Old/Retro LCD TV Internal Board Repair (for [Samsung] UN46D6420UF 46" LCD TV). Help with Datasheets, Component IDing, and "schematic" interpretation.

Thread Starter

Tentmaker

Joined Feb 16, 2019
14
Technical Details Summary:
Product:
Samsung UN46D6420UF 46" LCD TV
Board Model ID: SH120PMB4SV0 (Samsung?) (T Con Board)
Board Main Microprocessor ID: AMLCD LP410A1 (...)
Board DC-DC IC: RT9979GQW

Background:

Goal:
I hope I can get a positive helpful answer here.

Description: So, I tried to search online for a corresponding datasheet and/or schematics for the microprocessor, DC-DC IC, and the board to no avail. Found one video on YT describing a schematic for the DC-DC PM IC, but it was non-English (Hindi). I did find this one image floating around online on various websites of what appears to be a makeshift technician's servicing "schematic". It was essentially a picture of the board that was marked up with voltages and lines pointing toward the corresponding SMD component. Please see image below:

The image:

LTJ400HV06-L SH120PMB4SV0.3 BN41-01743B T-Con Datesheet Schematic Diagram.jpg

I am guessing it was made by resourceful and practical technicians from another country (say from Pakistan or India). From my guestimation, it looks like those are the expected measurements to see if the board is working fine.


Questions:
So, on to the questions:

Question 1: What do those voltage mark ups mean? (Is my aforementioned guestimation correct? Is this common practice with professional technicians in the West?)
Question 2: If I were to desolder the board to replace the SMDs (capacitors and resistors), could I measure them (capacitor tester + multimeter), and replace with an matching SMD component?
Question 3: Any suggestions on a good place to get a set of SMD components for a hobbyist (I'm not a pro!)

Thank you!
 

twohats

Joined Oct 28, 2015
447
Hi,
Question 1, I'm sure you are on the right track, someone has gone to the trouble to note those voltages.
Probably a very popular tv, or someone renting out that brand of tv. I would imagine those voltages are for a tv without any faults.
Question 2, You would only remove/replace something if it was faulty.
Question 3, Depends where in the world you are. Yes, you can purchase most things. Sometimes you have to buy in bulk.
eBay is good for small quantities, beware of fakes.
I'm not a pro.
Good luck........
 

Thread Starter

Tentmaker

Joined Feb 16, 2019
14
Hi,
Question 1, I'm sure you are on the right track, someone has gone to the trouble to note those voltages.
Probably a very popular tv, or someone renting out that brand of tv. I would imagine those voltages are for a tv without any faults.
Question 2, You would only remove/replace something if it was faulty.
Question 3, Depends where in the world you are. Yes, you can purchase most things. Sometimes you have to buy in bulk.
eBay is good for small quantities, beware of fakes.
I'm not a pro.
Good luck........
Thank you! I'm in Toronto. Ordered from "universal-solder.com", where about a 3-and-a-half-hour drive away because the price was right. There's another company called "Sayal electronics" in the Greater Toronto Area, but they seem kinda pricy. Thank you for the warning about eBay.:)
 

Thread Starter

Tentmaker

Joined Feb 16, 2019
14
Hi,
Question 1, I'm sure you are on the right track, someone has gone to the trouble to note those voltages.
Probably a very popular tv, or someone renting out that brand of tv. I would imagine those voltages are for a tv without any faults.
Question 2, You would only remove/replace something if it was faulty.
Question 3, Depends where in the world you are. Yes, you can purchase most things. Sometimes you have to buy in bulk.
eBay is good for small quantities, beware of fakes.
I'm not a pro.
Good luck........
Maybe, another question. For the SMDs, I recall that you don't have to be too precise regarding the type used? So if I needed an SMD cap of X capacitance, as long as my capacitance is higher than required, its okay (assuming it will fit on the pad space)? Is that a fair assumption?
 

Papabravo

Joined Feb 24, 2006
21,225
Maybe, another question. For the SMDs, I recall that you don't have to be too precise regarding the type used? So if I needed an SMD cap of X capacitance, as long as my capacitance is higher than required, its okay (assuming it will fit on the pad space)? Is that a fair assumption?
Not necessarily. If the value is required for the proper operation of the circuit, then a substitution may cause the device to malfunction. Without a schematic diagram and a knowledge of circuits you cannot make an accurate determination.
 

Thread Starter

Tentmaker

Joined Feb 16, 2019
14
Not necessarily. If the value is required for the proper operation of the circuit, then a substitution may cause the device to malfunction. Without a schematic diagram and a knowledge of circuits you cannot make an accurate determination.
Thank you. I guess this sounds intuitive else cap sizing/capacitance shouldn't matter. I did wonder if it was a cost thing as well that force people to focus on minimizing capacitance to a limited range for a particular circuit.

But it does make sense that it does depend on the circuit.

My issue here is that I likely won't find schematics for various projects, so there will likely be some guess work. Any other tips?
 
Last edited:

Papabravo

Joined Feb 24, 2006
21,225
Thank you. I guess this sounds intuitive else cap sizing/capacitance should matter. I did wonder if it was a cost thing as well that force people to focus on minimizing capacitance to a limited range for a particular circuit.

But it does make sense that it does depend on the circuit.

My issue here is that I likely won't find schematics for various projects, so there will likely be some guess work. Any other tips?
There are at least three basic reasons for using a capacitor:
  1. Timing circuits. Here the value is critical.
  2. DC blocking. Here the values are less critical, but they must present a high impedance to DC and a low impedance to AC signals of interest.
  3. Power supply decoupling. This application is less sensitive to particular values and several related values are often used in parallel. e.g. 10uF, 1uF and .1uF
 

Thread Starter

Tentmaker

Joined Feb 16, 2019
14
There are at least three basic reasons for using a capacitor:
  1. Timing circuits. Here the value is critical.
  2. DC blocking. Here the values are less critical, but they must present a high impedance to DC and a low impedance to AC signals of interest.
  3. Power supply decoupling. This application is less sensitive to particular values and several related values are often used in parallel. e.g. 10uF, 1uF and .1uF
Perfect! I think since I'm usually dealing with power supply problems, the timing-circuit consideration is never really a problem for me, hence why it seemed like capacitance values didn't really matter. So, thank you for this good explanation and detailed clarification. In my notes, I have:


1. Voltage Stabilization
2. Decoupling
3. Timing circuit/Timed charge
4. Switching operations

Is this exhaustive?

Also, to add what you've said:

1. Voltage Stabilization (capacitance value not critical)
2. Decoupling (capacitance value not critical)
3. Timing circuit/timed charge (capacitance values critical)
4. Switching operations (TBD)

Am I wrong to mention switching operations as its own category as its just a subset of timing circuits?

Thanks!
 

Papabravo

Joined Feb 24, 2006
21,225
...

Is this exhaustive?
...
NO. You wouldn't really expect a forum post to be exhaustive. The use of each and every component is the subject of entire textbooks, so I would never make such a claim for a single post. If you want to collect my 21,000+ posts into a single volume that might approach exhaustive, but even that would not be the complete story.
 
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