Y-board Buffer combo LJ92-01601A IC testing?

[Glitcher]

This is coming from a PN50B450

Does anyone know how to test the buffer IC's for shorts on this Ysus/Buffer board?

Tested all the common resistors that usually blow.

I found the usual 6 mosfets shorted, Then found the driver IC (irs21956s) had a short. Replaced everything that but still same problem, sound but no picture.. Retested everything on the board and there are no shorts but I still have the same sound but no picture.

I still haven't replaced the other smaller driver IC (irs21856s) but it ohmed out good. (Haven't ruled it out yet though)


Only thing I thought left were the Buffer ICs, but how do you test these? (No obvious burnt marks)

[tom66]

Re: Y-board Buffer combo LJ92-01601A IC testing?

If you have -Vsc and Vsch the buffers are likely OK, at least they would not cause no picture. Any glow from panel? Do you have access to a scope? Check voltage across the bank of red caps on Y-main should match X-main same caps. Also check if logic board LED flashing.

[Glitcher]

Re: Y-board Buffer combo LJ92-01601A IC testing?

I have 2 of these panels so tested by taking the y-main from the good one and putting it in this one and it fired up normally so im sure the original y-main is still bad somewhere...

I'm not leaving the panel on for too long as im testing cause I feel like something might blow soon. There is no glow from the panel. Vsch reads -49v. I can't get a reading from Vscan, its making my meter go crazy, even on the highest setting (600v) its reading off range??

The red caps on the y-main read 102.2V and 104v on the x-main.

Heres what I got from the scope.
I also noticed that the middle heat sink is hot while the left and right ones are still cool to the touch.

[tom66]

Re: Y-board Buffer combo LJ92-01601A IC testing?

Well you do appear to have Vscan as it's the lower part of the sustain waveform & your board is sustaining the panel. Can you try checking at one of the buffer outputs? It's plausible missing/shorted floating 5V could cause your issue.

[Glitcher]

Re: Y-board Buffer combo LJ92-01601A IC testing?

I tested with the scope on this circled cap, is that buffer output?

I found one of my old threads with the same panel and I found a photo of a working y-main for reference.

[tom66]

Re: Y-board Buffer combo LJ92-01601A IC testing?

That's a good place. What V/div scale is your "bad" scope shot taken at?

[Glitcher]

Re: Y-board Buffer combo LJ92-01601A IC testing?

Its at 5v/div X10 probe

[tom66]

Re: Y-board Buffer combo LJ92-01601A IC testing?

So that reading is OK, weird. Since you have the other board, could you check resistance across some of the little MLCC caps around the buffer chips?

[Glitcher]

Re: Y-board Buffer combo LJ92-01601A IC testing?

Heres the readings I got, its around the same for each batch near each IC.

The only difference I found was that the bad board had one reading of 10K while the good board was out of range for my meter. This was seen on each batch.

[tom66]

Re: Y-board Buffer combo LJ92-01601A IC testing?

Wish I had a bright idea, that's a stumper; don't necessarily think that 10k is a bad reading, could just be a production change from one board to another, any different rev #?

There should a little 5V 7805 regulator for the buffer logic supply, can you see one? Note, it floats wrt to -Vsc so don't short it or test it to ground.

Also, could you try taking a test at the buffer output - there appear to be metal pads at two parts of the board leading off the buffer ICs, this might provide more useful info.

[Glitcher]

Re: Y-board Buffer combo LJ92-01601A IC testing?

The bad board is Rev AA2 and the good is AA1, the only difference I see is that AA1 has bodge job which added a resistor and the AA2 rev has the resistor actually built in.

I might have to come back to this set another day.

[cliff_hootarski]

Re: Y-board Buffer combo LJ92-01601A IC testing?

Check resistance across the two power resistors below the third buffer IC from the top
of the board. Low resistance indicates one or more bad buffer IC's.... this is from a UK
seller of repair kits on eBay.

rev. AA2 board http://www.ebay.com/itm/LJ41-05905A-...item3f31f39fa0

rev. AA1 board http://www.ebay.com/itm/LJ41-05905A-...item3f32c25c1c

[JamesTV]

Re: Y-board Buffer combo LJ92-01601A IC testing?

I know this is an old thread but I did want to chime in anyway. There is a SMD 3 ohm resistor to the right of the first MOS-FET that opens and a 47 kohm SMD around the corner from it next to the heat sink. The Shorted MOS-FETs will open either both of one of these but you don't get them included in any kits available for sale.