Yaesu FT-920 backlight CCF replacement with LED bar - Page 1
Forum section: electronics / rf-microwave

author: DC1MC
author_rank: Super Contributor
timestamp: October 04, 2020, 06:47:03 pm
content: Hello everybody, recently I've got an Yaesu FT-920 transceiver in a neglected, grorrila assaulted state, so I decided to make some short to the point articles regarding individual repair tasks that I've performed on it.This topic is about the display backlight replacement, as the device is over 20yrs old, the old CCFL tube was almost fully off, just in one end some cadaveric light could be seen. Also the HV converter was producing some 580Vac/59Khzwith a self-oscillating two transistors/one resistor contraption, a wonderful source of QRMAs I've had anyway to repair the encoders (main VFO fully blocked, aux barely moving) I've ordered some LED bars for replacing laptop/monitors and did this project as German Reunification Day activity (after cleaning the garden).The service manual describes very well the device disassembly and YT has lots of videos about it, the bottom line is that the front panel (after removing the top and bottom lids) it has hinges that allows is to tilt forward and give easy access to the display PCB, encoders and front panel buttons (sadly not illuminated).The cable harness that goes to the (way too many) connectors is hold to the board with a thin zip tie, carefully cut it to be able to remove the display PCB. After cutting the zip tie, carefully remove the wired connectors, they are overlapped, so start with the ones on top, the cabling is reasonable well done and there is no chance to mess putting them back.Along with the wired connectors, there are a couple of flat ribbon cables, the connectors holding them have some kind of a collar, that must be raised 2-3mm to be able to pull-out the flex cables. Double your dose of carefulness here.Once the cable harness is out of the way, the display board itself is hold to the front panel wit just two screws on the extremities (right now make sure that you don't touch the other screws on the PCB).Now comes the most critical part: removing the LCD without breaking it:- Put the display board face down on a soft surface (sillicon mat, blanket) and remove all the remaining screws (4 pcs), they hold the reflector to the PCB.- Carefully straighten the metal tabs holding the LCD screen case to the PCB, make sure that they are aligned with the slots in the PCB and in vertical position.- Unsolder the two white wires from the CCFL to the output of the HW converter.- Carefully lift the whole LCD assembly of the board from the extremities, using both hands, in some situation the metal tabs may need extra attention to align with the slots in the PCB.The white reflector should remain "captive" in the LCD enclosure (it was shown here for instruction), the CCFL is hold in place by two silicone rubber grommets. Carefully (you see, I'm using this word a lot because is really needed), recover the grommets, they will be needed later and discard the CCFL.WARNING: The CCFL is a 4mm VERY fragile glass tube, also holding harmful chemicals, if it breaks in your hands it will cut your skin and act as a syringe needle, injecting you with really nasty mercury and whatever fluorescence stuff coats its walls. Better use gloves when removing the silicone pads on its ends, somewhere on your balcony or porch, in free air. If you aren't sure you could do it safely, use afterwards some BlueTac or similar to seal the light cavity, not worth risking !!!With the CCFL out of the way, is about time to focus on the LED bars, mine came as a pair with a driver PCB holding a boosted constant current source IC. As this thing produces even more QRM, was immediately discarded.Now the main problems are:- The size of the led bars of 490mm, is way longer than the CCFL, fortunately, they are organized as groups of three LEDs in series and all groups in parallel, the seller recommends to adjust the length by snipping multiples of three, use a marker and starting from one end mark on the backside the interval in between every three LEDs, be attentive, as it is easy to lose count and destroy a group. BTW, the resulting short piece it is still useful, as it has soldering contacts at the other end as well. Personally I've put the bar over the display PCB and make the cut in such a way that the bar was covering almost all the white reflective paint on the PCB, the LED spacing was ideal.- The problem of feeding the LED bar was solved in a simple way, the voltage is around 8.2V, the CCFL block is feed form the 12V rail, after considering different analogue driver ICs and discrete solutions I've decided to get a closer look at the original schematic, and cool, one can use it as it is with minimal. Just unsolder the R8516 (1.8K) from the side where it connects with the L8501 and use the hole to connect the + of your LED bar, the - goes in the former ground pad of the CCFL. Voila, all done, the bright current is around 200mA and the dim current is around 80mA, I have test it for hours and everything is stable, you even benefit from the fuse and the thermal cut-off and the L/C filter.I have fixed the bar on the PCB with some 3M 4mm dual sided tape, normally used for phone screens, but any adhesive will do it.Afterwards , put back the silicone grommets, they will serve as light blockers and seals for the light cavity. Of course put all the screws back and gently bend the metal tabs to hold the LCD in place. Make sure that all connectors are firmly seat in place and when done testing, zip tie back the cable harness to the PCB,The device looks like this with new backlight, the dim setting should be perfect for a normal room.Materials used:LED bars:https://www.ebay.de/itm/1Set-All-new-490mm-LED-backlight-strip-kit-update-CCFL-LCD-screen-to-LED-moni-G4/174271057091(this seller ships in a plastic tube, not the standard envelope, I have the remainining of the LED bars available for good home and price, but I foolishly threw away the tube, so pick-up only if some one needs it.)Dual sided tape:https://www.ebay.de/itm/MMOBIEL-4mm-Doppelseitiges-Klebeband-Tape-50-Meter-fur-Reparaturen-Handy-Tablet/173605014341Questions if any, gladly answered.Cheers,DC1MC

author: james_s
author_rank: Super Contributor
timestamp: October 05, 2020, 04:00:15 am
content: That looks really good. I remember when those CCFL backlights first started to appear, and I always thought they were so cool, prior to that it was common for LCD displays like that to be lit by small incandescent lamps. Today LEDs are a much better choice I think.

author: DC1MC
author_rank: Super Contributor
timestamp: October 05, 2020, 10:37:37 am
content: Quote from: james_s on October 05, 2020, 04:00:15 amIt does indeed look good, and because is driven by a pure linear load it doesn't produce any noise, if I would have searched more it is possible that I could have found a replacement CCFL, but I wanted to get rid of the noisy high voltage source. because of the nice difuser filter is not even a change of color, my camera struggles to actually capture the nice orange glow.Cheers,DC1MC