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Transistor Looking Device with G21 Marking

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bostonman:
I'm working on an oscilloscope and came across a section of the circuit that has different resistance from the other two working channels.

One section of the circuit goes into a device that I think is a SOT23 package (although don't quote me) three-pin device - I assume it's a transistor. When I (blindly) measure two pins, I get an approximate 1.4v drop, but, on a good device (but still in the circuit), I get a 0.7v drop.

The marking on the component in each channel is: G21, but one or two channels seem to have a sideways 32 printed on it. I assume the 32 is a lot date code since the other devices don't have the 32.

In any case, I did a search for 'G21' and found a datasheet on alldatasheet, it indicates the G21 is just a marking, but the actual part number is: DTD113ZK

Does anyone know if I found the correct datasheet, because, according to this datasheet, one pin should be ground, and I didn't measure either of the three pins as ground.

ataradov:
The one with sideways marking could also be DMG2301U.

The one you found is just a pre-biased transsistor, it can be used in many ways including without connecting anything to ground. You need to figure out a bit more of a schematic around those parts.

bostonman:
The four circuits are identical (it's a four channel oscilloscope) and all the circuits have identical components in the same locations.

It's why I assumed the sideways marking was a date code, but I'll look at the surrounding circuits more. This is an extremely difficult board because it's multi-layer with blind vias. It took several hours to trace the one point (now I discovered a second point elsewhere) that was measuring incorrectly, and, when I removed all the components, I realized this device was also in the circuit.

Sadly, this scope has an obsolete IC and I'm hoping the different readings is a surrounding component rather than the IC chip.

Looking at the datasheet for a DMG2301U, it alludes that the date code would be a letter and number rather than two numbers.

lordium:
One of my favorite sites for looking up codes:

http://chip.tomsk.ru/chip/chipdoc.nsf/vc1!readform&view=smd&cat=G&start=501&count=500

G21   2SK1958       SC-70   Small Signal FET(MOS type) Switching
G21   DMG2301U       SOT-23   P-CHANNEL ENHANCEMENT MODE MOSFET
G21   DTD113ZKA       SC59   npn digital transistor 1k0 +10k 50V 500mA
G21   MMBZ4621-V   MMBZ4621   SOT23   Small Signal Zener Diode (3.6V)
G21   DTD113ZN3       SOT-23   NPN digital bipolar transistor

so maybe last one is the likely one? Pull it off the pcb and try to measure the resistance to verify?

bostonman:
Attached is a circuit that I've traced. I've done the best I could to trace this circuit; the board is big with blind vias. From what I can tell, this is quite accurate, however, as noted, the scope probe input section is simplified.

I'm reluctant to remove a good G21 device to avoid damaging it or it flinging off somewhere. Ideally I'd like to know, and learn, what this device is.

A slim chance this component was replaced in one or two of the channels (remember two channels out of four are bad - but only in 50ohm mode). I noticed an AD823 IC was replaced in Ch2 and Ch3 (the bad channels) - at least it looks like they were replaced because it has some solder balls along with black marking rather than shiny silver as Ch1 and Ch4 (the good channels) have.

It's possible the device(s) marked with a sideways 32 are a (wrong?) replacement. I'll try looking at the part number later to see which ones have the 32, but wanted to point that out just in case it makes a difference.

Due to them be tiny, poorly carved part numbers, reflections under the microscope, etc... it's hard to read the part numbers.

On a side note, I assume the abbreviated marking (G21 in this case) is due to limited size to print the part number, however, why does a G21 have many different part types?

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