Tenma 72-7755 repair, sort of. - Page 1
Forum section: electronics / testgear

author: SeanB
author_rank: Super Contributor
timestamp: July 27, 2015, 04:16:37 pm
content: So, I paid Toploser for a Tenma 72-7755 late last month, sold for the cost of the postage.Arrived today ( actually Friday, but I didn't see the sun Friday), and I went to collect it from my friendly post office. Opened it there, as they were asking what a multimeter is, as most of my parcels are marked as either "gift" or" electronic". Demo of the meter, and see the random readings on the display as described by Ian.Come back to work, and look up the manual and software, missing inside as in the description. Rest was there, Type K thermocouple, test leads, serial data interface and cable and some nice croc clip test leads.So, go try the meter out. Turn on and DC volts random readings. Change to resistance, reads infinite. Diode the same, but bargraph says a little over half scale. Capacitance, 186pF residual. Frequency, temperature and all the rest show OK.So, read the manual, and the input impedance is described as being over 10M on all DC and AC ranges. Hmmmmmmmm, I wonder what it is actually. Switch to AC volts, and it stabilises at a few counts around zero.Back to DC, and connect another meter set to DC volts ( cheap OHL one for work use, also autoranging), and both settle down to showing zero plus or minus a few counts.Set the other meter to resistance, and it starts to do autorange hunting, showing alternately somethingf around 11M and OL. Tenma in the interim is doing the same on autorange, showing Ol and then switching to 4V. Put it in manual range, and it settles down to 440mV on the 4v range, and the other meter settles on 11.6M input resistance. Switch through to the mV range and the other meter and the Tenma both show OL.So the actual fault is in the manual, where they do not explicitly state that the 400mV range effectively is the input impedance of the ADC sample and hold amplifier, or something well over 10M, and suffering from charge injection and noise pickup from the environment. Short the input and it is happy at zero, and switch to AC and it is terminated correctly as well.Thanks Ian, nice meter! A problem because the manual does not explain this properly.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 28, 2015, 06:23:19 am
content: The Tenma 72-7755 is a rebadge of the Uni-T UT70B.http://www.uni-trend.com/en/product/2014_0729_813.htmlModemhead did a repair (simple) and included teardown pictures of the UT70B athttp://mrmodemhead.com/blog/gallery/uni-t-ut70b-dmm/In picture 33, you can see the UT70B shows -269.9mV with no probes attached. It probably jumps around wildly as well.

author: SeanB
author_rank: Super Contributor
timestamp: July 28, 2015, 06:49:01 pm
content: Yes, definitely a problem that should be highlighted in the manual, as it can be very confusing to a first time user, or somebody used to a meter with a fixed input impedance on all ranges. On ACV it does not do this, just has a small random noise pickup that varies as you wave a hand over it. Probably would be best to have programmed the Cyrustek chip to not autorange into 400mV range and only go into it in manual selection.