Cobra 148 GTL 120-Channels Base CB Radio Logo

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Mike Autry Posted on Mar 24, 2007

142 GTL Good AM Carrier Weak Modulation????

Weak AM modulation at 50% on Palomar PW-5000 meter. SSB has low forward swing with low modulation?? I have checked many voltages in the past and all that I have checked seems normal. Using SAM's Photofact. Re-align transmitter as per Sams with 2-tone audio generator and still low modulation. Replaced Audio IC TA 7222P no change. Any Ideas???

  • Mike Autry Mar 24, 2007

    UPDATE:
    IC4 TA 7222P Voltage Check:
    PIN1 = 13.76v RX / TX = 13.66v
    PIN2 = 7.04v RX / TX = 7.00v SHOULD BE 2.95v !! ??
    PIN3 = 0v
    PIN4 = 0v RX / TX = 0v
    PIN5 = 1.162v RX / TX = 1.162v SHOULD BE 1.95v ??
    PIN6 = 1.158v RX / TX = .8v (Gradual drop in voltage during TX Mic Gain @ Min. like capacitor unloading) SHOULD BE 1.95v
    PIN7&8 = GND
    PIN9 = 7.27v RX / 7.18v TX
    PIN10 = 13.10v RX / 13.00v TX

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2 Answers

A

Anonymous

Have you checked TR 26 AMC amp, and TR 24 & 25 AMC amps also for SSB TR34 ALC amp. make sure all voltages are right if all checks out try lifting out of the board or cut the collector of TR24 and try it. oop's that slipped out.

squirty whitney

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  • Posted on Apr 30, 2007
squirty whitney
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You may have to scope the finals circuit on the modulation to see if all the "inline" caps are installed correctly... be sure your pc board is grounded in all areas as required by specs in your manual. Make sure there is no bleedover and you are sheilding your crystals from the modulation platform. al

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