How to align the diode/continuity value in a Uni-t 33c multimeter - Page 1
Forum section: electronics / testgear

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: July 31, 2019, 07:54:35 pm
content: Hi all, I just got a used cheapo and found its way off in the continuity values.I posted the schematic and tried to reverse engineer what controls these values but after checking the various resistor values drew a blank. It has 5 variable pots but none seem to control the diode check values which are way off (approximately 30% high). This schematic is pretty accurate with a few modifications. The weird thing is, all the other range values are excellent. The only other quibbles I have are with a low beeper volume and back light intensity, which maybe related.Any help would be appreciated.

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 31, 2019, 09:22:18 pm
content: Are you sure ?Diode check is not showing resistance to calibrate it with resistors ... it is the voltage drop on the diode junction , and this value is not absolute , if the test current is lower or higher the junction voltage varies accordingly with the conduction formula.No multimeter has a diode pot adjustment .

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: July 31, 2019, 09:53:31 pm
content: Oh, thanks, yes I'm sure the thing stops beeping about 80 ohms and above. My Bryman 857s is loudest at 50 ohms and below.The question is why give a value on the meter if it doesn't correspond to the ohm value in the lowest (200 ohm range)? The number should be an exact value which matches in the lowest ohms range. Otherwise the display is meaningless. My other meters all match. They also agree (approximately) in the gain value when testing different types of diodes. When testing a 1N4007 the voltage drop should be somewhere in the region of .7 volts. The Uni-T is not even close.

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 31, 2019, 11:22:51 pm
content: Diode reading is the junction forward voltage not resistance ...So , if the others meters read 0.7V for 1N4007 , what is the reading on the Uni-T ?

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: August 01, 2019, 01:17:14 am
content: It´s on average 20% to 30% more for all diodes. Also the reading on voltage for continuity is about the same with a value displayed. But what is the significance of the number displayed when measuring continuity? Is it ohms or volts? Why is it wrong when measuring for continuity on a 50 ohm resistor. Why, why why?

author: CDaniel
author_rank: Frequent Contributor
timestamp: August 01, 2019, 04:52:24 am
content: For continuity is resistance , but from the schematic I don't see any difference between diode and continuity in this meter . In other meters the circuit is not the same .If you really want to play with it , the resistor R18 470J(ohm) seems to be responsible for the measurement current in diode/continuity mode . Maybe you have space for a 1K pot .

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: August 01, 2019, 11:53:12 am
content: Yes you're correct. My Bryman does use a different scheme to read continuity.Thanks for the tip about R18 I will look into it and let you know. Part of the problem is reading this type of schematic for multimeters. Its hard to understand where the connections go on the range switch, but I can see what you mean. I will try to reverse engineer the circuit by taking the meter apart. It may be possible to alter the value of this resistor. Or it could be a design mistake on Uni-t part. I just don't get it. If it is the correct value what and the design is right what is the meter trying to tell me with the number value?

author: CDaniel
author_rank: Frequent Contributor
timestamp: August 01, 2019, 06:19:10 pm
content: That's a low end very cheap multimeter , for troubleshooting simple circuits diode check for shorts and conduction ( even if it's not accurate ) is enough . The same with the continuity . Probably they did a compromise since the circuit is common for both ... so neither one could be made accurate .