Tenma 72-7720 malfunction - Page 1
Forum section: electronics / repair

author: psysc0rpi0n
author_rank: Frequent Contributor
timestamp: February 06, 2017, 02:20:17 pm
content: Hello...I have this Tenma 72-7720 equipment. I know it doesn't worth the trouble, but I would like to know what could possibly be the origin for the malfunction I'm about to describe!Originally this multmeter came with a fuse of 500mA for the uA/mA current circuit! At some point it broke and I switched to a 200mA fuse! But it always worked ok.And as I'm a student, there was some time now that I haven't used the equipment. Yesterday I needed it and I noticed that it was measuring current with a 10x scale factor error. I mean, if I have 4.8mA, the multmeter measures 0.48mA.I checked the fuse again and it was completely burned. Looks like there are small pieces of burnt material inside the fuse. It's not only the thin wire that is broke. The fuse is completely black.So I wonder if is there anything that might have happened in the past that the fuse could not protect the multmeter from!Is there any common/known reason for this to happen?

author: Armadillo
author_rank: Super Contributor
timestamp: February 06, 2017, 02:28:06 pm
content: Well maybe someone measured voltage while in the current mode and setting.

author: psysc0rpi0n
author_rank: Frequent Contributor
timestamp: February 06, 2017, 02:33:26 pm
content: Yes, but is that a possible reason to make a fuse go completely burned with pieces of black material attached to the inner side of the fuses's glass? It almost looks like tar in the fuse's glass!

author: Armadillo
author_rank: Super Contributor
timestamp: February 06, 2017, 02:49:44 pm
content: You are lucky it didn't explode on your face.... really...once, I saw a technician measured it on a 3phase 415volt Electrical Panel and the whole thing "BOOM" explode with me beside witnessing the measurement. The spark was so bright that it blinded my eyes... The probe melted and the cable was burned... luckily nobody was hurt. It's dangerous... but despite manufacturer made it safe by segregating the volt/current sockets.... human still make mistake... those little black speckle of materials...... nothing in comparison...

author: psysc0rpi0n
author_rank: Frequent Contributor
timestamp: February 06, 2017, 04:28:01 pm
content: Ok, nothing else to say...But it is curious that the multmeter PCB is not damaged at all, at least at plain sight!Going to look for a new scope!

author: Armadillo
author_rank: Super Contributor
timestamp: February 06, 2017, 04:58:42 pm
content: thanks to the fast acting fuse, it didn't burn up the board, but I am not so sure for some of the components that reacted faster than the fuse. Well, the fuse did protected the human.

author: psysc0rpi0n
author_rank: Frequent Contributor
timestamp: February 06, 2017, 05:00:58 pm
content: Quote from: Armadillo on February 06, 2017, 04:58:42 pmYes, indeed! And I ment I need to buy a new multmeter and not a scope!

author: Armadillo
author_rank: Super Contributor
timestamp: February 07, 2017, 10:12:55 am
content: One of the resistor is open circuited. Check for broken tracks and including solder joints.You can just measure those before you buy another.