Fixing Reverse Polarity Damage for IC2800H

Discussion in 'Radio Circuits, Repair & Performance' started by ABUHASHEM, Oct 9, 2019.

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  1. ABUHASHEM

    ABUHASHEM QRZ Member

    Hi all,

    This is my first post and I'm seeking help to repair my radio.

    I accidentally connect the power for my rarely used radio in reverse and damage the unit, I’m no expert in electronics and I don’t know how to diagnose using the circuit diagram but I can repair some easy to fix common power supplies and other electronics problems using multimeter and a soldering iron, but this damage is beyond my knowledge.

    When I opened the unit for the first time I noticed Zener Diode #D46 has blown out and a fried trace that I tried to patch as you can in the attached file, all other components looks visually OK, I replaced the Diode with the same part and fixed the trace and power the unit (with the right polarity this time) but the diode smoked and another part of the same trace fried again as you can see near part #L25 (R12J).

    Here is the service manual link: http://www.radiomanual.info/schemi/ICOM_VU/IC-2800H_serv.pdf

    I appreciate your help.


    Ahmed,
     

    Attached Files:

    Last edited: Oct 9, 2019
  2. G0GSR

    G0GSR Ham Member QRZ Page

    D45 is the diode that should have cleared the fault but may have gone short in the process.
    Remove D46 for now, that's for over voltage protection. Also check you replaced D46 with the correct polarity as it will do what you describe if reversed.


    Frank
     
    Last edited: Oct 9, 2019
  3. ABUHASHEM

    ABUHASHEM QRZ Member

    The new D46 that I replaced was with correct polarity.

    I removed D45 and test it, it reads OK with no short, I replace it though with a new one, and I patched the trace again and removed D46, then I connect the power, the 20A fuse blow now with nothing visible on the PCB, I found that the main power positive and negative are shorted but I'm not sure if they ware shorted before, which component could cause such short in this case other than D45?

    Thank you for your help.

    Ahmed,
     
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    No, the positive supply input should not show a DC short to the negative, one or more components are shorted out and dragging that line down.

    The trouble is that unregulated input supply voltage goes a lot of different places in the rig and any of the components along that line and possibly more than one component along that line could be the culprit. Take a good look at the schematic in the service manual you linked, the unregulated DC input bus is called HV and runs to many different circuits including a couple of voltage regulators, some various functional blocks and of course to the power amplifier.

    Best bet is to take a divide and conquer approach which is simplified a bit if you can actually measure a DC short or very close to it with an ohm meter. Follow that HV bus line on the schematic and where it goes through a connector, such as the jumper cable from J3 on the Main unit to J2 on the control unit, disconnect the cable or connection and see if your ohm meter symptoms change. That should help you narrow down what subassemblies or individual component(s) are shorting out the HV bus. Once you've isolated one or more shorts then look at just that circuitry and especially look for any active devices (e.g. transistors, diodes, ICs including voltage regulators, etc.) that could be shorted out. You'll likely have to desolder some of those suspect devices to see if it clears your short condition.

    For instance, to start I'd validate the short circuit (or very low resistance) condition still exists at the DC power input connector. Then disconnect the J3-J2 jumper mentioned above that connects the Main board to the Control board and see if the short still exists on the power input connector. If it has gone away then the short is on the Control board, if not then the short circuit is likely on the main board or in the power amplifier or somewhere else the HV line supplies directly. While that connector is off, also test Pin 6 of the Control board connector to ground as it's possible you have shorts on both the Main board or the Control board.

    It can be a tedious process, but it's not rocket science. Just follow the HV bus everywhere it goes in the schematic, locate those places on the various boards and keep testing with the ohm meter to try to localize the short remembering that a reverse voltage condition could damage more than one component.
     
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Maybe the 8 Volt Regulator or RF output transistors.

    A ohm meter with a very low range would be handy for testing.

    May be a good idea to use a 5 amp fuse for testing, Until you find the problem. Avoid smoking PCB traces.

    Good Luck.
     
    Last edited: Oct 9, 2019
  6. KD3WB

    KD3WB Ham Member QRZ Page

    You may have created a short circuit trying to repair the board. Undo the repairs, and if the short disappears, use insulated wires to bypass the burned traces.

    Ben
     
  7. VU2NAN

    VU2NAN Ham Member QRZ Page

    Hi Ahmed,

    Maybe a damaged capacitor between +ve and -ve.

    73,

    Nandu.
     
  8. ABUHASHEM

    ABUHASHEM QRZ Member

    Thank you all for the support, I'll try K7TRF suggestion by following HV bus after trying to locate the nearest point to the shorted component with ohm meter.

    Ahmed,
     

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