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| UT61E drift and recalibration |
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| Rick Law:
--- Quote from: PedroDaGr8 on December 22, 2013, 04:02:36 am ---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 --- End quote --- Arrgh, guess I need to "upgrade" my reading glasses yet again. I'm looking at the tear-down photos and not the PCB. Is it on the fuse side or the LCD side? |
| PedroDaGr8:
--- Quote from: Rick Law on December 22, 2013, 05:16:14 am --- --- Quote from: PedroDaGr8 on December 22, 2013, 04:02:36 am ---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 --- End quote --- Arrgh, guess I need to "upgrade" my reading glasses yet again. I'm looking at the tear-down photos and not the PCB. Is it on the fuse side or the LCD side? --- End quote --- Your glasses are fine, it's actually not labeled. I traced out the circuit using the schematic. It's the big black box on the fuse side. R39 is on the lcd side. Sent from my EVO using Tapatalk |
| stephenlm324:
I like the term archaeology. Adjusting VR1 may give unexpected results if my latest theory is correct. VR1 is used in a different way on the resistance range to volts. The resistance range needs a set of reference resistors. 100R 1K 10k 100k etc. The values in the divider are 1.0001k, 10.01K, 101.01k, 1.1111M and the 10M. The 1.0001k is close enough on its own but the 10.01k has the 10M switched in parallel to give 9999.99001k. The 101.01k with the 10M in parallel is 99999.901k and the 1.1111M in parallel with 10M is 999909.9991M. It saves money and is close enough so I should be impressed. But the 10M has a different effect on each range so if it was adjusted to 9M it would effect the 1M range by 1% but would effect the 100k range by 0.1% and the 10k range by 0.01%. Not quite what we want. It does however shed light on the chip data sheet page 9 section 1.2 which has been driving me nuts. |
| Rick Law:
--- Quote from: stephenlm324 on December 22, 2013, 01:20:24 pm ---I like the term archaeology. Adjusting VR1 may give unexpected results if my latest theory is correct. VR1 is used in a different way on the resistance range to volts. The resistance range needs a set of reference resistors. 100R 1K 10k 100k etc. The values in the divider are 1.0001k, 10.01K, 101.01k, 1.1111M and the 10M. The 1.0001k is close enough on its own but the 10.01k has the 10M switched in parallel to give 9999.99001k. The 101.01k with the 10M in parallel is 99999.901k and the 1.1111M in parallel with 10M is 999909.9991M. It saves money and is close enough so I should be impressed. But the 10M has a different effect on each range so if it was adjusted to 9M it would effect the 1M range by 1% but would effect the 100k range by 0.1% and the 10k range by 0.01%. Not quite what we want. It does however shed light on the chip data sheet page 9 section 1.2 which has been driving me nuts. --- End quote --- --- Quote from: stephenlm324 on December 22, 2013, 01:20:24 pm ---I like the term archaeology. Adjusting VR1 may give unexpected results if my latest theory is correct. VR1 is used in a different way on the resistance range to volts. The resistance range needs a set of reference resistors. 100R 1K 10k 100k etc. The values in the divider are 1.0001k, 10.01K, 101.01k, 1.1111M and the 10M. The 1.0001k is close enough on its own but the 10.01k has the 10M switched in parallel to give 9999.99001k. The 101.01k with the 10M in parallel is 99999.901k and the 1.1111M in parallel with 10M is 999909.9991M. It saves money and is close enough so I should be impressed. But the 10M has a different effect on each range so if it was adjusted to 9M it would effect the 1M range by 1% but would effect the 100k range by 0.1% and the 10k range by 0.01%. Not quite what we want. It does however shed light on the chip data sheet page 9 section 1.2 which has been driving me nuts. --- End quote --- Yeah, VR1 is used with VR2-5 for four different volt ranges which I wrote earlier. So, any change to VR1-VR5 will affect at least Volts and Ohms. (Don't see anything else, but there could be). 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) To do the job right, one probably has to make adjustments to all VR2 to VR5 individually, and then recalibrate it for either Volts or Ohms - in other words, if one is exact and the other is not good enough, a choice has to be made. What would be fun to do is for each from VR1 to VR6, to parallel each with one that is perhaps 10x the value, and serial a 0.1x VR value trimpot. With all 6 adjustable, one can play around till kingdom come and see how well the UT61E can perform. Not to be too self-centered... While I am not planning to such major surgury to my UT61E, the info I dug out are still turn out to be useful to me. The DMM Plus resistor measurements can be used to suggest how much the other voltage range are off - nowI know which resistance range and voltage range use the same divider resistors. In my case, the four DMM Plus Resistors are off by 0.11%, 0.19%, 0.09%, and 0.095%. So, the corresponding voltage range (using the same set of VR's) would likely be off the same amount. |
| Nuno_pt:
Hi, Has anyone play with the CW1,2,3 pots? |
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