Fluke 8800A - Page 1
Forum section: electronics / repair

author: neo
author_rank: Super Contributor
timestamp: September 04, 2017, 06:02:13 am
content: I have a fluke 8800A and one day it just stopped reading. So i followed the first commandment, Thou shalt check voltages.+35 is actually 30 and fluctuates by up to half a volt, ripple in the order of a quarter volt even with new cap. FAIL!-35 See above.+18 is actually 14.5, ripple inconsequential.-18 absent. 50 millivolts of ripple.-13 is actually +5 with 50millivolts of weird ripple.Voltage across IC, 5V.Guard has 2 volts of ripple.All ripple measured with scope set to AC and probe set to 10x.J3 is absent.How fubared is this thing?

author: Kryoclasm
author_rank: Regular Contributor
timestamp: September 04, 2017, 06:23:41 am
content: Your negative rail is in a bad way. You might want to verify that you have proper rectification of the AC to start with.After that, check the filter caps on the DC power supply side.

author: neo
author_rank: Super Contributor
timestamp: September 04, 2017, 05:34:36 pm
content: Of the three bridge rectifiers one was good.

author: deepskyridge
author_rank: Regular Contributor
timestamp: September 04, 2017, 06:56:08 pm
content: I just finished repairing a Fluke 6060B that had a burned out bridge rectifier on the power supply.It took out a lot of traces, lots of bodge wires later it works a treat.Gary

author: neo
author_rank: Super Contributor
timestamp: September 04, 2017, 07:33:46 pm
content: Quote from: deepskyridge on September 04, 2017, 06:56:08 pmWell then it is slightly odd because there is no visible damage they just do not work. I took them out and tested them with an external ac supply and oscilloscope, only one did what it should.

author: neo
author_rank: Super Contributor
timestamp: September 07, 2017, 09:06:13 pm
content: i bought new rectifiers, same model, i soldered them in powered it up and it did absolutely nothing different. Same problems only know the rectifiers work and i am sure of that much, I am beginning to wonder if i should just cut my losses and buy a different one at this point.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 08, 2017, 06:25:21 pm
content: My guess is that there is still something else broken, like some part on the -18 V that is causing a near short. To much current would lower the other voltages from the transformer too and could damage the rectifier.Not sure about the circuit, but there is a chance to have quite a lot of extra damage, e.g. from the -13 being at +5 instead.If possible I would isolate the power supply part from the rest of the circuit and check the supply on it own first.

author: jh15
author_rank: Frequent Contributor
timestamp: September 09, 2017, 05:53:19 am
content: If you are within east coast reasonable shipping costs to and fro, send it to me. I want to try new camera for future youtube stuff when retired at our new hackerspace.I and technicians in an aerospace lab used to repair them.Thought I could tear down and t-shoot on tape and learn from chat room, or just post the video for comments as I let the smoke out.Just a thought.Videos would be creative commons or non copyrighted.

author: neo
author_rank: Super Contributor
timestamp: September 09, 2017, 07:36:24 am
content: Quote from: jh15 on September 09, 2017, 05:53:19 amI have not given up yet, only been tempted to and even if i do i will keep the thing as a parts mule for the next one i buy because this is one of my favorite multimeters. That said, sadly i cannot send you this one but i can offer a the advice that if you are just looking for something to attempt to fix or blow up you can get some broken less specialized multimeters on ebay cheaper than you could get this one from me.

author: jh15
author_rank: Frequent Contributor
timestamp: September 11, 2017, 05:19:53 am
content: nope, just to help.Keep plugging away.

author: neo
author_rank: Super Contributor
timestamp: December 10, 2017, 04:52:19 am
content: The tants went bad, replaced them now to a separate issue, the OHMs. It refuses to get a reading anymore, it can't tell the difference between air and a jumper clip. The meter will read VDC same as always, only ohms is problematic.Now either i screwed something up or it went kablam when the power supply did. Suggestions please?

author: anachrocomputer
author_rank: Regular Contributor
timestamp: December 10, 2017, 12:03:07 pm
content: Are you dealing with the older model 8800A that has a pair of AA batteries to power the Ohms circuit? Or do you have the later version with a DC-DC converter to power the Ohms circuit? The AA cells are well hidden inside a plastic box or cover on the Ohms PCB. And there's no indication on the outside of the meter that it contains batteries at all (bad Fluke).I have one of each type, although the newer one, I haven't cleaned and powered up yet. I was fortunate that the older one had two dead AAs in it, but they hadn't leaked.EDIT: Photos of my 8800A:https://www.flickr.com/photos/anachrocomputer/albums/72157671108861210

author: intabits
author_rank: Frequent Contributor
timestamp: December 10, 2017, 01:58:46 pm
content: I have one of these. I only got it a few months ago, so I haven't done much work with it.But It seems good and very accurate. I've never had it open, but if you need to check a part, a voltage, or a signal from a known good unit, I'll be happy to try measuring it for you...

author: neo
author_rank: Super Contributor
timestamp: December 11, 2017, 12:30:02 am
content: No batteries.

author: macboy
author_rank: Super Contributor
timestamp: December 11, 2017, 02:10:23 pm
content: Quote from: neo on December 10, 2017, 04:52:19 amFirst, do you have the shorting bars across the two source and the two sense jacks? This meter does 4 wire measurement, so without the shorting bars, an attempt at 2 wire measurement ends in head scratching.Is there any difference between manual and auto ranging?Measuring Ohms requires two primary stages: Generating a stable known current to put through the DUT, and measuring the resulting voltage across the DUT. You need to figure out which half of that is broken. Try measuring the current by setting the range to the lowest range, then connect the leads to another DMM set to measure current in mA range. If it is OK, repeat for each higher range; you usually see 0.1 x the current for each higher decade range.

author: neo
author_rank: Super Contributor
timestamp: December 12, 2017, 12:28:02 am
content: I got the OHMs working, contact broke. Fixed it ans is now good as when i got it.Just curious, it bounces around alot when reading small values. For example, 1 ohm it'll read but every thing after the decimal place is moving so fast thats it's hard to read. Any fix?

author: macboy
author_rank: Super Contributor
timestamp: December 12, 2017, 02:59:39 pm
content: Quote from: neo on December 12, 2017, 12:28:02 amWith probes? That'll happen. Try a better, more stable connection, such as really sharp stabby probes, or good tight croc clips.

author: neo
author_rank: Super Contributor
timestamp: December 12, 2017, 04:21:39 pm
content: On 6 other meters,one being my 8400A with the sampling rate cranked, using the same clips i am able to get a stable reading, on the 8800A even with a dead short it swings between 2 and 4 ohms.Edit: It seems the OHMs are not quite sorted.

author: macboy
author_rank: Super Contributor
timestamp: December 12, 2017, 04:39:00 pm
content: That's not right. I don't have my 8800A any longer (found another good home for it this summer) but I do recall it was able to read low ohms very well. It seems you still have some issue.

author: neo
author_rank: Super Contributor
timestamp: December 12, 2017, 04:42:06 pm
content: Quote from: macboy on December 12, 2017, 04:39:00 pmI think that it's down to one of two possibilities, either when the power supply blew up something affected the ohms card or it just isn't installed correctly. I am only unsure of one wire inside though even that i am 90% sure got to where it belonged.So i'm going to go read the manual some more just to be sure.

author: neo
author_rank: Super Contributor
timestamp: December 12, 2017, 05:32:07 pm
content: OK i'm at a loss here, there is a plain white wire, the point i'm almost certain it belongs to is marked as TP14.

author: anachrocomputer
author_rank: Regular Contributor
timestamp: December 13, 2017, 10:24:50 am
content: I've just checked my newer 8800A, and there is indeed a white wire coming from the Ohms module and connected to a pin next to "TP14" on the main PCB. TP14 is right next to a diode. The white wire is screened at the Ohms PCB end, but the screen is terminated part-way along and a single white inner core connects to TP14.Hope that helps!

author: neo
author_rank: Super Contributor
timestamp: December 13, 2017, 10:27:27 am
content: Quote from: anachrocomputer on December 13, 2017, 10:24:50 amIt does actually, thanks alot.