Electronics > Repair

Fluke 8840A Faulty CPU

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bdunham7:

--- Quote from: View[+]Finder on May 24, 2021, 01:41:29 am ---The test was done with my best meter (HP3458) and the current best reference at 10VDC (sigma ~1.7microV). The Fluke range is 2V and 20V and the 6500 is 10V and 100V, so how about 1.999V? The 2V range doesn't much above 2V and the 100V range in most modern meters is not their best spec.

--- End quote ---

The 2V range on the 8842A is the native ADC range and thus the most accurate, so go for it.  1.9V is a calibration point, IIRC, so it should be at its best right there if you have a stable source. 

I once tested an 8842A vs my 8846A at 15.0000 volts and found them to be indistinguishable in performance at that point, although I didn't do any kind of statistical analysis.  And the 8842A spec is actually better at that point--35ppm + 2 counts  vs 38ppm + 6 counts, plus high impedance.  If my math is right, the 8842A would also have a lower spec uncertainty at 1.90000 volts-- 8 counts vs. 10. 

View[+]Finder:
The display of my used (are there any other kind?) Fluke 884X meters range from barely visible to good enough for most uses. A recent power outage in my area led me to uncover a way to improve the visibility without any effort at all.
 
Here's what happened. I was testing an eBay Fluke 8842 with a dim display to see if it was anywhere close to being usable when the power went out. The outage lasted a couple of hours and when i was putting my lab back on line, I noticed that the mains voltage was like 100V not the usual 120V. I also found that the 8842 display was blank--nothing lit at all--however, the relays were clicking as expected when changing functions. Wierd right? After a bit, the mains voltage was back to normal and the Fluke display was sorta visible again. This led to a review of the schematics, a browse of the eevblog forum and a leap of faith for the little meter. I unplugged the meter and used an isolation transformer to see what would happen if I set the ACV switch to Japan 100V. At 100V the display was dim but as I raised the voltage to 120 the display brightened considerably. No smoke, no excess heat, just brighter. Next I put the meter back on mains and observed it closely for a while. No problems, so I tried the "100V" setting on another dim 8842 with the same positive result.

I figure the 20% over-voltage in some way compensates for the lower output from ancient voltage regulator chips in the power supply. There does not appear to be any difference in calibration of volts or amps functions. Worst case, I might have to replace the voltage regulators sometime in the future, but since the display is unobtainable, I'll make do with what I've got.

bdunham7:
You are overdriving the VFD filament which is driven directly by the transformer.  The voltage regulators are LM78xx type and will simply run warmer (not all that good) and if you go high enough, there's a crowbar protection on the 5V supply that will blow the mains fuse.  The supply voltages on these are very stable--there's a 6800uF filter cap on the 5V line.

I'm working on an LED replacement, so don't burn your meters out in the meantime!  The mains transformer is truly unobtainium and pretty tough to replace even if you had one.

https://www.eevblog.com/forum/testgear/fluke-8840aaf-8842a-vfd-display-replacement/msg3557023/#msg3557023

I intend to get back on this project in a few weeks.

View[+]Finder:

--- Quote from: bdunham7 on June 02, 2021, 03:39:30 am ---You are overdriving the VFD filament which is driven directly by the transformer.  The voltage regulators are LM78xx type and will simply run warmer (not all that good) and if you go high enough, there's a crowbar protection on the 5V supply that will blow the mains fuse.  The supply voltages on these are very stable--there's a 6800uF filter cap on the 5V line.

--- End quote ---
Correct. The filament will be seeing a higher voltage, however the resistance of filaments (generally) will increase as voltage increases, so the current flow should not get to burnout level. Plus, all my meters are on UPS transformer/battery boxes that keep the supply under 115V and capture surges and spikes. Yes, I'm skating on thin ice and I'm certainly not suggesting that anyone 'over-clock' their meter without considering the consequences. As for the heat generated by the LM78XX regulators, that would depend more on the current flow than the input voltage, so unless there is some reason to to expect the measurement side of the meter to work harder because of the input voltage to the transformer, the heat doesn't seem to be a problem. In any case, the max input for the 7800 series is like 35V and that is not likely as configured.

BTW, I've read the posts on pulsing VFD's to wake up the phosphors or revive the filament; most don't end well. My post was more of a "what's up with this" than a suggested cure.

The real solution is your LCD panel concept to replace the VFD altogether. And, while you are at it, might I suggest an LCD for the HP3478? There are many for sale on eBay--some might need a battery replacement--and an LCD (or even a 7-segment) would make the HP3478 a more useful alternative.


Kleinstein:
A higher voltage can make quite some differene, as the voltage drop is often only some 30% of the voltage. So a 10% increase in the input voltage would make this a 30% increase in the heat.

Ideally there is a seires resistor that could be reduced to increase the filament voltage a little, if really needed, espeically if only for a temporary higher intensity.

The HP3478 has a LCD to start with, but often with poor contrast and without backlight. While it was not so good when new, there is relatively little aging with the LCDs.

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