Fluke 8845A reads double the voltage in 100V range - Page 1
Forum section: electronics / testgear

author: kb0thn
author_rank: Contributor
timestamp: August 08, 2013, 02:12:03 am
content: Hi Group,I have a Fluke 8845A from about 2008. Meter reads essentially perfectly in the 10 voltage range. When it switches to 100V range, voltage reads close to exactly twice what it actually is.The problem seems to be related to the front / rear input selector switch. I had never used that switch until about a month ago when we attempted to use it for an application. My software guy that was developing the ATE software mentioned that it was reading "funny" and we ended up using an stand alone switch to select voltage source to measure, rather than the front / rear switch like we planned. I confirmed today that by banging back and fourth on the front / rear switch I can get it to periodically read correctly on the 100 volt range.Fluke will repair and re-calibrate it for $565, they claim. A bit steep for a what was originally a $950 meter, if you ask me. Other instrument repair centers are no-quoting repair on it; instead they just want to sell me a new meter.Before I crack open the case and start investigating, does anybody have any advice? Is the service manual available somewhere on the web?Thanks in advance,-James JarvisAPRS World, LLC

author: staxquad
author_rank: Frequent Contributor
timestamp: August 08, 2013, 03:34:05 am
content: http://us.flukecal.com/literature/manuals/search?keyword=8845a

author: kb0thn
author_rank: Contributor
timestamp: August 08, 2013, 11:12:12 am
content: Quote from: staxquad on August 08, 2013, 03:34:05 amI see the calibration manuals, but no service manuals. Did I miss something?Thanks!-James Jarvis

author: fmaimon
author_rank: Unknown
timestamp: August 08, 2013, 01:07:33 pm
content: I don't think you will find any service manual for this meter. They are non-existent in all recent ones.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 08, 2013, 01:14:04 pm
content: Hi,here you'll find the complete manual of the 8840A, the 5 1/2 version and perhaps similar.http://www.ko4bb.com/Manuals/Fluke/Fluke_8840A/Fluke_-_8840A.pdfGoto p. 166, there's the input switch S301 (front / rear).Normally, it's not possible that the 100V range reads twice the applied value, as there's a 9.9M / 100k divider, which is not directly dependant on the front/rear switch.K301 bypasses the 9.9M resistor by 100k and interrupts the 100k to ground - a volatile design, perhaps.(In HP DMMs, the 100:1 divider is completely switched in or out).If the 8846 is similar in this aspect, there may be the root cause:I'd suspect malfunction / contamination of relay K301, or oxydization of the bottom/ left part of S301, which signals the µP the state of the switch, and may irritate the µP, which in turn leaves K301 in the 10V input mode, even in the 100V/1000V range.What exactly is displayed, if you apply 10V in the 100V and in the 1000V range?Frank

author: kb0thn
author_rank: Contributor
timestamp: August 08, 2013, 01:23:35 pm
content: Quote from: Dr. Frank on August 08, 2013, 01:14:04 pmFrank - thanks for the reply and the link to the 8840A manual. That manual is actually doubly great, since we have 8840/AF here that needs some work.Apply 10 volts from my Fluke 343A (which is cold) give me the following readings:(10 volt range) 10.00041 nice and stable(100 volt range) 19.8412 and the last two digits are bouncing all over the place(1000 volt range) 19.841 nice and stableI will pop open the case and see if I can figure out what is going on. Thanks for the suggestions.

author: kb0thn
author_rank: Contributor
timestamp: August 08, 2013, 05:22:59 pm
content: It appears the problem was with the front rear switch and that I fixed it.The front rear switch is a terrible implementation, IMHO. Little pads on the bottom of the circuit board and a plastic slider with spring contacts to bridge pairs of those pads. Despite having only been switch < 100 times, the switch pads were somewhat torn up and had decent witness marks on them.I really have no desire to use the rear inputs, so I just removed the slider switch and jumpered the 10 connections that it would normally make. Jumpers were placed on the top side of the board, so if I really wanted to in the future I could add the front / rear functionality back.Meter now reads pretty much correctly on all ranges.Attached are some photos of the repair, the switch slider (didn't take any of the bottom of the board, unfortunately), and evidence that it is reading correctly on 10, 100, and 1000 volt ranges.I never did find schematic or service manual online. As close as I found was the calibration manual, which actually was somewhat useful in helping figure out how to dis-assemble the meter.Questions:Given that this thing is 5 years old and has never been calibrated, I am thinking of sending it in for calibration. I am wondering if Fluke or whatever cal lab will have any objections to performing calibration on this slightly modified meter? And recommendations for a good cal lab? Fluke quoted $171 and 5 days turn-around on a calibration. I'm also looking for someone to calibrate my 343A calibrator.Thanks for the advice,-James Jarvis

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 09, 2013, 07:34:36 am
content: Quote from: kb0thn on August 08, 2013, 05:22:59 pmHello James,the cal manual should indicate, if front / rear switching is required during calibration.I think so, because normally, the offset voltages on the shorted input jac´ks both for rear and front are included in such DMM calibration procedures.Therefore, Fluke might stop the calibration, if there's an error due to your modification.Perhaps you may ask them before.They will also calibrate the 343A for a similar price.I recommend to clean the surface of the switch springs and the pads on the PCB.Depending on the metalization, (gold on the PCB? brass as spring material?) there's oxydization or just dust on the surfaces , which can be removed either by alcohol solvent, or by a very fine polish paste.Afterwards, the switch will work like new.On the photograph, one of the springs seems to be twisted. If so, try to bend this spring to its correct form.Frank

author: MrAureliusR
author_rank: Unknown
timestamp: August 09, 2013, 07:50:03 am
content: I would agree with the above post completely. Cleaning both sets of contacts will, in my experience, almost always restore a switch to original working condition. Pick up a bottle of Electrolube IPA or Contact cleaner, give 'er a spray and a scrub (gently) and it'll be like new.Its much better to keep the switch in place. Most cal labs will claim the modification will make it difficult, if not impossible, to guarantee the accuracy/longevity of the cal. So even if they do cal it they won't guarantee it if you have problems down the line. They'll just say "Sorry, it must be due to your modification." If you absolutely must remove the switch then try and do the cal yourself...