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Cobra 148 GTL from 96.. see any mods?

Discussion in 'CB Radio Modifications' started by PA770, Nov 7, 2018.

  1. PA770

    PA770 W9WDX Member

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    Plugged in and it works.

    I got a few radio checks. They say I sound distant.. of course that varies..



    When I turn the RF gain up the noise goes up abit like it should but the RF meter really don't move at all (like the 29).. on the 29 I can go from zilch on the RF meter to about 3db on the meter.. this does nothing like that.

    So here's the pics of the guts..

    IMG_20181107_172141333.jpg
    IMG_20181107_172152729.jpg
    IMG_20181107_172158289.jpg IMG_20181107_172207374.jpg IMG_20181107_172213948.jpg

    I don't see anything jumping out at me. It does have the RCI 8719-A chip.
     
    #1 PA770, Nov 7, 2018
    Last edited: Nov 7, 2018

  2. PA770

    PA770 W9WDX Member

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    Also note Made In The Malaysia and board is EPT014811Z and has a VR7 pot and it appears that R131 is intact.
     
  3. loosecannon

    loosecannon Sr. Member

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    looks completely stock.
    nice find!
    definitely worth restoring.
    LC
     
  4. PA770

    PA770 W9WDX Member

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    The darn inner knob on the SWR Cal/RF gain is cracked inside where it slides on. Knob won't stay on.. the pot is straight and fine.. anyone got an inner knob from a scrap radio laying around??

    IMG_20181107_202517319.jpg IMG_20181107_202615469.jpg

    So this from what I've been reading is kinda an odd board maybe. Someone mentioned 148 GTL-F model having same board.. I dunno..
     
  5. PA770

    PA770 W9WDX Member

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    I checked the DK on this it appears to be about 3-4w.. which I would prefer to be lower like 2w for my TS350 and then swing up from that..

    Anyone know which pots specifically that if I decide to turn up would do it.?

    So many conflicts online.. mine has a VR7 pot which from what I can tell doesn't exist on some of these.

    I think I see the VR10 in rear right near the transformer.. but the numbers are covered abit.

    I sure as heck am not going to clip anything..

    I did email DTB radio and may just send off to him to tune and align..
     
  6. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Have him look around R121 too, found an older - what appears as a workaround, for the older 5-pin to 4 pin reworks.

    This was from at time when Grants and 148's used 5-pin - the Pin 3,4,5 and 1,2 were and had to be, kept separate due to the audio lines' own RF pickup and subsequent unstable squeal you'd get from certain microphones. Pin 2 used an RF choke, while Pin 4 - which was the MB3756 Control pin ground in TX, also used 3 and 5 which served as Speaker Return off of Pin 5. Pretty much any of the radios built in this style, like Grant and 148's even the Gemtronics too...used a choke of some kind to keep Pin 1 and 2 from audio squeal problems.

    By this area is', the Audio in for the Mic amp from the front panels' own Mic Gain control...
    Pin345from12.png

    So you are aware, and if the mic you use has problems working in this setup - start there...
     
  7. PA770

    PA770 W9WDX Member

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    Thanks. I saw that winding and thought odd.. didn't know what it was.

    I will have tech check. Just using factory coffin mic.. seems to work fine. I just need the RX to be straight as I feel I have more noise floor than the 29 from 95, and it's in the same setup with the same A99.
     
  8. sp5it

    sp5it Master of puppets

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    I would check driver bias, because PR is on it's limit.
    Mike
     
    #8 sp5it, Nov 8, 2018
    Last edited: Nov 8, 2018
    kopcicle likes this.
  9. kopcicle

    kopcicle Sr. Member

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    ^^^^^
     
  10. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    I get yelled at for this stuff, but here's what I would offer DTB - see if he can help you get the BIAS set right in a radio you just can't get Driver and Finals for anymore - at least not like the OEM ones they were designed for originally.

    BIAS Example...
    BIASSSBFinalDriver.jpg

    And what your radio and pretty much every other radio in CB world does, they use no divider in the DRIVER section just plain trimmer.

    BIASSSBFinalDriverLRG.jpg

    Well that's fine for many OEM stuff but when you get an operator in there and they're long winded or not paying attention - they can kill the radio on the very next keyup due to poor bias settings.

    This 1/2 rail idea gets you thru a bias mA setting problem lots of older radios have - and it can provide a level of protection from regulation problems or thermal overheats - it just simply PARES down the current available for use and although affects the BIAS trimmer range - you don't need a ton of current power reserve to provide the SSB signal a push over to be amplified and to make it Class AB.
     
  11. loosecannon

    loosecannon Sr. Member

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    Andy that is a great idea!

    I never really even thought about it before, but you are right, they never set these biasing schemes up with a voltage divider!

    and with the trimmers themselves getting poorer and poorer in quality, it makes even more sense to add that resistor.

    im going to start doing this as a matter of course!

    thanks!
    LC
     
  12. PA770

    PA770 W9WDX Member

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    Question is will the tech take the suggestion??
     
  13. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    They use discretion - and so should you.

    The mod is for survival - not necessarily a pass for the FCC Part 95 test. Many a typical CB radio can benefit from some form of better Bias power filtration - a simple low-pass filter and as needed an extra BIAS filter cap and buffer resistor can offer even more levels of protection which takes care of that FCC Part 95 Free Pass...

    Below is something I've been seeing lately - as in the past Decade, so since they took to the idea a longer time ago when I found it worked - maybe the radio you have can benefit from such an upgrade.

    See below...
    SS3900Galaxy88switcheroo.png

    A brief explanation...

    The Mods above are to take the Ferrite Bead that is on a simple wire, take that jumper out and put in a buffer resistor of a low-ohmic value with that SAME BEAD instead - to allow the BIAS to pass into the part, but as heat and demand for more power due to that thermal effect of the voltage drop is now increasing - the resistor "holds back" some power from arriving to the part to keep equilibrium - as it makes the Transistor operate more into Class C than Deeper into Class AB.

    This is also reciprocal - it keeps more of the RF out of the Bias circuit - this is important in MOSFET designs for it can (lack of impedance to the flow of RF) generate a feedback loop and make the part self destruct. Keep the Gate where it belongs - Bias it yes, but don't let RF back into the Bias.

    There is also the discussion of D90 (Which doesn't apply to your radio but it's included) and the Asymmetrical effects of power demands placed upon the Bias network as the operater talks louder and or longer - so it affects the duty cycle and the ability of the radio to maintain forward bias when the operator is driving this into clipping (saturation)
     

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