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| Some old school instruments showing how it's done (HP 3325A and Fluke 8506a) |
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| guenthert:
--- Quote from: SilverSolder on January 23, 2021, 08:26:03 pm --- --- Quote from: guenthert on January 23, 2021, 07:05:54 pm --- I bought a few older DMMs and all drifted less (or rather the accumulated drift was less) than the original specification would have allowed for (one HP 34401A even seems to have been "spot-on" in the 10DV range). That might have been survivor's bias or an indication that the manufacturers specification are wider than necessary for instruments living an easy life. In any case, I haven't seen one, which drifted so far as yours. I'd watch it for a while and observe the current drift, which might indicate some fault needing a repair before calibration. --- End quote --- All of the ones that I have were bought very cheaply - as junk, basically... - and they were all still within spec, unless they were outright broken. When fixed, they came back to spec. --- End quote --- Well that might be the case, but joeqsmith, who asked about calibration, has a 8506A which is quite off. (I should have been more clear whos "yours" is :-DD |
| SilverSolder:
--- Quote from: guenthert on January 24, 2021, 05:06:41 am --- --- Quote from: SilverSolder on January 23, 2021, 08:26:03 pm --- --- Quote from: guenthert on January 23, 2021, 07:05:54 pm --- I bought a few older DMMs and all drifted less (or rather the accumulated drift was less) than the original specification would have allowed for (one HP 34401A even seems to have been "spot-on" in the 10DV range). That might have been survivor's bias or an indication that the manufacturers specification are wider than necessary for instruments living an easy life. In any case, I haven't seen one, which drifted so far as yours. I'd watch it for a while and observe the current drift, which might indicate some fault needing a repair before calibration. --- End quote --- All of the ones that I have were bought very cheaply - as junk, basically... - and they were all still within spec, unless they were outright broken. When fixed, they came back to spec. --- End quote --- Well that might be the case, but joeqsmith, who asked about calibration, has a 8506A which is quite off. (I should have been more clear whos "yours" is :-DD --- End quote --- I was (badly worded) trying to agree with you that it is not normal that these meters have drifted as far as @Joeqsmith's (and it looks like the problem may indeed be a broken resistance module). As an aside, I had not noticed the Xdevs article/teardown/repair of the Fluke 8506 (linked to by @Joeqsmith). I had been wondering why there haven't been any cheap ones on eBay for about the last year... I guess this is yet another case of prices shooting up when a previously "hidden gem" gets discovered by EEVblog and its friends! :D |
| joeqsmith:
I finished up the DC alignment (through 4-85) without any issues. The Fluke pretty much matches my old HP34401A in all the DCV ranges. I wouldn't trust using the HP as a ACV reference so I'll leave that section alone. An extender would be very helpful but for a one shot repair, time to add some test points and start working through it. |
| joeqsmith:
The relay K1 on the resistance board will not change states. The interesting part is how this area of the design has at least three approaches. One design used 4 transistors to drive the two coils. My schematics show a single coil with a FET and two transistors. The board have uses 2 coils and three transistors. The software/logic to control the relay drive circuit seems the same. GAL of 4000 series, both use a single pin. My board uses the old 4042. Low-activates the resistance mode. The relay checks out. I can change the state manually to disable the resistance mode and DCV works fine. They have this thing AC coupled to use the edges to fire the transistors. It's a bit Rube Goldberg. Maybe this section was assigned to the intern. Time to reverse engineer the madness. |
| SilverSolder:
The manual for the 8505A also works for 8506A for all the modules (except Thermal RMS). The reason I say that, is that some of the 8505A manuals are from different years than the 8506A ones, so you get a broader range of circuit diagrams - one of which may fit! I think the attached diagram may be for your board - |
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