FIXED: Fluke 335A trips on higher voltages - Page 1
Forum section: electronics / repair

author: enut11
author_rank: Super Contributor
timestamp: May 02, 2017, 08:08:11 pm
content: Hi AllI now have a Fluke 335A DC Voltage Standard / Differential Voltmeter / Null Detector all-in-one. Seems to behave well on the 10v and 100v ranges. On the 1000v range it works up to 70v, higher than that it trips the Current Limit relay. Before I get into this beast, I am putting it up here for some ideas on how to proceed. Meanwhile, I am researching all info on the Forum.Thanks in advanceenut11

author: dacman
author_rank: Frequent Contributor
timestamp: May 02, 2017, 10:20:42 pm
content: There are lots of aluminum electrolytic capacitors in it. They can all be tested in-circuit with a good LCR meter.

author: enut11
author_rank: Super Contributor
timestamp: May 03, 2017, 11:52:24 am
content: Hi dacman, can you or anyone suggest a good reasonably priced LCR meterenut11

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 03, 2017, 02:27:02 pm
content: I just went through my 332B awhile back, most of the electrolytics were bad (1970s), while some of them were okay and in good shape, I replaced all of them since I was in it. The high voltage caps were the most difficult to get at. Since these units are usually run 24/7, those caps have a lot of hours on them, I'd start with changing them first unless you see something obviously wrong on one of the boards. There are some circuits which are not on plugin PCBs, those are a little harder to get at. If this unit is in the 30+ year group, the electrolytics are first on the list.

author: enut11
author_rank: Super Contributor
timestamp: May 04, 2017, 06:38:47 am
content: Thanks Edwin. So the current trip may be caused by leaky caps? What equipment did you use to to test the caps? I started out with an el-cheapo $20 tester but it died, probably from a partially charged cap.Where do you set the Trip Vernier and Current Limit controls on your 332D for normal use?enut11

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 04, 2017, 05:05:19 pm
content: Most of the caps in my unit were visibly leaking or bulging, it had been working okay and then suddenly stopped one day. Since it wasn't the easiest access, I didn't open the unit up very often for no reason as such. I'd say nearly 3/4 of the caps were obviously not in good shape so I just changed all of them out while I had the unit open. I did use two of my lab grade LCR bridges to check the caps out of curiousity, they were either bad or good, no in between. The high voltage caps were good but despite the difficulty of removing them and replacing them (no exact replacements), I did it just so I could ignore them for another few decades hopefully. Since they were mounted to a heavy PCB, they could not be visibly inspected, the board was hard-wired so it could not easily be extracted but if you think the caps are questionable then it is worth the effort.If your unit is over 20 years old, mine is closer to 40, then it is likely worth your time to just change out the caps and be done with it, if you only change out the 'bad' ones, most likely you will have to get back into it soon enough to change out more of them, given the complexity it is worth the effort to just change them all at once, not that expensive. I would recommend putting in 105°C rated caps where possible, since my unit is all linear power, I didn't have to worry about specific low ESR types. If you have any switching power in your unit then you'll have to be careful about the replacements.The trip and current limits should be set a bit higher than your expected worst case parameters for whatever you've connected to, it isn't so much that you're protecting the 335 as what it is connected to. The 335 will run just fine up to maximum settings but whatever you are connected to may not be able to tolerate higher voltages or currents, the trip/current limits are to protect the external load more than anything else.

author: cncjerry
author_rank: Unknown
timestamp: May 04, 2017, 06:18:42 pm
content: I assume you checked the variable trip control on the front panel? I think the 335d does as well. I had this problem as well and I can't remember why, but it had to do with the relay contacts. I cleaned them and the problem was corrected.

author: enut11
author_rank: Super Contributor
timestamp: May 05, 2017, 12:09:57 am
content: Thanks @cncjerry. I did fiddle with the front panel variable trip but I must admit I had not read the manual. As I was only feeding a 10Mohm DMM at the time I was not expecting the 335 to source any current. I will try the relay first as it is likely to be easier to do than the electros.Anyone know how to tell the age of this old bit of equipment? Is it likely to have any SMPS?enut11

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 05, 2017, 03:11:56 pm
content: I checked the date codes on components. I did clean relay contacts while I was in there but I doubt the contacts are your problem. Mine was also tripping at low settings with no load.

author: cncjerry
author_rank: Unknown
timestamp: May 05, 2017, 06:34:23 pm
content: I remember what was happening with mine. When I put it back together I had the power on/standby switch out of sync with the front panel. When the unit is in standby or off, the relay shorts the output terminals but if the contacts are dirty and the relay isn't working, then the contacts aren't open causing a high resistance (if they are dirty) or short if clean. So the problem was the relay on my unit but it showed up as a current issue under high voltages because my contacts were dirty. Had they been clean, then the current would have tripped on any voltage. Make sense?Make sure the relay is open when powered on.

author: enut11
author_rank: Super Contributor
timestamp: May 11, 2017, 09:41:43 pm
content: Some progress on the Fluke 335A Voltage Standard repair.The advice to change the old electrolytic capacitors was spot on. The 335 was tripping at low voltages and re-setting the trip pot did not help. When I checked the PS to the trip circuit it measured 26v but was supposed to be 35v. The current limiter PCB which feeds the trip circuit shows a number of large electros. On testing cap C1 feeding Q1 the ESR was through the roof. When I replaced it the regulated output went up to 37v, a little high but much better. No doubt all the electros need replacing but I was happy to get the 335A working again.A question about safe working voltage for an electro: I only had a 470uF 63v electro to replace C1. The unregulated voltage at the rectifiers was 60v. Is it safe to leave the 63v electro in circuit or should I replace it with a higher voltage version?As an aside, working on the Fluke 335A is not easy. It weighs over 20Kg and has a live inner chassis at output potential (up to 1100v). I made up a simple stand from chipboard to support the 335 on its front panel. This gives improved access and allows safe testing of the circuits.enut11

author: pelule
author_rank: Frequent Contributor
timestamp: May 11, 2017, 10:15:40 pm
content: IMHO the 63V is not enough. Lifetime will quite short./PeLuLe

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 12, 2017, 06:26:46 pm
content: Those innards look a lot like mine in the 332B I've got, that 63V cap is a bit closer to rated voltage than recommended while I doubt you'll see a shorten life span from it operating at 60V, but if the voltage wanders up over 63V very much, you'd want a little higher overhead rating like the original. The line voltage here is running over 5 volts higher than nominal and that could be a problem if your electrolytics are rated too close to nominal operating voltage. I wouldn't worry about replacing it quickly but I'd replace it reasonably soon with at least a 75V rated.Yes, that inner chassis is quite hot and really demands paying attention while working on it while on, it can give you a pretty nasty shock. Make sure the high voltage is discharged before poking around in there.