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| UT61E drift and recalibration |
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| stephenlm324:
--- Quote from: Monkeh on December 20, 2013, 10:28:48 pm --- --- Quote from: stephenlm324 on December 20, 2013, 01:55:41 am ---I have drawn a blank on calibration of the resistance range. The voltage divider network could be replaced with a Caddock 1776-C4815 $23 from Digikey postage of arround $50, gasp. But I suspect that even this might not help. Page 9 of the ES51922 data sheet referring to the "divider ratio" mentions R6, so where is it? I had been under the impression that it was referring to the voltage divider resistors but there are only 5 of those. The only conclusion I can come to is that the divider is "on chip". I am open to suggestions. --- End quote --- R7, 100R, connected to pin 22 (OR1)... It's under the display. --- End quote --- Already tried that, tacked a resistor across it but nothing changed. |
| Rick Law:
--- Quote from: stephenlm324 on December 21, 2013, 01:20:02 am --- --- Quote from: Monkeh on December 20, 2013, 10:28:48 pm --- --- Quote from: stephenlm324 on December 20, 2013, 01:55:41 am ---I have drawn a blank on calibration of the resistance range. The voltage divider network could be replaced with a Caddock 1776-C4815 $23 from Digikey postage of arround $50, gasp. But I suspect that even this might not help. Page 9 of the ES51922 data sheet referring to the "divider ratio" mentions R6, so where is it? I had been under the impression that it was referring to the voltage divider resistors but there are only 5 of those. The only conclusion I can come to is that the divider is "on chip". I am open to suggestions. --- End quote --- R7, 100R, connected to pin 22 (OR1)... It's under the display. --- End quote --- Already tried that, tacked a resistor across it but nothing changed. --- End quote --- Stephen, Let me make sure I understand what you said. Do you mean you changed the reference resistor (connected to OR1) and nothing changed? -OR- do you mean you tacked a resistor across specific divider and nothing changed? Thanks Rick |
| stephenlm324:
I tacked a 1k across R7 (OR1 pin) and checked the meter with 1K 10k and 100k 0.1% to find no change. I have redone this fiddle to find that the 200ohm range does change, meter reading 110ohm with 100ohm connected. I now have 68k across R7 giving a correction I can live with. 100ohm 0.1% reading 99.92 instead of previous 99.79, which means my resistors are in spec now. Unfortunately this only effects the one range they are all out by about the same percentage. Longer test leads would have the same effect. I have tried a resistor across R8(VR5 pin). This appears to be the reference resistor for the 2k range as well as part of the voltage divider. This should be a value of 1.0001k for the divider making it less than ideal for use a reference resistor but would explain why the ranges are all on the low side. |
| Rick Law:
--- Quote from: stephenlm324 on December 21, 2013, 01:20:02 am --- --- Quote from: Monkeh on December 20, 2013, 10:28:48 pm --- --- Quote from: stephenlm324 on December 20, 2013, 01:55:41 am ---….. Page 9 of the ES51922 data sheet referring to the "divider ratio" mentions R6, so where is it?... --- End quote --- R7, 100R, connected to pin 22 (OR1)... It's under the display. --- End quote --- Already tried that, tacked a resistor across it but nothing changed. --- End quote --- From my research: I also gathered from other docs that VR6=OR1, but that is NOT part of the divider. Read onward to my reply below to your other post below: --- Quote from: stephenlm324 on December 21, 2013, 10:17:52 pm ---I tacked a 1k across R7 (OR1 pin) and checked the meter with 1K 10k and 100k 0.1% to find no change. I have redone this fiddle to find that the 200ohm range does change, meter reading 110ohm with 100ohm connected. I now have 68k across R7 giving a correction I can live with. 100ohm 0.1% reading 99.92 instead of previous 99.79, which means my resistors are in spec now. Unfortunately this only effects the one range they are all out by about the same percentage. Longer test leads would have the same effect. I have tried a resistor across R8(VR5 pin). This appears to be the reference resistor for the 2k range as well as part of the voltage divider. This should be a value of 1.0001k for the divider making it less than ideal for use a reference resistor but would explain why the ranges are all on the low side. --- End quote --- Again, from my research (one may call data archaeology... this is a lot of digging), this is what I gathered: 220ohm range uses VR6 (aka OR1) ONLY 2.2KOhm and 2.2KV range use VR1 & VR5 (for 1/10000) 22KOhm and 220V range use VR1 & VR4 (for 1/1000) 220KOhm and 22V range use VR1 & VR3 (for 1/100) 2.2MOhm and 2.2V range use VR1 & VR2 (for 1/10) 22MOhm and 220MOhm (no Volt range on these two) uses VR1 only. VR1=R34=10MOhm VR2=R11=1MOhm VR3=R10=100K VR4=R9=10K VR5=R8=1K VR6=OR1=R6=100Ohm To adjust 2.2Kohm, 22Kohm, 220Kohm, and 2.2Mohm in one shot, adjusting the 10MOhm (VR1) would do it. The 10MOhm (VR1) should be R34 from the schematic. I can’t find R34 (which should connect to R39 and R39 in turn connects to the input jack). I think it is on the underside (see teardown link initial post by stdnn, 2nd from last picture in the original post), but I don’t see a resistor there. https://www.eevblog.com/forum/testgear/uni-t-ut61e-multimeter-teardown-photos/ I don't have the skill (or just plain lack of courage) to attack that problem particularly when I can't find R34. But I think it would be helpful to your quest. Looks like adjust VR1, you change all ranges between 22K to 220MOhm, or you can adjust VR2,VR3,and VR4 for 22K, 220K, and 2.2M individually. If you found R34 and have a go at it, keep me posted. I am trying to build up my courage - that I destroyed my daughter's phone when I last attack something this small was...well, something I am trying to get over. Rick |
| PedroDaGr8:
Hate to say it but R34 is staring you in the face. Based on the schematic and tracing the board it's the divider. Sent from my EVO using Tapatalk |
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