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Thread: ICOM IC746 now dead

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  1. #1
    Join Date
    Sep 2009
    Location
    Bocas del Toro Panama
    Posts
    7

    Default ICOM IC746 now dead

    Was working well went through a period of iffy / slow startup now wont startup at all. External and Internal fuses ok. I was trying to troubleshoot and I have he service manual. After the fuse power branches and one branch goes off to IC1 which is the 5 volt regulator , presumably it supplies the startup/logic circuit which in turn energises relay 1 to turn the radio on. I cannot find IC1. !!!!!
    The parts list says it is on the PA Board but I have searched and cannot find it. IC2 and IC3 are there on the board but no IC1 .
    Any help would be much appreciated.
    73 Tony vp9ne

  2. #2

    Default

    Quote Originally Posted by VP9NE View Post
    Was working well went through a period of iffy / slow startup now wont startup at all. External and Internal fuses ok. I was trying to troubleshoot and I have he service manual. After the fuse power branches and one branch goes off to IC1 which is the 5 volt regulator , presumably it supplies the startup/logic circuit which in turn energises relay 1 to turn the radio on. I cannot find IC1. !!!!!
    The parts list says it is on the PA Board but I have searched and cannot find it. IC2 and IC3 are there on the board but no IC1 .
    Any help would be much appreciated.
    73 Tony vp9ne
    What's the part designation for IC1? Many 5 Volt regulators are in a TO-220 transistor type case, and will probably be found attached to a heatsink or some side of an enclosure that serves as a heatsink. Check around the edges of the PC board for a likely position.

  3. #3
    Join Date
    Sep 2009
    Location
    Bocas del Toro Panama
    Posts
    7

    Default

    Yes it is a TA7805S in a TO-220 package so it should be attached to some heat sink somewhere. I will have to do further dismantling to find it UGH!!!
    Thanks
    Tony

  4. #4

    Default

    Quote Originally Posted by VP9NE View Post
    Yes it is a TA7805S in a TO-220 package so it should be attached to some heat sink somewhere. I will have to do further dismantling to find it UGH!!!
    Thanks
    Tony
    Have you first ascertained that the regulator is bad?

    This can be done by finding any conveniently located point where you can take a 5V measurement with your DMM. That could be the + terminal of an electrolytic tank cap located near any chip or circuit powered by the 5V (check schematic) or the power pin on an IC. Check for very nearly exactly 5VDC and suspect the regulator if that reading is below about 4.8V. Also set the DMM to AC and look for ripple. Should read pretty close to zero volts AC.


    73

  5. #5
    Join Date
    Sep 2009
    Location
    Bocas del Toro Panama
    Posts
    7

    Default

    good suggestion from ke3wd thanks. I still cannot find the IC1 but I found the ribbon cable that it supplies and 5 volts is there . I also discovered some 14 volt signals are missing so I must investigate further
    Tony

  6. #6

    Default

    Already described in detail by Pat:

    http://forums.qrz.com/showthread.php...ghlight=746PRO

  7. #7
    Join Date
    Sep 2009
    Location
    Bocas del Toro Panama
    Posts
    7

    Default

    thanks for the reference This is for a 756pro and mine is non prro. Also thesymtoms on mie are just totally dead will not power up.
    Thanks
    Tony

  8. #8
    Join Date
    Sep 2009
    Location
    Bocas del Toro Panama
    Posts
    7

    Default

    I have done some further testing and have separated the front assembly from the radio disconnecting the ribbon cable. All the push buttons like ant and tuner etc short their line to ground. The power switch seems to have failed and is constantly at ground . Would this have the effect of never allowing a startup ?
    Thanks
    Tony

  9. #9

    Default

    Quote Originally Posted by VP9NE View Post
    ...The power switch seems to have failed and is constantly at ground . Would this have the effect of never allowing a startup ?
    Thanks
    Tony
    Yes.

    To test, try to find a way to disconnect switch from the circuit (may have to remove it) and then check to see if both pads are still grounded. If so, keep troubleshooting as something active may be shorted. If not, try firing the rig up on the test bench by shorting the contacts momentarily as if a switch was present.

  10. #10

    Default

    Probably too little too late, but IC1 on the PA board is the little rectangular 3-pin device just to the right (when looking from front of rig) of the round IC2. It's the 5V regulator for the startup circuit and should be putting out 5V anytime the power connector on the back of the rig has 12V on it. How do I know this? Last night my 746 made a couple of bursts of static while sitting idle on 6m while I was on a different rig running my 2m amp. I think some rf got into the 746. I shut it off to stop the noise and she never came back on afterwards. Either I killed the cpu or killed the startup circuit for the cpu over on the main unit (2) board (behind the front panel). There appears to be a little toggle circuit on the main unit (2) consisting of IC1706, D1704 and Q1703 and I believe once the cpu lights up it sends a signal all the way back to the PA board to pull in RL1 which is the main power relay. Still investigating....

    dale kd7uo

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