| Products > Test Equipment |
| GW Instek GDM-8034 bench DMM: looking for schematic or calibration notes |
| << < (3/6) > >> |
| floobydust:
I don't think it's calibration to blame. OP I think your issue is called "rollover error" which is a difference in reading between +ve and -ve values. It should be less than one count. Check Reference capacitor C603, it should be a 0.5-1uF part. 0.1uF is a bit small and cheap or maybe it is bad with leakage current. Beyond this, for the ICL family, a "rollover resistor" and diode can be added in series between INTegrator OUT and analog COM. Like 1N4148 (cathode to INT OUT) and 100k to analog COM at ICL7106 pin 27. I don't see it on your board. It's in the old Harris/Intersil datasheets. The multimeter is likely the same front-end as the LED version GW-8035, and GW Instek did white label under a few other brands like Metermaster, Amprobe, Wavetek etc. in your search for a service manual or schematic. It's just hard to guess their model numbers, use image search maybe. Failing that, it is not hard to figure out the calibration trimpots by tracing out some of the circuit. |
| mqsaharan:
--- Quote from: floobydust on July 25, 2024, 02:39:54 am ---... for the ICL family, a "rollover resistor" and diode can be added in series between INTegrator OUT and analog COM. Like 1N4148 (cathode to INT OUT) and 100k to analog COM at ICL7106 pin 27. I don't see it on your board. It's in the old Harris/Intersil datasheets. --- End quote --- Hi floobydust, Could you please upload the old Harris/Intersil datasheet here that discusses/presents this solution. I downloaded a few old datasheet from different sites but couldn't find it in any of them. Thanks. |
| floobydust:
I can't find the one Intersil datasheet or app note with schematic for it I found last night. It's mentioned as text in others. Was it here... long, slow download over 1,000 pages: https://usermanual.wiki/Document/1981IntersilDataBook.1668550145/view http://www.bitsavers.org/components/harris/1991_DB301.1_Harris_Data_Acquisition.pdf OP on second thought I think you have no rollover problem. "... measuring a 0.250 V reference on the 2 V range, it reads +0.251 or -0.249 ..." You are looking at the bottom of the 2V range. Def'n: "Rollover Error -difference in reading for equal positive and negative voltage near full-scale." It's usually 0.5-1 count as a spec. Intersil attributes problems there due to leakage currents on a dirty PCB or an integrating or ref capacitor with higher dielectric absorption. |
| Kleinstein:
The roll over error has in the ICL71xx has mainly 3 components. One is an offset, e.g. from input bias and protection resistors or the divider. A 2nd is the ref. capacitor from leakage and pump out from parasitic capacitance - so rel. high capacitance in a not too large case is wanted. Today possibly even an X7R of maybe some 2.2 µF. The voltage is constant and DA is thus not an issue. The final part is from the difference in the direction for the comparator. The 4.5 digit version 7135 has some hint's on this with an suggested adjustment for the steps around 0. For the 3.5 digit version this was less of an issue. While the error is still part of the turn over the bigger issue is the DNL around +-0. It is a very basic meter and chances are it will work for quite some time with no issues. If there is an accuracy problem this would be more like a defect (e.g. old electrolytic capacitor, cold solder joint, dirty PCB or spiders home under the chip). In this case a calibration alone would not fix the issue, but it would need a full repair. Chances are one would than ditch the meter or than look for a schematics or do a reverse engeniering with the help of the typical circuits. Nothing to really worry now as the meter may fail in different ways, like failing display, blow up from a lightning stike, get lost in a flood or fire or get smashed beyond repair. |
| coromonadalix:
some sheets basic circuit, woth to read https://electronics-diy.com/ICL7107_volt_meter.php |
| Navigation |
| Message Index |
| Next page |
| Previous page |