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| Ronan calibrators. |
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| jchw4:
Stability. The final issue that I had was switching from the 1V range to 100mV range yielded permanent 13V on the output. With the help of the other forum members (see https://www.eevblog.com/forum/beginners/strange-lm312h-behavior/), it seems that the easiest solution was to replace the opamp with the modern one. But even with the original one, the device is stable within the DMM range, and noise is beyond what is displayed. The current source does not have feedback and depends on the 0.01% 10 Ohm resistor, which dissipates 0.1W of power and nearby transistor drops 12V at 0.1A and heats up too. Which is unfortunate, as the voltage source is very good. |
| jchw4:
Improvements. The interesting question was whether we can increase voltage stability even more for cheap, because precision voltage source is very useful, even if not very accurate. With the help of the forum members in the same thread (see https://www.eevblog.com/forum/beginners/strange-lm312h-behavior/) I replaced source zener with MAX6225 and the result is much much better now. |
| jchw4:
Final impressions on the X85: X85 Calibrator - Good: Will give you stable 4.1 digits of output voltage and resistance. - Good: Should give you stable 4.1 digits of output current, but there is no feedback. - Good: Can be bought for cheap. - Good: Easy to fix. (But may need a lot of work.) - Bad: Looking at Ebay photos I assume they all have leaked batteries. Depending on how it was stored (vertically, horizontally, on which side) and your luck, the battery could corrode it through into total garbage. - Bad: PCBs are very old and they are easily damaged. Don’t ask how I know it. You can see my final repair result on the photos. |
| alm:
Thanks for posting this nice teardown, repair log and review of this unit! I like your creative solution for the battery replacement. Just one tiny thing suggestion for the future: I would prefer to see noise measurements reported as ppm of output. This makes it easier to relate to DMM digits and specifications, and also allows you to compare different ranges better. |
| coromonadalix:
Thks for the teardown, never saw that brand :-+ |
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