Screens Gone Blank


 

J. Winn

TVWBB Fan
Voltage between center pin of the pot and ground should be .4V right? Im reading almost 5V thats not right is it?
 

J. Winn

TVWBB Fan
I have continuity between 1 and 5, 2 and 3, 2 and 15, 3 and 15. If I jump 3 and 16 I get the display back but the pot does nothing.

Also, jumping 3 and 16 with a wire vs with the multimeter. With the wire the screen is pretty faint but with the multimeter it is much darker. :confused:

Also, not sure that it matters or not but its a 4.2.4 not 4.3.
 
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RalphTrimble

TVWBB Diamond Member
It looks like the contrast pot is a simple voltage divider, so the outer two legs are gnd on one side and +5VDC on the other, the wiper (center leg) should sweep from gnd (0v) to 5VDC as you rotate it, so that is what you should see on Pin 3 of the LCD connector. And since Pin1 is gnd and Pin2 is +5v on the LCD header you are correct, you should have continuity between the legs of the contrast pot and the first 3 pins of the LCD header.
 

RalphTrimble

TVWBB Diamond Member
Voltage between center pin of the pot and ground should be .4V right? Im reading almost 5V thats not right is it?

The center leg of the contrast pot should read 0V (gnd) when rotated fully in one direction, and 5V when rotated fully in the other direction. I assume 5v would be fully clockwise but it could be the other way around. So try rotating the pot in the opposite direction, does the center leg go to 0V? If so the contrast pot is doing its job. You should see the same voltage on Pin3 of the LCD as you see on the center pin of the contrast pot.
 

J. Winn

TVWBB Fan
The center leg of the contrast pot should read 0V (gnd) when rotated fully in one direction, and 5V when rotated fully in the other direction. I assume 5v would be fully clockwise but it could be the other way around. So try rotating the pot in the opposite direction, does the center leg go to 0V? If so the contrast pot is doing its job. You should see the same voltage on Pin3 of the LCD as you see on the center pin of the contrast pot.

The pot seems to be working as it should. I stumped. Would a clean install be of any help? My only other option at this point is to jump pin 3 to pin 16 and deal with the crappy contrast i guess. Also, this only works if I use the top row of LCD contacts that arent connected to the HM. If i jump the pins that are connected to the HM it doesnt work. If anyone else has any ideas of what i can check/troubleshoot please chime in.
 
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Bryan Mayland

TVWBB Hall of Fame
No software changes are going to make any difference, so don't waste any time reflashing software. You have continuity between pins 2 and 3 which is a definite problem. Luckily, pin 3 comes directly from that center potentiometer pin so there is only one PCB trace involved and it is shorting to the 5V line (pin 2). I would think it has to be happening at the LCD connector so solder suck out pin 2 and 3 and just lightly retouch them with solder.
 

J. Winn

TVWBB Fan
No software changes are going to make any difference, so don't waste any time reflashing software. You have continuity between pins 2 and 3 which is a definite problem. Luckily, pin 3 comes directly from that center potentiometer pin so there is only one PCB trace involved and it is shorting to the 5V line (pin 2). I would think it has to be happening at the LCD connector so solder suck out pin 2 and 3 and just lightly retouch them with solder.

Fixed the continuity between pin 2 and 3 and I see the voltage change on pin 3 now from 0 to 5V when I change the pot position. Still no text on the screen. Its displaying text because if I do the jump pin 3 to pin 16 on the unused row of contacts i can see it faintly. Any other ideas???
 

RalphTrimble

TVWBB Diamond Member
If you jump those pins on the unpopulated header and it makes a change in display, but when you jump the same pins on the soldered pins it makes no change, then I suspect a busted trace on your LCD board. Try measuring continuity between the two headers on the LCD (with power off) you should see continuity on each pin respectively. If you don't then try putting a jumper wire on the LCD board between the pins that do not have continuity, that may fix you up.
 
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Bryan Mayland

TVWBB Hall of Fame
I'm assuming you don't have continuity on pins 3 and 15 any more either, right?

It is very strange that you're seeing the voltage change on pin 3 but it does nothing. Are you measuring on the soldered side or the unpopulated side? If you're checking on the soldered side, try on the unpopulated side. I'm wondering if there is the right contrast at the solder point but it isn't getting through to the LCD for some reason. If there is different voltage on the unpopulated side, then try running a wire from soldered pin 3 to the unpopulated pin 3 and see if that works.
 

J. Winn

TVWBB Fan
Back in business. Jumped pin 3 on the bottom of the LCD to pin 3 on the top of the LCD. The trace must have broke. Thanks for helping me troubleshoot.
 

 

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