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Help needed-148GTL driver bias issues

Discussion in 'General CB Services Discussion' started by Dmans, May 20, 2020.

  1. Dmans

    Dmans Sr. Member

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    So I have accumulated 3 or 8 148GTL's in the last couple of years. One of which will be repaired due to the cosmetic condition of the faceplate, case, circuit board etc. using components from the others as needed.



    So the "restoration/repair" project is nearing it's end for the lucky unit (it is a "Made in the Malaysia version with a EPTO14811Z board). It is the same as the Texas Ranger 296. The unit has been fully recapped all modifications removed and the receiver aligned.
    During alignment of the transmitter I found the output is very low. Am keys about 1 watt with negligible forward swing. SSB output is about 5 watts. The driver will bias at 30mA and final at 50mA (both transistors replaced with new 2166 and 2312 Mitsubishi's)
    When checking the transistor voltages per the service manual, I find the driver transistor base voltage when keyed in AM at -1.2Vdc (yes, that is negative 1.2Vdc!). In SSB the voltage is per the service manual. I can increase the bias voltage to read .47Vdc on the base of the driver but then the bias on the driver is about 400Ma. (Output power at this bias level is about as expected-swings to 16-18 in AM and peaks about 20 in SSB)
    I did replace an open MV1Y diode on the driver transistor with no change. I tried another 2166 in the driver position with no change. All voltage readings on the final transistor are as per the service manual. Voltage readings on the driver transistor in SSB mode are also per the service manual.

    Any ideas what I am overlooking?

    73
    David
     

  2. Handy Andy

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

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    Got a Spare pic of your Divers Foil side?

    I have a hunch...

    Diode may not be grounded...or at least not grounded to Foil ground - it could be floating earth chassis - back panel.
     
  3. sp5it

    sp5it Master of puppets

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    Some transistors need even 100mA to work.
    Mike
     
  4. Dmans

    Dmans Sr. Member

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    HandyAndy,
    Here is a picture of the foil side of the board. The pointer is on the MV1Y diode cathode connection to the board.

    Mike,
    I wouldn’t have a real issue with 100Ma bias on this transistor but at nearly 400Ma it gets hot pretty quick. Incidentally, this transistors gain tests at 84 on my Atlas DCA. One of the strongest in my handful.
    20762A97-BF94-40E0-997A-9BB2683F479D.jpeg

    73
    David
     
  5. Handy Andy

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

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    Got it - thanks...
    upload_2020-5-20_21-6-37.png

    Not to be a bother but where are you "plugging in" the diode?
    upload_2020-5-20_21-10-6.png

    Remember, the diode needs to be - forward biased.

    It's just "odd" to see the mA requirement for the Final in AM go so high so quickly.

    Older days, we set it in Grant and 148GTL's for no more than 180mA in AM - about 120mA for Driver in AM mode.
    They still heated up like banshees.

    When you said you got negative voltages, this was why I posted that "180 ohm" resistor mod.
    Cobra140-142TXsection.jpg
    For some ungodly reason, they took care of Finals by trimming voltage across the transistor from Voltage source to Ground . But they neglected the Driver - and I don't know why - losing the 2166 or the 1306 was easy enough - they never bothered to balance out or at least send the trickle current to ground when at idle.

    Heck even the 138 as well as the 146GTL are affected (offended) by this omission, Hr2510's onto the PC-122 and even Grant...

    Well, you'd ask why worry about idle current - for the very effect you're having now. NEGATIVE Base lead voltages will damage the Driver

    Simply put - for the bias itself wasn't the problem - the ability to BALANCE the Bias to keep the 2166 or the 1306 off or stay stable and drain off the 8VTX (floating) across the diode - that current - was the problem.

    Galaxy does this on the RECENT entries for MOSFET designs
    upload_2020-5-20_21-39-36.png ,
    Gotta love that they think a 100K resistor deserves a Capacitor designation...SIGH - their hearts in the right place...

    But Ranger/RCI - New home of Galaxy and Cobra...don't...
    upload_2020-5-20_21-33-2.png

    I'd put one a 180 ohm or even 220 ohm (note the 330 ohm R182) just to take and DROP some of the voltages so the Driver doesn't kick on, in high-gear forcing the Final into full on. Your 400 mA reference...

    If you have one 2166 driver in the Hand, it's worth far more than two in the trash...

    Ok once installed now what? Well, you should be able to easily trim the mA between 50 to 70mA on the meter and KNOW you're not pushing the Driver with that much, more like 1/2 less mA depending on the Value of your bleeder resistor - the higher the ohmic value - up to the value of the bias buffer resistor in there - you know you're dividing the mA draw by 1/2 between the two resistors - lower value makes the bleeder resistor take in more current - The Diode will take care of the rest. The 1306/2166 or whatever you use in there - will thank you...
     

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  6. Dmans

    Dmans Sr. Member

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    Andy,
    Thank you for the pointers on this unit. I installed a 180 ohm resistor in parallel with the MV1Y (actually NVS-01 on this unit) on the driver side and it dialed right in to 30Ma and base voltage of .64Vdc on SSB and .45Vdc on AM. With a new 2312 in the final position I am seeing 17 watts output with a two-tone signal injected in the mike jack that is very clean on the scope.
    Again, many thanks. This has been an on again off again project for about 18 months or so for me. I initially wanted to put this chassis into a very clean TRC-49/Navaho Pro-Niner but may leave it as is. The remaining carcasses of 148's that I have (with ugly faceplates and rough cases) may find a new home in the Navaho case/cabinet.

    73
    David
     
    tecnicoloco, Handy Andy and Robalo like this.
  7. Handy Andy

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

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    Yo're most Welcome - glad you can use this trick to keep the driver driving "safe"


    Only thing I wanted to mention
    You're not alone in this - I have several radios with broken bezels - rought shape that need a little TLC over to work on myself.

    So I wish you good luck with your endeavors!

    Check! Check!
     
  8. Robalo

    Robalo Well-Known Member

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    . This is my collection last time I checked, half work—- half doesn’t work, and I’m fixing them when I have nothing to do. Or what else can I do
     
    tecnicoloco and bobl9355 like this.
  9. Low_Boy

    Low_Boy Sr. Member

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    You can use a galaxy bezel on that radio. Just have to get a face plate sticker because the face plates come off the old bezel pretty hard(usually) Sometimes you get lucky. They have to be straightened. You can get them flat but they will need a sticker to hide the rub marks from straightening.
     
    Dmans likes this.

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