| Electronics > Repair |
| Fluke 8840A Faulty CPU |
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| bdunham7:
--- Quote from: View[+]Finder on June 02, 2021, 07:50:48 pm ---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. --- End quote --- Trust me on these observations--I've gone over the 8840/42s pretty thoroughly when testing my LED display, and trying to revive VFDs before that. You can turn up the voltage enough to make the VFD filaments visibly glow red, which they normally don't--so they definitely are getting hotter. And I can assure you that the VFD filament voltage is the only one that is budging when you do this. I can also tell you that reducing the VFD segment/grid drive voltages from 30 to 24 volts, which happened when I tried to run the LEDs in parallel with the original drivers, has almost no effect on the brightness. I never burned any VFDs out since the you can only boost the line voltage so much before the power supply crowbar kicks in. As for the regulators the heat produced is the product of the current and the voltage drop across them. It's not what they're rated for that counts, it's what they're heat sinked for in their actual implementation. The regulators should actually just shut down rather than roast, so I'd worry about the transformer more, or if the filter caps are weak they might decide to retire. I found that while you can improve the VFD a bit with an overnight burn at a 15-18% overvoltage, the result is very uneven and doesn't last. --- Quote ---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. --- End quote --- Of course you mean LED. I have an HP 3478 but I didn't know that they had display failure issues? Or do you just want something backlit and more visible? I'm not sure how much demand there would be for that model. It's basically a $100 meter and I don't think people will buy a better display for it unless theirs quits. Maybe I'll have a look once I'm done with this one. Proof of concept is done, now it's just details. And finding the parts that everyone keeps running out of! |
| View[+]Finder:
Yes, LED. My mistake, different problem; different solution. As for the display brightness, I was just reporting what I saw (and a bit of "let's see what this does"); not as part of an attempt to find a way to brighten the display. I observed the display 'off' after the return of power; dim on the 120V setting; and brighter on the 100V setting. I did the test several times, just to verify. So I left that meter on the 100V setting and it is fully functional as far as measurement and display. You have far more experience with these meters than I, so I trust the observations I have no plans to fool around with the voltage on the display. I don't remember exactly, but my usual practice when working on meters is to test for hot spots, particularly on voltage regulators, either with a finger on a recently unplugged unit or with an infrared thermometer on a live one if there might be an issue. There was probably a finger-check in this case. I just now checked the two 8842's running the 1.999V test @bdunham7 suggested and the outside case temperature is 37C for the meter under review and 41C for the other (the 37C is on top of the 41C, no surprise there) and no hot-spots over the transformer area. Yes, it would be a sad day if the transformer got smoked; not as sad as scrapping a good meter because of a dim display, however. I would be happy to double-check any of the voltages for interested readers as well as verify the temperature of the heat sinks and transformer. |
| View[+]Finder:
The problem with the HP3478 is twofold: the calibration is maintained by a lithium battery and the display is useless in dim light. The Fluke 88xx series has neither of these problems. Both are 'closed-case' adjustable for calibration purposes. In meters 'of a certain age,' specifications matter less than actual performance. Frankly, I don't know whether the Fluke is a better meter, although the voltage reference in the 8842 seems to be held in higher regard. How about a voltage-measure face-off? I have a GPIB network and 2 each of HP3478 and Fluke 8842. And a Python test script to trigger simultaneous readings on GPIB. What should the parameters be? Hmmm . . . |
| View[+]Finder:
--- Quote from: bdunham7 on May 24, 2021, 03:31:02 pm --- --- 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. --- End quote --- Attached are results of a test with both Fluke 8842's at 1.9999V from an HP3245 precision source. The Keithley 6500 meter was pretty much spot on for the 10,000 observations: 1.99989765V |
| bdunham7:
The 8842A is clearly superior for the vast majority of users as long as it isn't broken and has an in-cal AC Option 9 board installed. The 3478A is limited to 300 volts (something I didn't know until it was pointed out by another member to my surprise) and the high-impedance only goes to 3 volts. It does have some slight advantages elsewhere (AC bandwidth and 30M ohms instead of 20) but those don't make up for the shortcomings in practical use, IMO. Measuring the filter cap voltage of a PFC SMPS may blow it up. The calibration battery is a one-time issue and you need a recalibration if you aren't careful. The references aren't enough of an issue to worry about for most uses, the LM199 in the HP is pretty decent, although that version is probably not as stable as the LTFLU-1 long term. I think the Fluke will beat the HP in accuracy tests, but I'm not sure most users should care. The display does suck, but for what these sell for, it's still a good cheap meter if you can live with the limitations. |
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