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General Stonewall Jackson No modulation on am,ssb,fm & pa

Discussion in 'General CB Services Discussion' started by 2la547, Jun 12, 2019 at 8:48 AM.

  1. 2la547

    2la547 New Member

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    I have a General Stonewall Jackson base (Board# EPT360014B) that the modulation would cut in and out every so often . It now has no modulation at all. It receives and keys perfect . The talkback does not work but the roger beep can be heard through talkback when unkeying. I've tried multiple mics , tested the mic jack , wires from the mic jack to mic gain pot. back to the connection on the board and searched the board for cold solder joints. I've also tested and replaced TR51 , IC7 , IC8 with no luck . Any help diagnosing this radio would be greatly appreciated


     
    #1 2la547, Jun 12, 2019 at 8:48 AM
    Last edited: Jun 12, 2019 at 9:27 AM

  2. nomadradio

    nomadradio Analog Retentive

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    I really don't try to troubleshoot this sort of problem without a 'scope showing what's coming out of the transmitter while it's keyed. And having a 'scope with a probe to see what comes out of pin 1 on IC4 is an unfair advantage. The DC voltage there should be around 4 Volts, more or less.

    But the one test that won't require even that much is your finger laying alongside the shaft of a small screwdriver.

    Key the mike in AM mode and touch the tip of the screwdriver to pin 3 of IC4. Your finger touching the shaft provides a pretty good hum and noise source into the chip.

    This should get you a raucous squeal out of the transmitter's modulation.

    If you don't, unsolder the center pin of TR32 so that the lead doesn't touch the rim of the hole in the foil pad, and try again.

    If THIS gets you the feedback noises from the "manual digital signal injector", there is a fault in the modulation-limiting circuits.

    And if you don't get any noise at all from the pin 3 test, the problem is downstream from that point.

    Simply changing parts before the fault's location is established creates the risk of adding a new fault to the first one. If you're scrupulously careful and never create any accidental solder bridges between adjacent foil pads, the risk is smaller.

    We have seen a few radios where the owner followed internet advice to "just change" some likely part. When that didn't fix it, the radio comes to us. Handy thing we're good at identifying multiple faults in a radio.

    If it starts out with only one fault, the troubleshooting is simplified if no other faults are added to it. Have a very close look at the foil traces leading from where the center lug of the mike pin is wired to the front edge of the main circuit board. We have seen solder faults there more than once.

    73
     
    Shadetree Mechanic likes this.
  3. nomadradio

    nomadradio Analog Retentive

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    Oops. just noticed you said the roger beep can be heard.

    That pretty well puts the problem R187 and the mike jack. That would include the wires from the mike jack to the mike gain control, and from the mike gain to pin number 46 at the front edge of the main pc board.

    Can't remember if the Stonewall has an echo board. If so, that board can also stop your mike audio dead if a connector is loose, or the power to the echo board is interrupted.

    73
     
    Shadetree Mechanic likes this.
  4. 2la547

    2la547 New Member

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    Nomadradio thanks for your help . I've traced the wires from R187 to the echo board (p#ept0ssb50f) . I'm starting to believe I have a bad echo board .
     
    #4 2la547, Jun 12, 2019 at 11:59 PM
    Last edited: Jun 13, 2019 at 12:07 AM
  5. nomadradio

    nomadradio Analog Retentive

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    We got in the habit of first unplugging the 3-pin input and 3-pin output plugs from the echo board. We then take three of the 1/10-inch spaced pins from a "snap-off" strip and plug the two 3-pin sockets into each end of the pins to bypass the echo board altogether.

    If you don't have any of those on hand, this is not so easy. But it settles the question of whether the echo board is killing your mike audio.

    73
     
    2la547 likes this.

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