Notes on testing Y-Main LJ41-04516A and any other Y-Main sustain

We often receive a question similar to this:
(it was sent via our eBay listing for Samsung LJ41-04516A test and repair service)

Question: "Hello, I would like to know if I have a bad Y-main board, I have tested Q5004, Q5005 Q5006 Q5007 Q5012 Q5013 Q5027 Q5028 Q5029 Q5030 are good.

Does it mean the other board is defective instead of Y- main board?"

Answer:
This answer is valid for pretty much any other sustain board I am aware of ... as well as any other electronic module which can not be (easily) tested in standalone mode, but only as part of a set made of several modules working together.

You can't tell if a module - Y-Main LJ41-04516A in this case - is working fine just by NOT finding a problem on it.

It is first because of the simple reason that you have not checked every single component on it and you can't be certain that they all work and second because you may have tested all of them individually (hardly, but theoretically possible), but you haven't tested them under load and that still leaves quite a good possibility for some of them NOT working.

The most (and for practical reasons the ONLY) reliable way to know if a sustain board is working is to to put it in a working set and see it function properly.

It is theoretically possible that you may put a sustain in a set without , say, a display (or with disconnected buffers) and measure the outputs of the sustain with a multimeter or, better yet, a scope. That gets close, but in reality is still far enough from seeing actual good image on a plasma display.

A lot of experience with a particular board - like we have with the boards we offer repair services for - can, to a degree, work as a substitute to testing on a real set, but for the same quoted first reason above it can't be a full, guaranteed substitute.

We only offer repair services for sustain boards that we can test and see working on a set.

Note that the opposite test is more applicable and beneficial: if you do find a failed component on a board (specifically one that you know tends to fail) by means of testing then you do not really need to test the board in a set. It is bad.

But the only practically certain way to know if a sustain board is good is to test it in a live, known working set.

The alternative is undefined and unprofessional.

By chance, during our recent move into our new location, we lost the testing unit for that same sustain board, LJ41-04516A. We instantly increased prices of all listings to a practically prohibitive level and told all customers who had already submitted a board that we are unable to provide a proper service and have to cancel it.