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{
    "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/GMCOJ6F6PAWCWTGPUGL2AAWDHMEJRIZU/",
    "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/",
    "message_id": "[email protected]",
    "message_id_hash": "GMCOJ6F6PAWCWTGPUGL2AAWDHMEJRIZU",
    "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/GMCOJ6F6PAWCWTGPUGL2AAWDHMEJRIZU/",
    "sender": {
        "address": "nx7u (a) cox.net",
        "mailman_id": null,
        "emails": null
    },
    "sender_name": "Scott Townley",
    "subject": "[amsat-bb]  IC-821H TX problem on 70cm--solved",
    "date": "2007-09-07T00:41:13Z",
    "parent": null,
    "children": [],
    "votes": {
        "likes": 0,
        "dislikes": 0,
        "status": "neutral"
    },
    "content": "I had written the list awhile back regarding an intermittent TX problem \nwith my IC-821H on 70cm.\n\nSomeone pointed me to the eHam reviews, where PE3HMP mentions thermal \nproblems with Q43 causing the 70cm TX to drop out.\n\nI opened the radio up, and found significant thermal browning on the sheet \nmetal and PCB in the left-side rear corner, where several voltage \nregulators are.  Q43 was confirmed on the schematic as controlling the 70cm \nTX, and is found on the underside of the MAIN board, sure enough in the \narea of thermal browning.  Quick bench measurements confirmed that the uP \ntriggers for TX were all present up until Q43...so it was definitely the \nculprit.\n\nQ43 is an internally-biased PNP switching transistor in an SOT-23 \nsurface-mount package (Toshiba RN2425), rated at 200mW and will switch \n800mA.  Of course it's unobtainium nowadays.   Which you could argue is a \ngood thing, since it failed to begin with...\n\nTo replace it, I searched out a P-channel enhancement-mode MOSFET.  These \ndevices can be drop-in replacements for switching PNP transistors (same \npolarity relationships between DGS and CBE).  The device I chose was a \nZetex ZXMP6A13F, rated at 625mW and will switch 1100mA.  Also the switching \nperformance was very good, with Rds(on)=0.4ohms.  I was happy to find an \nup-rated device from the RN2425 too.  They are available for USD0.63 in \nsingle quantities from Digikey.\n\nA quick solder job (which finds my SMT skills deteriorating with age) and \nthe '821H was back on the air with reliable 70cm TX.\n\nJust a further note, that today's MOSFETs are really incredible switching \ndevices, worthy of consideration in many applications.\n\n\nScott Townley NX7U\nGilbert, AZ  DM43di\nmailto:[email protected]\nhttp://members.cox.net/nx7u \n\n\n",
    "attachments": []
}