Fluke 8000A measurement period way out of spec

[kc8apf]

I picked up a Fluke 8000A at an estate sale.  All the ranges appear to work and display reasonably sane values.  I started going through the calibration procedure in the manual which starts with the measurement period.  The manual says TP5 should have a 100ms period on a frequency counter.  I'm seeing 146ms and R20 can only adjust from 146ms to 150ms.  I suspected C14 had gone bad so I removed and measured it.  It measured at 0.222uF which is spot on so I soldered it back in.  The 5V rail is holding steady at 4.8V.  If Q6 failed, I wouldn't expect pulses at TP5. U3 is a custom Fluke part but from measuring both sides of C14 (see attached oscilloscope screenshots), it looks like pin 6 is held at -10V and pin 8 charges the cap and then lets it dissipate through R20 and R56.  Any ideas what is going wrong?

[jeroen79]

Too much ESR in C14?
What if you replace R56 with a lower value?

[Paul Moir]

Have you checked the other rails, the 15, -5, and -15 rails yet?  Check for absolute value and ripple using a DMM set to VAC.

Actually it looks to me like the troubleshooting flowchart is pretty good on about page 25 of the PDF you linked.  Probably best just to follow it and see where it leads.

To me, C14 looks like a very unlikely candidate for trouble.  Since it's part of a matched set of U3, C14, C16, CR20, R56, R57 & R58  (pg 47) that gives you a clue to the responsible actors.  CR20 would be worth looking at if the rails check out OK.

[kc8apf]

All points measured at U3.
-15V rail: -16.5Vdc w/ 65mV ripple
5V rail: 5.5Vdc w/ 65mV ripple
15V rail: 16.8Vdc w/ 70mV ripple
CR20: 6.3Vdc w/ 58mV ripple

While trying to figure out where C16 was, I realized the board layout in the manual I linked above is slightly different from the board I have.  I found an updated manual on Fluke's site that matches my board.

I ran through the flowchart and ended up at "Check Period, VCO, and CR20."  CR20 and VCO check out fine.  It's just the measurement period that is off.  In a fit of madness, I removed and verified R20 (period adjustment pot) and R56.  R56's value isn't specified in the manual. Mine is a MIL spec 442k 1% part which seems a bit odd as none of the other components are MIL spec.

Looking at the pieces of the matched set (U3, C14, C16, CR20, R56, R57, R58), they seem to be part of both the VCO and the timing control which are both part of U3.  I can't find any indication that the VCO's operation is related to the timing control though.  Pin 5 of U3, where C16 connects, shows the same frequency as the VCO test point (TP1).

If the VCO and timing control are independent, then the period should only be affected by U3, R20, R56, and C14.  I don't own an LCR meter so I checked C14 with my Fluke 87 which doesn't report ESR.  That could be an issue though I'm starting to suspect that R56 was replaced with the wrong value at some point.

[Tim T]

I am a bit late responding due to being away on business and my new to me 8000a needing replacement Ni-Cads and bulb. But on dealing with the batteries/bulb today, the period measurement is comparable to that of the OP (but the wave form looks different) and the reading from a poster with the same question in the Test Equipment forum about a couple months ago.

So could it be that the Fluke manual is in error? Have to wonder how often that happened in the old days.


tim