TENMA 72-1023 = UNI-T UT805

[costas]

Hi ,
I bought this tool from E-bay, built primitively, hoping that it can be easily repaired, I see that inside the PCB are from UT805, it means that Tenma 72-1023 is a clone of UT805. What is the problem: at start-up everything is fine, no error occurs but if I switch it to any measuring range it crashes and does not recover until after restart, I suspect that something is blocking the communication bus… I replaced the measuring circuit ES51986A but in vain the defect persists, I checked the voltages and supply capacitors with the ESR-meter, they are ok, a scheme would be welcome or a troubleshooting idea.
Thank you .
Costas

[Twoflower]

In the last image the connectors in the upper right corner (the lower one) seems not to be sitting correct. It either never was fitted correctly or someone has played with it. Check if it helps to plug it correctly.

[costas]

In the last image the connectors in the upper right corner (the lower one) seems not to be sitting correct. It either never was fitted correctly or someone has played with it. Check if it helps to plug it correctly.

I checked the all plugs are inserted properly !

[Kleinstein]

With more moden equipment there is usually no official schematics available. So you have to make up your own and do at least a partial drawing from the pictures.

A frist step would be to get a general idea which parts are isolated or share the same supply. An isolation barrier is usually easy to spot. It is not sure there is one, but chances are good.

The second point is where are ADC and reference. It looks like the ES51986A does most of the conversion. Quite often such meters are relatively close to the standard circuit proposed by the main chip datasheet. There can be a few variations, but it is a good starting point.

[timenutgoblin]

In one of the photos (IMG_7154 ok.jpg) there is a large DIP IC under a bunch of cables, near the front panel. Try reseating the IC. It looks like the IC has come out of its socket. Maybe the unit was dropped? Check the other socketed IC devices.

[costas]

With more moden equipment there is usually no official schematics available. So you have to make up your own and do at least a partial drawing from the pictures.

A frist step would be to get a general idea which parts are isolated or share the same supply. An isolation barrier is usually easy to spot. It is not sure there is one, but chances are good.

The second point is where are ADC and reference. It looks like the ES51986A does most of the conversion. Quite often such meters are relatively close to the standard circuit proposed by the main chip datasheet. There can be a few variations, but it is a good starting point.

that's what I thought, that's why I checked and replaced the measurement chip, but in the digital area with two Atmega and a NEC for the display it doesn't work very well with the use data sheet  schemes ,  a firmware defect is also possible ...

[Kleinstein]

The  ES51986A is supposed to be only 6000 counts. The other point is that there is quite a good ADC (AD7710) on the PCB. So the circuit starts to look odd. The At89S52 should be a 8052 compatible µC, so not an AVR.

2 µCs would mainly make sense if one is at the input side and the other at the output (e.g. interface, if there is one). But I see no isolation barrier - at least not a good one.

[costas]

The  ES51986A is supposed to be only 6000 counts. The other point is that there is quite a good ADC (AD7710) on the PCB. So the circuit starts to look odd. The At89S52 should be a 8052 compatible µC, so not an AVR.

2 µCs would mainly make sense if one is at the input side and the other at the output (e.g. interface, if there is one). But I see no isolation barrier - at least not a good one.

very interesting observations, only I want to fix it not to make it more efficient, so I have to work with what IC's is on the board...

[Kleinstein]

Of cause you only want to fix it, but it helps to understand at least the basic working to have a good chance to find the problem in a more systematic way than randomly exchange chips as chance.

A possible point to watch would be the digital ouput signals from the µCs - especially those that change and indicate activity. Candidates would be UART or SPI pins. Chances are that when the DMM gets stuck, one of the µCs no longer shows activity / sends data.
Still it helps to know which chips sends data which way.

Poor contact at the sockets are a possible candidate  - so worth moving the chips a bit out and back in.