Author Topic: Transistor Looking Device with G21 Marking  (Read 4943 times)

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Offline bostonman

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Transistor Looking Device with G21 Marking
« on: March 24, 2021, 11:37:37 pm »
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.

 

Offline ataradov

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Re: Transistor Looking Device with G21 Marking
« Reply #1 on: March 25, 2021, 12:05:31 am »
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.
Alex
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #2 on: March 25, 2021, 12:58:31 am »
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.
 

Offline lordium

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Re: Transistor Looking Device with G21 Marking
« Reply #3 on: March 25, 2021, 02:40:54 am »
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?
« Last Edit: March 25, 2021, 02:47:25 am by lordium »
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #4 on: March 27, 2021, 02:18:31 pm »
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?

 

Offline ataradov

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Re: Transistor Looking Device with G21 Marking
« Reply #5 on: March 27, 2021, 07:13:22 pm »
Pin4 of the AD736 is VSS. Given that it is a front-tend, it is likely to be a negative voltage.

So pin 3 of the device is connected to ~VCC/2 and pin 2 is connected to ~VSS/2. Both are static voltages. And pin 1 is then connected to the signal path.

The only way this makes any sense to me if this is a diode array for clamping. Try to remove a good device and measure 2-1, 1-3, 2-3. You should see a single diode drop in the first two measurements, and a double diode drop in the last.

You did not mention which pins measure 1.4 V, but it may make sense if you measured 2-3.

The reason markings are conflicting is that there is no common database, each vendor comes up with their own scheme. The goal of the markings is not to identify a completely unknown device, but to confirm that they match to the expected device.

Alex
 

Offline lordium

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Re: Transistor Looking Device with G21 Marking
« Reply #6 on: March 28, 2021, 09:51:10 am »
just looking at the tests in your pdf I would say it's the DTD113ZN3. The first result seems like a broken one, but the second seems right, about ~1.7V and ~10k Ohm (depends on the current output of your multimeter etc)

« Last Edit: March 28, 2021, 09:54:22 am by lordium »
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #7 on: March 28, 2021, 03:04:37 pm »
This is a huge help, but, according to the datasheet, the package dimensions don't match my measurements.

Unfortunately even if this (assumed) DTD113ZN3 is bad, the problem with this scope is an Agilent hybrid chip. I've measured several points and the area around this DTD113ZN3 measures differently. After probing more in another area, I found the AD823 IC had much lower resistance on the bad channels. My guess is that a previous owner found the same issue and attempted to replace these in hopes to fix the channels; and why they appear to have been replaced.

Sadly once I removed the IC, the circuit still measures low and all surrounding components are not contributing to the problem. The only unknown is the hybrid. Trying to find a non-counterfeit part is next to impossible, and trying to find one is harder. Also, they are not easy to replace because of a ground pad underneath.
 

Offline ataradov

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Re: Transistor Looking Device with G21 Marking
« Reply #8 on: March 28, 2021, 06:12:16 pm »
If it is a transistor, then your schematic makes no sense.

Measure the voltages on the pins of the working channels while it is powered on.
Alex
 

Offline lordium

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Re: Transistor Looking Device with G21 Marking
« Reply #9 on: March 29, 2021, 01:34:35 am »
Yeah I take the readings and measurements with a grain of salt and read some between the lines. But Rohm makes a DTD113Z in SC-59 package (G21 marking) similar to sot-23 but a little bigger, so fits your measurement. But a transistor on some oscilloscope input seems likely (as some kind of protection I assume). And since you do have good channels to work with, it's all about how far down the rabbit hole you want to go (start swapping!).
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #10 on: March 29, 2021, 02:00:16 am »
I can't disagree on this circuit not making sense, especially since I have two unknown components: the one marked G21 and a hybrid. I measured point-to-point several times, so a possibility exists I made an error in transition from measuring, to writing, to drawing the circuits, but I've checked everything several times. I made a small error on the schematic - the 100 ohm resistor is before pin 1 on the G21 - I've redrawn that section - and added the second circuit in question on the bottom. Attached is a picture of the board - I drew a red circle around G21 and two lines pointing to the vias that are connected. So I've pretty much have one pin that's correct because it's visible on the board (the 100 ohm resistor), and I am confident the two vias are connected making two pins correct.

I removed the G21 from a good channel and measured it - see attached for the updated list of measurements.

From what I can tell, the AD 823 on both Ch 2 and Ch 3 (the bad channels) was replaced by someone, and, it appears the G21 component too. My measurements led me to these two components, and, after looking at the soldering, it appears they were replaced by someone finding the same differences in measurements. I plan to replace both components in both channels, so I'm still interested in knowing what this G21 component is.

What I also noticed is the AD 823 in Ch 2 and Ch3 has a dark marking instead of a shiny silver one; and they are AD 823A instead of 823 (not sure if it's an issue, but I'll look later, just wanted to mention it). The interesting thing is the G21 in Ch 2 and Ch 3 have the sideways 32 suggesting they were replaced with the wrong component because Ch 1 and Ch4 have just a plan G21.

Attached is a picture of each G21 showing the 32 on two of them to avoid any doubt on what I'm referring to.
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #11 on: March 29, 2021, 02:36:36 am »
Quote
Yeah I take the readings and measurements with a grain of salt and read some between the lines. But Rohm makes a DTD113Z in SC-59 package (G21 marking) similar to sot-23 but a little bigger, so fits your measurement. But a transistor on some oscilloscope input seems likely (as some kind of protection I assume). And since you do have good channels to work with, it's all about how far down the rabbit hole you want to go (start swapping!).


I replied to the thread before I got notification you sent a follow up - don't want to seem I ignored your response.

Not sure I want to swap as I think the main issue is in the hybrid chip. I'm guessing someone blew the input which damaged the hybrid, or maybe they replaced the G21 when they got different measurements like me, but replaced it with one with the sideways 32 and blew the hybrid. I'm uncertain the AD823 is bad, but this is irrelevant since it's obviously an AD823.

I'm on the fence with how I'll proceed with this scope: I can replace the components (once I can confirm what this G21 component is) and leave it with a bad 50ohm input on two channels, or gamble with the ones being sold on Ebay. Gambling is around $160 for two chips that could be counterfeit, and the cost to have someone replace them. Probably a $300 total gamble - and it's not to say the replacement chips will last since they could be counterfeit.
 

Offline lordium

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Re: Transistor Looking Device with G21 Marking
« Reply #12 on: March 29, 2021, 04:08:38 am »
You can see picture for some samples when I check this part, they all have those numbers on the side (most likely date code or similar).
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #13 on: March 29, 2021, 04:24:23 am »
Quote
Take the readings from the various pins of the transistor and evaluate the approximated values with the datasheet. So that this might help to know whether the datasheet referred is correct or not

Well technically I've done that. Between two pins, I should get 11k, but looking at my measurements, I didn't get that on any of the pins.
 

Offline lordium

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Re: Transistor Looking Device with G21 Marking
« Reply #14 on: March 29, 2021, 05:11:37 am »
When I set up test with regular transistor (2n3904) with external resistors, and measure resistance I get the following:

~11k using UT-61E (both directions)

~11k using 3457A positive on emitter

but

~6.8k using positive on base, because the 10k is in parallel with the diode(PN junction), so the 10.24k could be due to your multimeter.

also, the 11k is not super important, since it's more likely trimmed to the ratio of the resistors, not actual value (might differ from one manufacturer to another).
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #15 on: March 29, 2021, 12:36:03 pm »
I think you're looking at my in circuit measurements.

If you look at my last message with the attachments, the updated spreadsheet has a section for the G21 component removed from the circuit and measured on the bench.
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #16 on: April 02, 2021, 02:14:47 am »
I'm kind of at the end of the rope with this component and based some findings on the website provided above:

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

I believe I've narrowed the possibilities to three components based on package size (see attachment that includes all the measurements of a good component and the three possibilities).

The most practical one seems to be the DTD113ZK. If it were this component, I'd assume two pins would measure 11k out of circuit. If so, seems it's back to the drawing board with figuring out what this component is.
 

Offline ataradov

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Re: Transistor Looking Device with G21 Marking
« Reply #17 on: April 02, 2021, 02:41:34 am »
Can you actually measure voltages on the pins when the board is powered on.

If your schematic is correct, it makes no sense to have a transistor there.

Also, how does the channel behave when the part is not in circuit? It is not breaking the signal path, so you should see something.
Alex
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #18 on: April 02, 2021, 01:25:46 pm »
I was hoping to solve this before powering the board as I've removed a few components for troubleshooting purposes, however, it seems some questions will be answered if I take your approach.

My concerns are: I power it without the components, and they turn out to be something that controls a voltage thus causing a high voltage on a component and damaging something; or I install the "bad" components and need to remove them again to replace which could damage the board. I know on Ch 1 (the good channel) a pad lifted on pin one of this G21 while I was removing it.

In any case, I think the best approach will be your suggestion and just power it to confirm voltages.
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #19 on: April 10, 2021, 01:37:45 pm »
I'm waiting for some replacement components from Digikey. I am replacing the AD823 and resistors I removed to measure and test the circuit in both channels.

Once those arrive and are installed, I'll power it without the G21 and provide updates.
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #20 on: May 03, 2021, 03:44:23 am »
Hopefully this topic hasn't gone stale.

Unfortunately I've been unable to get time to solder new components on and take measurements, but this weekend I set aside time.

Attached is the schematic (which I've previously posted) of how I interpreted the connections based on hours of tracing. Second file is the measurements of two out of the four G21 components (one per channel) measured out of circuit (note: one is G21 and the other is G21 with a sideways 32 - I suspect channel 2 and channel 3 had these replaced because these are the only two channels which are bad, have flux indicating someone replaced these, and have a '32' on them - possibly being the wrong component).

These measurements were done in both resistance and diode settings as noted.

Third file are voltage measurements in all four channels (channel 1 and 4 work in 50ohm mode, and channel 2 and 3 don't and are considered "bad"). Two channels don't have the G21 installed (noted in the file), so I was able to measure the pads only. You'll see that the two G21 components removed are the ones I measured out of circuit. Since they were already removed, I kept those channels without G21.

I switched the front channels from high impedance to 50ohm to see if that affected any voltages (noted on the attached).

It seems this G21 doesn't contribute to the normal function of the circuit, but may be some sort of protection device.

My guess: these components are not the cause of channel 2 and 3 failing in 50 ohm mode, however, trying to figure out what they are so I can replace them with the correct "G21" components I think is important.

 

Offline ataradov

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Re: Transistor Looking Device with G21 Marking
« Reply #21 on: May 03, 2021, 04:27:55 am »
All of this further confirms that this is just a diode array. It clamps the input voltage to ~1/2 of voltage rails.

For the first pass its presence should not affect performance on small scale inputs. If something does not work, then it is not the fault of that component. You can confirm that by moving the good one between the channels.

And then once you figure out what else is broken, replace it with BAT54S and don't waste any more time on that.
Alex
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #22 on: May 06, 2021, 02:50:37 am »
I agree that it must be some sort of protection device (especially since it's tied to the input signal path side), however, based on the package size I measured, it only matches that of a transistor type package based off of:

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

Offline ataradov

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Re: Transistor Looking Device with G21 Marking
« Reply #23 on: May 06, 2021, 02:59:09 am »
That list is far from complete. It also has not been updated in ages. You measured a diode, it makes sense that according to the schematic that it would be a diode. Why are you refusing to believe that this is a diode?
Alex
 

Offline bostonman

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Re: Transistor Looking Device with G21 Marking
« Reply #24 on: May 06, 2021, 03:08:27 am »
Not refusing to believe it at all. I just wanted to remind you (and/or anyone reading) about past postings.

Much like myself, we follow different messages and/or message boards, so some messages within the thread can be forgotten and wanted to remind you that a list was provided; and that I looked through that list thoroughly checking each package size to see if it matched mine.

Since nobody commented on that list, I assumed it was accurate and what this component type would be limited to.

At this point I accept your answer as being correct. Ideally I'd like to replace this with the same manufacturer. It would also be nice if I could understand what the sideways 32 represents since it's on the "bad" channels. This could be the cause of the channels failing or maybe coincidence.

 


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