Help:Video Transcoder, LMH1251, possible ground problem???

Question

Hello,I have built a YPbPr (component video) to RGBs (analog, composite sync) transcoder using the LMH1251. This also includes the LMH6739 tripple video buffer.The probelm is that the screen goes black after just a few seconds, but the image is stable and clear while it is on. Here is some more diagnosis info...When I tried it out for the first time the image had a rolling black bar that was solid (not waving) and took up half the screen, rolled very slowy. If I touched a grounded area on the pcb the black bar would increase and get full of horizontal lines on the top and bottom of that bar, as if the image was compressed into the center of the screen. Again, this was only when I touched it. When it was 1st plugged in, for a few seconds, the screen was fine then the black bar started. If I picked the pcb board up, not touching ground or anything conductive, and tilted the board, the black bar would consume the screen and become distorted in a simlar way to when I touched ground. The amount of distortion would be directly proportional to the angle and amount of tilt.Now for the odd part. The next day I pluged it in a went to try it out and experiment. IT WORKED PERFECTLY! I didn't change a thing. I didn't even unhook it. I would leave it on for an hour and worked beautifully. I could touch GND, tilt it and worked 100%.  Now, again, it is not working.This time the screen goes black after about 2 seconds and nothing has changed.  Could I have a bad Capacitor? How much grounding is required? I basically have one main vein that runs though the pcb and branches off where needed.  When the screen goes black, I need to let it sit for at least 5 seconds before I can turn it on again and get the image.I am using a LM1881 to get comp sync.  Pretty much used the schematic from the National web site with the exception of the LM1881. The screen is PSOne lcd (Playstation 1 LCD). Power supply for the transcoder is an old Zip drive power box 5 Volts 1.0 amp. Power supplu for LCD is...

Jan Panteltje · Accepted Answer

On a sunny day (Sat, 15 Sep 2007 22:15:04 GMT) it happened "Jerry Lynds" wrote in :Sound like a floating CMOS input to me :-)

GPG · Answer

A possible candidatePOWER SAVE MODEThe LMH1251 is equipped with a power saving mode whichis controlled by Pin 21. This pin is a logic level input. The de-vice will enter a low power mode when the power save pin isapplied with a logic high by the MCU. Under these conditions,the IC reduces its current consumption to a minimum as spec-ified in the Electrical Characteristics section. However, thesync processor and switch will always remain active. Duringpower save mode, the RGB video outputs are held to theblank level, while the sync signals are allowed to continue tobe processed and/or passed through. Based on the absenceor presence of sync signals at the output of the LMH1251, theMCU can determine whether to bring the system to a lowpower consumption state.

Jerry Lynds · Answer

I have thought of this, but I left that pin floating so that feature is disabled.  I was wondering if it needed to be grounded though for a logic low, instead of just left floating...

GPG · Answer

It is not disabled unless a logic low is applied.

Jerry Lynds · Answer

Got it! Thanks for the help to everyone between the three groups I sent this to. You were right GPG, I left the "Power Save" pin Floating (thinking that was ok for a logic low) to disable that feature. I connected the power save pin to GND via a 1K resistor (Thanks to a schematic by Michael Schwingen) and now it works great...so far anyway!

Thanks guys,

Jerry Lynds Halifax, Nova Scotia

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Help:Video Transcoder, LMH1251, possible ground problem???

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