[HELP] Unknown components

[Twerp]

Hello,

Newly registered, but I've used this site in the past when google has directed me here. I'm trying to reverse engineer a schematic and I have the majority of it completed. There are a few components that have no visible markings making it pretty difficult to find. If I am in the wrong forum area (maybe analog would have been a better choice?), please let me know.

There are three components. Q001 looks to be in a standard SOT-23 package while Q305 and Q306 appear to be in a SOT428 (maybe a SOT89???). So I have checked continuity, diode tested, and ohm'd out the leads, but having difficulty reaching conclusions.

So those last three pictures show my thoughts. The middle pin on the top is connected with the bottom pin for both the Q305/306 components.

Q001: It looks like it is a simple BJT since the diode check showed it was open between the collector and emitter in both directions. The problem with this is that with every diode package I have seen, the base is always in the same position (see below picture). The base in my design will have been in the emitter position. Am I being a bit too paranoid with this component?

Q305: It looks it is a PNP BJT, but I am getting weird readings going across the collector and emitter terminals. I've heard there are prebiased transistors so I believe that is the case for Q305. Correct?

Q306: It looks to be like two diodes in series. The problem is the forward voltage from pins 2 to 3 never really settle. It slowly keeps increasing and I stopped probing at 1.8 volts since there was no sign of stopping.

So I'm basically looking for any indication that I'm on the right track (or completely off). It's difficult to know what's inside these black boxes. Are these packages mostly for discrete components or can IC's be packaged in here as well? (in which case I would really have no way of finding that out).

Thanks for any assistance,
Twerp

 

[asimlink]

Hi Twerp,

Although I am not an expert in reverse engineering but, whenever time comes where I need to reverse engineer a circuit i follow following steps:

1. Begin with identifying ICs or the multiple pin components in the circuit that can be identified easily.
2. Download IC datasheets, their application notes or schematics from google search
3. now start tracing the circuit and keep comparing the devices like transistors, diodes with the application circuits.

In reverse engineering it really helps you if you also look at application circuits provided by the ic manufacturer or the schematics from wherever you can find. And also while you are trying to trace and identify parts, keep making the schematics in parallel and see if it makes sense to select a particular component at particular location! Otherwise components are very difficult to identify correctly.

Have a great day!

 

[Twerp]

Yeah I've pretty much done all that. This is the tail end of it where it's pretty much a brick wall. I've tried simulating what I thought would work out (comparing the simulated voltages to the actual ones). The MOSFETs are being driven by a pin on the LED driver (11 V source).

The left one was just to see what would happen and it turns out to be the most accurate, even though I don't understand why they would do that. The 3.3 V source comes from another pin on the driver. I haven't detached the components Qx05/Qx06 and scoped it (so I'm not 100% positive the 3.3V and 11V are coming from the driver or not).

So I'm not really sure why this layout would be used. It seems really unintuitive. Could anyone shed light on this?

 

[asimlink]

Hi Twerp,

you are in difficult times no doubt! the small black devices in 3 pin SOT can be voltage regulators or voltage references. I have seen some LDOs and voltage references in SOT packages.

Regards,
Asim

 

[Twerp]

Yeah, that was what I was afraid of... Was hoping for a nice easy solution, but it looks like it's not the case.

Thanks for the replies, asimlink.