LG 42PQ30 no prime

[Caleb]

Picked this one up from the recycle yard yesterday, Pulled the back and there is no visual problems. All found fuses tested good on power supply, ysus, buffer and control, powered it up and all supplies are within spec with the possible exception of the -178v as I didnt find the test point for it yet. And yet I get no panel prime. Per the training manual I checked the crystal on the control and Im getting 24.8 khz which Im assuming is within spec for a 25khz crystal. I can hear relays click on the main when changing inputs.

The only thing I can see thats out of place is there is no light from the diode on the control so at this point Im thinking the control board isnt working properly. But I was wondering if anyone had any other ideas before I order another control (btw the two regs on the control were outputting 3v3 and 1v8)

[tom66]

Re: LG 42PQ30 no prime

Measure VSC and -VY across the points on the Y-sustain board, confirm if they meet the sticker ratings.

If the xtal is oscillating, and power supplies are there, it seems unlikely to be a control board issue. Not impossible, but seems unlikely.

[Caleb]

Re: LG 42PQ30 no prime

Ok found the vy test point it measures 178.8, vsc is 140.5, it appears all voltages are ok. I guess the next step is to get out the scope and look at waveforms unless you have another idea for me to try?
On page 121 of the training manual it states that if the led is no blinking on the control board and 5v supply is present replace board ,although the layout of the control in the manual is different than mine is there another way to check the control? Ive removed the lvds from main and shorted auto gen to no effect. I just checked the second reg again and it is 1.2v I typo-ed the 1.8v above
and it seems that crystal is supposed to be 25mhz, so I tried measuring again and cant seem to get the frequency counter on my meter to pick it up now.

[tom66]

Re: LG 42PQ30 no prime

Yes I would use a scope you can check the VS_UP and similar signals to determine if control board is sending right waveforms. You can also test the output of the Y and Z. You can test the xtal with a scope but you must use a10X probe.

[Caleb]

Re: LG 42PQ30 no prime

Ok ysus is outputting waveform to buffers attached pic of waveform, sorry its so blurry trying to hold the probe without slipping and take a picture at the same time. I get no waveform on the zsus test point ( does have 12.5vdc) and am not sure what to probe on the control to verify waveform, there doesn't appear to be any voltage present on the control pin labeled zsuson which Im thinking is supposed to turn on the zsus? as I cant seem to find vs up on the control on which board might I find its test point?

[tom66]

Re: LG 42PQ30 no prime

That's not really what a SUS output should look like, it looks like you have a problem on the Y-main, check the Vsc and -Vy under a scope to see if they pulsate too, and check this with and without the buffers.

[Caleb]

Re: LG 42PQ30 no prime

I checked the -vy and it was steady, but I will check it again tonight, couldnt get the probe to the vsc, will pull the ysus tonight and add in a test loop. I couldnt remember which output I could not connect ground on scope pretty sure it was vsc and buffers, that waveform was tested on the buffer test point with no ground connected. Scope was acting funny too, wouldnt resolve the 25Mhz crystal signal even though its a 250Mhz scope, might have to grab the old Heathkit out of the garage and try again.

[Caleb]

Re: LG 42PQ30 no prime

Tested with scope getting less than 1 volt of ac on both vsc and -vy with buffers connected and disconnected, tested the waveform on the buffers again and noticed its only a couple volts. After perusing the training manual some more I found a note that said the y gets its signals from the control board as well- is this not all pointing at the control? what am I missing that you are seeing?

[Caleb]

Re: LG 42PQ30 no prime

Well I went ahead and ordered a control crossing my fingers that will fix it. Will let you know if it works.

[Shinju]

Re: LG 42PQ30 no prime

Were you getting audio from any of the inputs?

[Alastair E]

Re: LG 42PQ30 no prime

Have you checked any of the IGBT's and MOSFET's on the Y-Main for shorts....?
--It Looks like whatever drive the board is making is being damped/clamped down by some short somewhere...

BTW, Xtal oscillators may stop if you probe the crystal connections due to the capacitive loading of the probe.....
--Not something I would recommend probing--Especially on a Control-Board, as it conceivably could kill the Y or X Mains--or both, as the xtal forms the clock that runs it. Stop the clock with something on a SUS turned on in its normal cycle, and there you go, Bang!

[Caleb]

Re: LG 42PQ30 no prime

I tested the crystal on the control as the training manual said to, In the future I will avoid it.

Yes it has sound but appears to be no waveforms being produced by the control board, no Im alive led lit on the control, no shorts found on the ysus or zsus, all voltages appear to be good through each board. We will find out in a few days if the control is responsible, if not it wasn't an expensive board.

[Caleb]

Re: LG 42PQ30 no prime

My control board came in today and the set is now working. However there is a section of the screen that seems to be prone to red sparklies. Its on the right side of the screen and only is apparent on white portions of the picture. It seems to affect about a 4" wide section the most, will tinkering with the voltages work on this type of fault?

[fixerupper75]

Re: LG 42PQ30 no prime

Have same set. with no picture and I checked and no led on the control board and I don't have 5v coming in to the control board. Any help or just replace the y sus?