UBC780XLT Dodgy Screen

UBC780XLT Dodgy Screen

Postby centralcoastscanman » Wed Sep 23, 2009 10:49 am

Hi Guys & Gals

Recently my UBC780XLT screen has decided its going to play up, and by this i mean there is light on the screen but nothing is displayed when the scanner is powered up.

any ideas on how this can be fixed, if it can as I really don't want to depart with my ubc780xlt anytime soon, as other than this its been faultless for 6 years.
Posts: 750
Joined: Fri Oct 31, 2008 7:58 pm

Re: UBC780XLT Dodgy Screen

Postby citabria » Wed Sep 23, 2009 11:16 am

I've fixed this one a few times before. Take it apart, and resolder the LCD flex where it meets the main board.
Site Admin
Posts: 1036
Joined: Thu Aug 14, 2008 8:22 pm

Re: UBC780XLT Dodgy Screen

Postby Mike Alpha » Wed Sep 23, 2009 4:18 pm

There's also a resistor on the back of the front board that sets the contrast of the LCD. It may need replacing.

Info here: http://myweb.accessus.net/~090/bc-780xlt.html#contrast

Mike Alpha
Posts: 643
Joined: Thu Aug 14, 2008 8:22 pm

Re: UBC780XLT Dodgy Screen

Postby Mike Alpha » Sat Feb 22, 2020 11:43 pm

A proper fix for a faded screen on a Uniden 780XLT, 785D or 796D was posted on the RR forum a few years ago.

It turns out the screen fades because the LCD module internal voltage converter goes faulty and it is a -ve voltage that sets the contrast level via a SMD resistor. So an isolated DC DC converter is needed to replace that -ve voltage.

The RR forum explanation and fix is here https://forums.radioreference.com/threads/bc780xlt-dim-display-fix.360267/

But I've managed to do the same with $10 worth of parts.

I used a Tracopower TBA 1-1221E DC DC converter module with a -5V & +5V output. I took the 13V input voltage via a 100ohm resistor from the scanner available on a regulator transistor that sits on a heatsink in the 780XLT. Then the isolated -5V through a 10K resistor and then into a 10K variable resistor to adjust the contrast level at the 8.2Kohm SMD resistor point on the circuit board. The isolated 0V output can be grounded to any point on the circuit board but I prefered the point closest to the decoupler capacitor just in case there is any noise that needs to be shunted.

Here's a pic showing the mod. Double sided foam tape stuck everything to the PC board.

(107.44 KiB) Not downloaded yet

I've resurrected 2 x 780XLTs from no screen to perfect displays now.

Mike Alpha
Posts: 643
Joined: Thu Aug 14, 2008 8:22 pm

Re: UBC780XLT Dodgy Screen

Postby cartman » Sun Feb 23, 2020 7:21 am

Great stuff Mike.
My only experience in electronics is blowing stuff up.
I think i have a blacked out 780 somewhere in addition to the 780s i am using. Will have to locate it
Professional Scanner nut. Ibis bin chicken of radio scraps
Uniden 325P2, Whistler TRX-1, GRE PSR800 x 2, Uniden 780 x 3, Uniden 796, Uniden 396 x 2, Uniden 246,
DSD v1.101, WinDSDv2, ADCR25 v5.10, Unitrunker, Trunkview
User avatar
Posts: 2055
Joined: Wed Aug 13, 2008 12:54 pm
Location: Liverpool, NSW, Australia

Return to Uniden

Who is online

Users browsing this forum: No registered users and 1 guest