decoupled power line for any of the ICs that handle digital signals.

Thank you for the response.  I hope to pickup the caps tomorrow and install them over the weekend.  That was on the list anyhow, as was the cleaning of the contacts for the switches.  I was waiting to do all of that "maintenance" type stuff all at the same time, hoping it would resolve the problem.  If it doesn't, then the only remaining thing I can think of would be a corrupted OS.  But it sounds like your suggestions are kind of the route I was going, so that is nice to know.

Again, thank you for the response.  I'll post my results once I've got the new caps installed, etc.

I wanted to give an update for the benefit of anyone else that might find this thread, and who is experiencing the same problem.  I managed to buy the four capacitors today (three 10uF 50V and one 100uF 6V) and install them.  As mentioned, the right one (C1319), a 10uF, was missing entirely.  One of the legs was on the pad, but that was the only evidence of the cap.  The left cap, also a 10uF (C1302) was leaking.  I remove the three installed caps and the leaking one had goo on the board.  Fortunately, none of the delicate traces were damaged.  It took a lot of work to get it nice an clean, but eventually I did.  I then installed the four brand new caps.

I turned the meter one with eager anticipation (Christmas in October).... And, lo and behold.... It all works!  The AutoSet no longer ratchets to the 100V/div.  I should also note that it turns out that many of the other buttons on the left side of the keyboard also would cause the cycling to 100V/div.  My guess is that there must have been a lot of noise or something.  But, in the end, all that I really care is that it is working now.  Total cost to fix?  About $1.25.  It was a good day!

Hopefully this thread will help another soul.  And thank you, AVGresponding, for giving me your thoughts.  I appreciate that.


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