Fluke 8506a - ranges down but will not range up - Page 1
Forum section: electronics / repair

author: enochRoot
author_rank: Newbie
timestamp: August 08, 2019, 04:58:57 pm
content: I recently acquired a Fluke 8506a. Some previous owner had made a bit of a mess of it. Modules were in the wrong place, wrong lids on modules, etc. I got that sorted out and am left with some problems.It seems like it will need to be adjusted, but before that the main issue is that the auto ranging will not work when ranging up. It will not display an overlimit or anything, it will just hit the ceiling of that range and be stuck on that measurement (1.95v, 19.5v, etc). If you start off at the maximum range, and dial down the voltage, it will range down just fine.I've looked over the manual and schematics and it would appear a series of relays and hybrid ICs control the ranging. I tested the coils on all of the relays and they are fine. Beyond that I am lost.This is not a particularly easy device to work on given the plug-in modules. And of course this is all on the Thermal RMS board which is a double-board sandwich jobbie. Testing voltages (other than exposed test points from the top) would be next to impossible without rigging some kind of extension ribbon cable. In the manual they call out using special module extensions, but I seriously doubt I would find those.As a side note, I was able to start the calibration process and did the mechanical adjustment of the 10V range. It is spot on now. The others will need to be taken care of, but I will need to send it away for that as I do not have the equipment here to handle it. I just want to make sure that it is going to work ok before I spend the money on a calibration.All of that aside, it's a nice meter with great specs and I'd love to have this working! I actually received a full refund but was able to keep the unit (as it was sold "working fine"). So putting some money into it would definitely be worth it. But there would be a ceiling to that.I tried to look up the relays and this is what I found:https://www.parttarget.com/5945-01-164-2459_5945011642459_R5828-1.htmlandhttps://www.parttarget.com/5945-01-242-7009_5945012427009_603340.html/-CAE71A4B-FD7A-45A0-B62B-5C4C742AD227They both appear to be 1 pole normally open.The hybrid IC is a 544361, and there are some on ebay. But they are quite pricey!https://www.ebay.com/i/122793513208/How should I proceed to test this further? Any ideas what it could be? Am I looking in the right place? I generally just build audio gear, so I have never had to trouble-shoot an instrument this complex.Oh, I guess I should add that the only mention of ranging I could find in the manual is to test the auto-ranging function. It is a single paragraph and basically just said to test if it works. No mention of what to do if it doesn't work.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 08, 2019, 07:17:33 pm
content: If the decimal point is not moving when reaching the upper limit, chances are the problem is detecting the point to range up, not with changing the range. So the relays should not be the problem.The problem could be something like a supply that is too low, so that an amplifier can not reach the tripping point. Also a way of calibration might also cause this.It the trouble ranging up also with negative voltages ? It is well possible that there is a separate limit - so this may work.If it is just the auto-ranging one may still use the meter with manual ranging only.p.s. besides the main supply also a not working bootstrapped supply for the amplifier could limit the range.

author: enochRoot
author_rank: Newbie
timestamp: August 08, 2019, 08:14:59 pm
content: Thanks for the reply! I tested it w/ negative voltages and the same thing is happening. Since it is also not showing an over or under-voltage indicator as it should...it does make sense that it could be detecting the point to range up.I'm not sure what you mean when you say "...a way of calibration might also cause this.". Could you elaborate?I guess I'll try and check supply voltages. After reading about a number of repairs...I should have checked that first! Well, the first thing I looked for was mechanical failures. But it often seems that supplies/caps are often the culprit. I'll investigate more and see what I can find.And yes, I can use this by manually adjusting ranges if all else fails. But that would bug me probably.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 08, 2019, 08:29:18 pm
content: way off calibration would be an cal. error of more than some 10% so that the 19.5 V max values shown would actually be some 22 V which could be the actual limit for some stages.Checking the supplies would definitely be a first. A next point could be the bootstrapped supply for the input amplifier (some U/2 +-16V). Another test would be a low frequency triangle wave of some 2.2 V peak, using 1 V range and follow the signal through amplification, to see how much actually reaches the ADC. Chances are there are comparators or so to check for overflow somewhere in the circuit.

author: Testtech
author_rank: Regular Contributor
timestamp: August 08, 2019, 08:56:51 pm
content: Is it in CAL mode? If so, autorange may be disabled.

author: enochRoot
author_rank: Newbie
timestamp: August 09, 2019, 02:33:55 am
content: Checked the voltages at the power supply and they were all fine. I'll have to check them on the sub-boards. Specifically the input amplifier like you mention. I can try the triangle wave as well.Testtech...thanks, but it is not in CAL mode. I was hoping it would be something simple that I'm overlooking, but I fear not.