In today’s market, and expensive high-precision bench meter will have a host of features: graphs, alarms, averaging, and more. It will probably even use an operating system. However, old meters can still get the job done at a price that you can actually afford. A case in point is the Fluke 8842A, solid meters with 5.5 digits of resolution and the ability to do two or four wire resistance measurements. They are built like tanks and are surprisingly affordable, especially if you consider what they went for when new. [Illya Tsemenko] recently updated a log about repairing such a meter, and there is a lot of good information about them if you own one or are thinking about one.
The biggest problem with repairing these meters is that there are several custom parts including the display that are essentially unavailable. For that reason, [Illya] took a meter with a broken display and used it to source parts for another meter.
Someone had already worked on this meter and there were some suspicious mods. For example, a matched set of JFETs gave way to four individual FETs. Perhaps they were hand matched, but perhaps they were not, which could cause lots of measurement errors.
The meter uses a patented recirculating remainder A/D technique. The patents give a good overview of how it works and are linked in the post. The issue with the meter turned out to be cracked glass in a thin film resistor network that had caused the resistors to change values. This is one of those components that would be very hard to source, but luckily it was good in the donor meter.
These are great old met ..
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