Author Topic: #51 Fluke 8520A How far does this rabbit hole go? Part 1  (Read 2798 times)

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#51 Fluke 8520A How far does this rabbit hole go? Part 1
« on: May 16, 2016, 02:10:52 am »
In part 1of this video we tackle a few tantalum capacitors that has gone dead short. We also have to replace a

faulty LM340T-12 regulator after checking the voltage rails. Just how many problems does this "Working EBay Find"

have? At a stand still till I can figure out the next step on what is causing Err01.




Fluke 8520A service manual:
http://assets.fluke.com/manuals/8520A___smeng0100.pdf


In the first picture we have to replave C238 due it is shorted. We also had to replace R256 and 257





Here we find C450 is also shorted. This was dragging the +15 volt rail down.




We find that U704 has a dead short from input to output. This is a LM340T-12





When you boot the 8520A on it is showing a voltage on the display. This should show"000"



If you hit 2-wire or 4-wire to test the Ohm setting you will get Error 1. This means high voltage when selecting

ohms scale




After repairing the voltage rails we still have the error 1. But if you pull J19 we get no errors. But ofcourse the

meter does not function.



And again with J19 removed we can go into ohms mode.



So with J19 removed the error goes away. The unit does not function but it looks like we are closer to the problem.
We still have to trouble shoot error code 01.
Err01 HV - High voltage present with ohms function selected.

The video

« Last Edit: May 16, 2016, 09:15:25 am by Radio Tech »
 

Offline Len

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Re: #51 Fluke 8520A How far does this rabbit hole go? Part 1
« Reply #1 on: May 16, 2016, 04:25:19 pm »
It's a cliffhanger! Waiting to see how it turns out...
 

Offline wn1fju

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Re: #51 Fluke 8520A How far does this rabbit hole go? Part 1
« Reply #2 on: May 16, 2016, 06:14:41 pm »
My Fluke 8520A had two shorted tantalums, both in the ACV circuits, as well as an electrolytic.  And after I got the power rails straightened out, I still had problems like yours (although I can't remember if it displayed Error 1).  Remember that they use JFETs to switch the various signals around (and various gain factors) and it sounds like one (or more) of your switches isn't functioning correctly.  The schematic is a bit confusing because they run bootstrap voltages to the JFETs so they will switch whatever is thrown at them.  At least in my unit, the problem was not with the JFETs themselves, as I would have expected, but rather the transistors and logic driving the JFET gates.  My problem was in the DC Buffer circuit and had faulty U208, Q225, Q226, Q229 and Q230.  Not that this is your particular problem, but I'm trying to provide an example of what can go wrong.
 

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Re: #51 Fluke 8520A How far does this rabbit hole go? Part 1
« Reply #3 on: May 16, 2016, 08:22:28 pm »
It's a cliffhanger! Waiting to see how it turns out...

Lol, I guess you could say that. Had to end the video there. Pulled an all nighter on it Saturday.
Cannot wait to see how it turns out myself.

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Re: #51 Fluke 8520A How far does this rabbit hole go? Part 1
« Reply #4 on: May 16, 2016, 08:29:33 pm »
My Fluke 8520A had two shorted tantalums, both in the ACV circuits, as well as an electrolytic.  And after I got the power rails straightened out, I still had problems like yours (although I can't remember if it displayed Error 1).  Remember that they use JFETs to switch the various signals around (and various gain factors) and it sounds like one (or more) of your switches isn't functioning correctly.  The schematic is a bit confusing because they run bootstrap voltages to the JFETs so they will switch whatever is thrown at them.  At least in my unit, the problem was not with the JFETs themselves, as I would have expected, but rather the transistors and logic driving the JFET gates.  My problem was in the DC Buffer circuit and had faulty U208, Q225, Q226, Q229 and Q230.  Not that this is your particular problem, but I'm trying to provide an example of what can go wrong.

I am glad you replied with this. As I remember while testing the unit I never saw any switching.  Like it is stuck in one mode of logic. Do not even remember seeing any activity on the relays. I think you are close to the issue.
I also have a couple of scenarios in mind.  It is possible there are two failures here. Then again it may had been one failure and a second failure caused by who ever was troubleshooting the first failure.  It sure has had work done in the area where the problem originated. You just never know on equipment like this.

I agree on the service manual. Written like stereo instructions from the 70's lol.
Someone also suggested I make copied of the Eproms in this unit. Surly glad I have the hardware to do that.


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