I am in the process of troubleshooting a Therma 1 Thermometer and need help identifying 3 components please

[Techie_hobby1]

The fault at present when it switches on reads Error. Sometimes the screen shows a temperature that quickly cycles (drops) to a negative value before entering Error mode.

First component is in SOT23 package and has marking code DMF with a D1 (or 01) written sideways..
I think it might be a P-Ch Mosfet since...
Battery +ve is directly connected to Pin 3 (top single pin), Pin 1 (bottom left pin) connects to GND, and Pin 2 (bottom right pin) connects into the 2nd compoent that I am unable to identify. Both Pin 1 and Pin2 connect through a large (0805) ceramic capacitor.

The second component is also a SOT23 package and has marking code 1UAW
I think this might be an NPN Transistor, however I thought this was the issue and swapped out with a different NPN transistor (marking code J3Y) but I get no power whatsoever. It could be that th J3Y is not of the same pin layout as well though.

The last component is an 0603 black SMD, although close by on the silkscreen is THa. So I am assuming this is an SMD NTC thermal resistor / thermistor. I tested resistance then place my finger over the component and took a fuerther reading, and the resistance had dropped quite dramatically, so think that confirms that component.

[Techie_hobby1]

harp I decided to do a reflow of a couple of areas containing numerous components and have fixed the issue. Must have been a broken solder joint that I didn't spot under the scope.
Thank you for all your help

[Techie_hobby1]

I have attempted to produce a schematic in hope this will answer some questions - and also taken a couple of images of the 2 essential components.

[harp]

I dont see from your solarized image, if 1u.. have silkscreen label on pcb. Maybe it is some IC if noted say U3... I not sure about that because the marking is not same style on package, not match...

[stj]

they are obviously being used as low-drop diodes.
cant find them in my databook though.

[Techie_hobby1]

harp That is an awesome find thank you. The DMF is spot on.
In regard to U1AW marking, it is identified as U2.
Now that you have identified the DMF that tells me that the Drain (connected to Bat +) outputs 4.7v, and source outputs 4.7v., since Gate is connected to GND…. Then from Source this connects into the Collector of 1UAW NPN transistor.
The Base is tied to GND, but the Emitter is only outputting 2.7v.
I am pretty sure the MSP340F4250 requires 3.3v to work properly, since the switch FPC will drop a small amount of voltage..(albeit not a lot)
So that is why the other end of C1 is only giving me 2.4v.
Which component is the culprit - could it be the NPN transitor?

[Techie_hobby1]

stj yes these are definitely a bit of a challenge

[R_J]

1UAW could be TI TPS7A0527PDBZT See page 34 of this datasheet

[harp]

U2 means that is a IC, not transistor, so, it have input, output and ground.

[harp]

And Q1... it is p-mos so, it not likely to have gate on ground... before it will be your problem, some gate drivers shorten to ground... U2 is probably voltage regulator for that IC, and mosfet Q1 is enable it sometimes.

[harp]

Like R_J find: The TPS7A0527P is a fixed-output, 2.7 V, 200 mA low-dropout (LDO) regulator from Texas Instruments with ultralow quiescent current (1 µA typical) and an input range of 1.4 V to 5.5 V... I am a bit confused with your schematic, it seems like you do trace it while it is powered on? If thermometer has sensor on wire, check first that is wire do not intermitten over bending. Can you give as a picture of whole device and pcb?

[Techie_hobby1]

R_J That is spot on, and now I understand the logic with silkscreen notations (although I am guessing this is not always the case) I always wondered why some SOT23 are labelled as Q1, U1 and even D1, obviously D1 would be a Diode, and Q1 should be a Transistor, but I just thought U1 for a SOT23 is just the company using a U and instad of the D or Q.. now it seems to make a bit more sense..
harp in answer to your question, no I do not trace while it is powered, and use Continuity mode to trace pretty much most of the circuit, however I had to use resistance mode to identify the button traces since the ribbon cable is so thin, in continuity mode it would not pick up when the button is pressed.

Thae images are both front and back of the whole device...Note: I left off the crystal in the schematic and its additional components as I did not see any relevance to the issue I am currently experiencing with the Error code.

[Techie_hobby1]

And just incase I really have missed read the pcb, here is a close up of the entire board in stages.. but I am fairly sure I have it drawn correctly

[harp]

Strange... so they tie mosfet to allways be open... Ok, is that error has some number to identificate in manual? If you unplug sensor, is device show error, or ambient temperature? My first guess (if batt voltage are ok) is something about cable to the thermocouple sensor, or on connector itself. I dont see clearly, but it is possible that connector is smd mounted and loose contact. If you gently bring a force in some direction, is error changing? I think that power on pcb is ok, and problem is somewhere else.

[Techie_hobby1]

harp Apologies I missed a little bit of your response, there is no sensor on the actual wires that go to the type K plug. It is just 2 short leads and the type K plug at ht eother end (approx 10cm length)

[Techie_hobby1]

harp I decided to do a reflow of a couple of areas containing numerous components and have fixed the issue. Must have been a broken solder joint that I didn't spot under the scope.
Thank you for all your help