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_posts/2022-05-11-TTI1906-Repair.md

@@ -71,3 +71,23 @@ The 200uA to 200mA current shunts are contained in RP2. The 200uA scale is switc
 V5 prevent high voltage transients from occurring if the current is fed from an inductive source when the fuse blows; these could otherwise prevent the arc from extinguishing within the fuse body which could therefore result in instrument damage. VR5 uses a TVS symbol on schematic, however physically it's a MOV (Metal-Oxide Varistor)[^afd6].
 
 D1 to D5 protect the Multimeter current input for overload/overvoltage or connection mistakes, that may damage the switch MOSFET or the shunt resistor. In current measure mode a fuse can protect the multimeter input, however there are some limitations. One of this limitation is fuses do not blow sufficiently fast enough to protect the shunt resistor and MOSFET switch from destruction in case of a fast voltage transient. For this reason a 5-diode protection circuitry[^c6df] is added as a protection mechanism.
+
+# Troubleshooting
+
+The transistor F6 was tested and no issues was found. Same for the shunt resistor RP2, Fuse FS1 and MOV V5.
+
+Testing the protection diodes D1 to D5, I found D2, D3 and D4 damaged. Diodes D2 and D4 were dead short circuited, while Diode D3 shows a vary high leakage current.
+
+![]({{ site.url }}{% link images/tti1906/input_fail.png %} "mA input circuitry faulty components"){: width="800"}
+_mA input circuitry faulty components_
+
+The main PCB showed some previous soldering work performed on the input diodes (D5), it maybe means the unit experienced similar fault before and was repaired.   
+
+![]({{ site.url }}{% link images/tti1906/d_location.jpg %} "mA input circuitry faulty components location on PCB. Soldering signals on D5"){: width="600"}
+_mA input circuitry faulty components location on PCB. Soldering signals on D5_
+
+I decided to replace all protection diodes D1 to D5 to avoid any further problems cause for potentially almost-faulty part.
+
+After replaced the diodes, the unit was retested and the 200uA scale works ok.
+
+Even as an uncalibrated[^906a] instrument, I found all the TTI1906 multimeter measures very closer to the reference voltage source and to my Fluke 45 bench multimeter.