Electronics > Repair

Fluke 8050a True RMS U32

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tango17:
My report of success was somewhat premature. I had the scope on the 2 volt/div scale when I thought it was on the 1V/div scale. Everything looked more or less correct, but in reality, the repaired 8050a was reading about 50% low on all AC scales.  By chance, the numbers seemed to match up.  I'll be using Modemhead's numbers to track down what's going on.  At least I've got something other than zero on the display. :)

tango17:
I took some readings from 1 volt to 200 VAC on another meter and the 8050a, then X-Y plotted them in Excel. They lie accurately on a line, so at least I know the error is just a linear scaling error.

wiss:
You must get really bad matching-errors from the two hacked-in transistors!
These circuits are annoyingly temperature dependant. That's why no one try to build one with discreet transistors.
Even when you get a 3046-replacement I would guess that adjustments will be needed.

ModemHead:

--- Quote from: wiss on March 11, 2015, 08:14:56 am ---Even when you get a 3046-replacement I would guess that adjustments will be needed.

--- End quote ---
For those who haven't seen the schematic, there is a configurable voltage divider and trim pot on the output of this module. Hopefully when the matched array is installed the output will fall within the expected output range.

tango17:
Yes, there are 4 resistors for the DC calibration, and two more resistors and two trim caps for AC calibration, plus one adjustable resistor on the U32 TRMS board itself.  I've been looking at the calibration procedures, but I decided not to touch anything until the transistor array is installed. I ordered three of them today. With luck, one of them will bring it within calibration range.

Again, I'd like to express my thanks for the help.  I'll let you know how it goes.

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