Author Topic: Fluke 8845A(may apply to 8846A as well) intermittent "Initializing" loop  (Read 2637 times)

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Offline Muxr

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So a few days ago I received an "as is" 8845A for a decent price, and was surprised when it fired up without issues:



It didn't however take too long for it to start exhibiting one. You probably know but 8845A/8846A has I think kind of a nifty feature where even when you turn the meter off using the front soft power off button the meter doesn't completely shut off. The reference (LM399) and most of the system remains in standby for a quick boot up and warm up time.

Well, I randomly noticed the meter come alive from this standby mode into a continuous Initializing (boot up) loop. The meter is on a UPS and there was no power brownout at the time as none of my other gear was affected. Powering off the meter using the hard switch located in the back of the instrument and waiting couple of minutes resolves the issue and the meter comes back and boots normally but after awhile the issue randomly pops up again.

I've only done a cursory look at any obvious issues and I will document my process here.. I am open to any suggestions, as I don't have a service manual for this unit.

I've checked the unit with a thermal camera after it's been warmed up for any obvious signs of thermal issues. There are rows of linear regulators in the power regulator section of the board and one particular LM317 gets warm, but nothing is running particularly hot.



The unit is nice enough to offer well labeled test points for rail voltages:



I waited till the problem reappeared and started measuring. All low voltage rails appear fine.. +60V test point however showed an issue. The rail dips from ~65V to 32V every time the Initializing boot loop starts.

Located the DFO6S bridge rectifier in the vicinity:



And it measures fine.. the rectified output does not dip as the test point shows which is a bit odd. The AC input also looks fine without dips.

Anyone knows what the 16 pin IC marked "PE-65728 0733-J CHINA US. PAT 5015981" is? I looked up the patent and it pertains to IC inductor packaging:



Next to it there is this TN4CW MOSFET, one of the pins looks a bit suspect, maybe a cold joint so I took a pic for a closer inspection:




« Last Edit: May 26, 2017, 11:39:48 pm by Muxr »
 

Offline Muxr

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Looks like that TN4CW MOSFET regulates the 77V DC coming from the rectifier down to 61V.

So that pin really looked weird, waited till the issue occurred again.. I pushed on it with a test lead and it seems like I got it to boot from it and then back to boot loop. So I just touched it up a bit with a soldering iron, which was a pain (curse you lead free solder!).

So far so good, been up 15 minutes without issue, will continue to monitor.

Well, spoke too soon. The issue is back. The gate of the TN4CW has huge dips, I think it goes from 0V to 63V on a 1 second interval. Will have to pull the board out so that I can trace and figure out what's driving it.
« Last Edit: May 25, 2017, 10:04:18 am by Muxr »
 

Offline Muxr

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A bit better closeup of the 60V section. I swear I could barely see those pins when I touched them up, they came out alright though.

Anyways trying to figure out what that 8J (with a flipped 4) component is. Silk screen says VR1 as in "Voltage Regulator 1".. some datasheets I found suggest it's a Zener, but not sure: http://www.mouser.com/ds/2/308/MMBZ5221BLT1-D-79158.pdf

Voltage across it appears to be 6.25V so that corresponds to the 8J value in the table in that datasheet.

« Last Edit: May 25, 2017, 11:18:54 am by Muxr »
 

Offline Muxr

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Today I am not able to reproduce the issue, although after having slept on it I am starting to suspect the 47uF 100V electrolytic next to the bridge rectifier. When I measure the unregulated output of the rectifier which 47uF appears to be in charge of filtering, there is about 360mV AC component to it. At 77V that doesn't seem like a lot for a full bridge rectifier, however I seem to remember measuring a couple of volts of AC component last night.. I will order a replacement just in case and will wait to see if I can get the issue to appear again.
 

Offline Muxr

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The underside of the 60V section is empty next to it however there are some 47uF 10V Tantalums that I am contemplating recapping as well. Maybe I replace them with the newer Polymer Tantalums while I have it all taken apart.


 

Offline Muxr

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Wow, that was impressive from Mouser. I ordered the parts late last night.. like 11PM EST, 2nd day saver and I just received them. Impressive.
 

Offline mzacharias

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A bit better closeup of the 60V section. I swear I could barely see those pins when I touched them up, they came out alright though.

Anyways trying to figure out what that 8J (with a flipped 4) component is. Silk screen says VR1 as in "Voltage Regulator 1".. some datasheets I found suggest it's a Zener, but not sure: http://www.mouser.com/ds/2/308/MMBZ5221BLT1-D-79158.pdf

Voltage across it appears to be 6.25V so that corresponds to the 8J value in the table in that datasheet.

The soldering on the tab of Q5 looks REALLY bad (cracked).
 
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Offline Muxr

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A bit better closeup of the 60V section. I swear I could barely see those pins when I touched them up, they came out alright though.

Anyways trying to figure out what that 8J (with a flipped 4) component is. Silk screen says VR1 as in "Voltage Regulator 1".. some datasheets I found suggest it's a Zener, but not sure: http://www.mouser.com/ds/2/308/MMBZ5221BLT1-D-79158.pdf

Voltage across it appears to be 6.25V so that corresponds to the 8J value in the table in that datasheet.

The soldering on the tab of Q5 looks REALLY bad (cracked).
I only touched up the bottom right pins (2) of them. There are some test probe marks on it, but it looks good to me. Maybe I should touch up the gate and the top pin (tab)?

edit: I see what you mean, will do that, thanks!
« Last Edit: May 26, 2017, 05:59:35 pm by Muxr »
 

Offline Muxr

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Overkill perhaps a bit, but I ordered 3 of each and hand picked the ones with best tolerance and lowest ESR. I have some other recapping projects for less sensitive equipment in the works and I am sure some of these will come in useful.

 

Offline Muxr

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Recapped it a few hours ago. It's been running for the last 2 hours without issue..

I hate intermittent problems.
« Last Edit: May 27, 2017, 12:25:49 am by Muxr »
 

Offline Muxr

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Well recapping didn't fix it. The issue is back today. I finally broke down and ordered me a differential probe.. it's time to dig into this deeper. Will probably also try to trace and reverse engeneeir this circuit over the following weeks/months.
 

Online TiN

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Re: Fluke 8845A(may apply to 8846A as well) intermittent "Initializing" loop
« Reply #11 on: August 18, 2018, 06:32:22 am »
Did you fix it yet? :)
I'm getting sick 8846 today as well  ;D
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Offline Samogon

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Re: Fluke 8845A(may apply to 8846A as well) intermittent "Initializing" loop
« Reply #12 on: August 18, 2018, 07:52:57 am »
Intead of fiddling with 60V which is probably VFD related, i would check outguard processor its power rail and all around it.
 

Offline Muxr

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Re: Fluke 8845A(may apply to 8846A as well) intermittent "Initializing" loop
« Reply #13 on: August 18, 2018, 06:06:04 pm »
I did fix it. It turned out to be a dirty/failing power switch. |O

Seams to be a common issue with these: https://www.eevblog.com/forum/repair/fluke-8846a-repair/msg1456626/#msg1456626

I do have another sick 8845 which fried its transformer. It's on my pile of things to work on. I got a replacement transformer for it directly from Fluke but I need to spend some time and troubleshoot to see where there is a short. Will be making a new post on that repair.
 


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