Icom 746 display still bright but loses all characters; anyone else have this?

Discussion in 'Radio Circuits, Repair & Performance' started by NH7RO, Nov 28, 2017.

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  1. NH7RO

    NH7RO Ham Member QRZ Page

    My 1998 Icom 746 suddenly lost all of the characters displayed on its screen. Several years ago I updated the display with the "dim or dead screen" transistor fix that worked fine until now.

    My display is still bright (100% brightness setting as recommended) but there is nothing to read---all the characters, frequency, etc. are gone.

    My display looks like a blank white box now.


    Anyone else ever have this exact same problem? I think I have some leftover transistors that I did the original mod with (had to buy minimum order of 6) so I will try to find them and try doing that fix again---but maybe that will not fix this at all----would like to find out more first...

    73,

    Jeff
     
  2. WE6C

    WE6C Ham Member QRZ Page

    Does this happen even when the brightness is turned down? Curious since I have a 746.
     
  3. NH7RO

    NH7RO Ham Member QRZ Page

    I don't know---since I can't see any characters on the display I have no way to go through the menu settings.

    I did find the leftover 2SB1202 transistors last night so I am going to try replacing the one that I installed about ten years ago as a dim display fix. No idea if this will take care of my new problem but I figure it's worth a try.

    If that doesn't work then I'll have to look into replacing the display (and there I wonder if a 746 Pro display is the same part or not?).

    I was just about to look up the dim display fix again and get to work on this when I got the alert from QRZ---thanks!---I'll post my results here later.

    73,

    Jeff
     
  4. WE6C

    WE6C Ham Member QRZ Page

    Yes very curious. A retired radio tech friend has done several of these display fixes. He said for me, never doing one, for me to count on 4 hours. It doesn't sound like this is your same problem however. Since I bought mine used I do not know if this upgrade/fix has been done.

    When I get around to doing mine I was thinking about using a better transistor, not a surface mount, and attaching it to that shield.

    My friend says one of these days he'll come over and "coach" me and I won't let that opportunity get away!

    Hopefully someone that's seen this chimes in.

    Bob WE6C
     
  5. NH7RO

    NH7RO Ham Member QRZ Page

    I ended up spending eight hours on replacing the single transistor (2SB1202) I had installed about ten years ago---it was a lot more tedious and confusing to disassemble, unplug all the various ribbon cables, take apart the layers of circuit boards nesting within the facia, undoing the nuts on the NR and audio/rf gain pots, remove the shield box and transistor, install the new transistor, route and reconnect the ribbon cables (after remembering where they went) and reassemble completely than I remembered.

    My eyesight is not as good as it was and my memory is not as sharp these days so these compounded my difficulties. Fortunately some of the ribbon cables had already been marked by Paul Hanson, W6XA when he performed a minor repair several years ago unrelated to the display issue.

    For anyone considering such a fix I strongly advise marking all ribbon cables as well as circuit boards (top/bottom) with a Sharpie to ease reassembly. Btw, all of the knobs (including the big VFO knob) pull off---but if your 746 is old and lived in a humid environment most of its life the knob parts affixed to the shafts will undoubtedly get rusty or corroded---making removal quite difficult. I finally resorted to covering part of the facia with electrical tape and slowly prying up the knobs with a pair of wire cutters! The blades were just right for fitting underneath the knobs on two sides so they worked well at levering them off.

    The good news is that I have my display back as good as ever---but when I first powered up my 19-year old rig I still had the blank white screen. I did notice a faint semblance of a readout so, just for kicks I decided to see if I could get into the menu and adjust the contrast and brightness levels---in case it might make a sliver of a difference.

    Bingo! As soon as I reduced the contrast via the big tuning knob my display with characters suddenly reappeared!

    So now I am wondering if my repair really made the crucial difference or if my recent powering up/down the rig by flipping only the power supply switch was the cause of the loss of characters to begin with. During my web searches I discovered someone remarking that bypassing the rig's on/off switch is not a good idea since it shocks the display transistor with excess current (makes sense!).

    I had tried to adjust the contrast and brightness before yesterday's repair session but was unable to do so as I could not read anything that might have been otherwise displayed.

    Bottom line: I'll never know,the exact cause but at least my old 746 is working as good as new once again.

    To anyone debating whether or not to replace the NTE2525 with a higher current and voltage capable transistor I can still recommend doing so but be fully aware that it is tedious and requires great care with a thin tipped low wattage soldering iron. There are reasonably good tutorials on the internet but some of the work requires a good bit of intuition, too.
     

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