Hi,
I have a Data Precision 2480 DMM I am trying to fix. The schematic is here:
http://meanmutha.com/DataPrecision2480Schem.pdfNote that any component labeled "FSV" in the schematic is not present on the board. I don't know what "FSV" means. Also, this unit has the RMS option for AC, if that matters.
Everything works fine EXCEPT the DCV 100V setting. Ohms works fine.
If I try to measure 5v DC it reads correctly when I have the range set to 1v or 10v or 1000v. However, when I set the range to 100v it reads ~6.89v (it varies sometimes). If I feed it 1.26v it reads 0v in the 100v range. The error offset is not linear that I can tell.
I checked the input voltages per the limited service info in the manual, and they read fine. (Manual is paper, and I do not have the whole thing scanned.) I checked the reference voltages, and they were fine too.
The logic seems to be working fine, as the readout is displaying the voltage as seem at E37 on the schematic (right after the attenuator at H12). The manual also provided some logic waveforms, and they checked out OK on the scope.
The input to the meter is on the left hand side of the schematic. Everything DCV related is nicely labeled DCV.
A4 is a precision resistor pack used in the attenuator. When in 100v range pin 4 reads the voltage displayed by the meter (the wrong voltage). When it any other range it reads the correct voltage. As the A4 resistors are also used in the Ohms measurement I do not believe they are at fault. Also, other ranges work fine, and some would use the same resistors.
I tried removing C2, C3, C4, C5, C8, C9, C10 (used for AC tuning for ACV). This had no effect on the problem.
The decimal places work correctly (lower right side of the schematic, controlled by the same switch.) The 100v range switch is S10 (you can see these ranges listed right next to the attenuator on the schematic).
I am beginning to suspect it is possibly an issue in the switch S10. I put tons of contact cleaner in there, and in all the other switches, but that did not help. These are those ganged push button switches, where pushing one in makes the others pop out. They are a massive pain to remove, and that would be my last resort after seeking help here.
Any help would be appreciated. Any suggestions for where to look would be helpful.
The since the difference between the correct voltage and the displayed voltage is not consistent this makes me think a transistor or opamp is involved. Would some kind of crud in a switch cause the offset to vary like I described above?
Thanks in advance.