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Cobra 148GTL (A.K.A. Texas Star TR-296)

Discussion in 'General CB Services Discussion' started by Dmans, Sep 10, 2018.

  1. Dmans

    Dmans Well-Known Member

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    I recently acquired a "Made In The Malaysia" 148GTL. Upon inspecting it today, I found a 68 ohm resistor bridging the base of TR-37 to ground. I believe the original part was supposed to be a 4700 ohm (R226).
    Is this somebody's idea of "strapping" it for more power? Maybe a swing kit? Or is this one of those "Manufacturers Discretion per build" thing that happens during assembly and testing?

    Schematic is here.

    http://www.cbtricks.com/radios/rci/tr_296gk_dx/graphics/tr_296gk_dx_sm_sch.pdf
    TR-37 is at the lower left of the schematic near the final transistor TR-36.

    My inclination is to remove the 68 ohm resistor and install the 4700ohm in its place per the schematic as I am not able to get the final Bias set below 70Ma.



    This small "deviation" is not all of the problems with this unit. Cracked tuning slugs, worn out SO-239, bad RF Gain pot, bad CT2 trimmer, broken plastic mount screws for Audio IC and Balanced Modulator IC.....................

    All help is appreciated.

    73's
    David
     

  2. Handy Andy

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

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    Your thoughts on replacing it?

    I'd agree - remove and reinstall HIGHER value...why?

    TR496-Malasian148.jpg
    And you'd be correct...

    Cobra 148/Cobra 2000 thereabouts...
    Comparison148.jpg

    Good catch, does affect bias trim.

    Also make sure you do not have R181 SHORTED, if it's possible, remove R181 - that 10 ohm and REINSTALL in line (in series with) L40 - this method is how I verified that Unidens' "Diode Drive" mod works like it does and why I now install a low-value resistor in the Bias line to ISOLATE the bias and Transistor from each other so if the diode or bias failed, I don't have a ton of current flowing into the Base of the Final.

    :+> Andy <+:
     
  3. Robb

    Robb Yup

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    Umm, the 296 added that part to the schematic and used a 1kohm bias pot, whereas the 148/2000 schemo shows no resistor there and used a 5kohm pot.

    Think I would check the bias diode too.
     
  4. Handy Andy

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

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    The resistor swamps the Bias diode - and with no diode in there? YIKES!

    IT's why I recommend pulling R181 to see if it even brings down bias trim current. If it soars upwards - the Base was tuning on because of the Bias was not regulated (using the Diode as that level of reference).

    This may be a bigger problem. It deals with polarity of the Base lead going to ground.

    The diode needs to be in there - no resistor - why? IF ever the base lead RECTIFIES RF power - this can send a voltage into the bias circuit - the diode acts as the one-way valve keeping the rising voltage on the output side of Emitter from punching past the Base into the Collector. The diode pushes that power into ground, keeping - a resistor does too but with restrictions.

    Then once that's done, reconnect R181 with say a 5.6 ohm resistor so the bias can get to Final.

    That 496 may already be shot - the resistor on the base and the resistor on the feed to this part, is the only thing keeping ALL the voltage from dumping into the Base of the Final. Voltage divider sort of thing....

    Now, the above is presuming that your testing on SSB modes, not AM - for on AM - the mA drive changes to like 180mA or so - no greater.
     
  5. Dmans

    Dmans Well-Known Member

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    Andy,
    Isn’t R181 already in series with L40 paralled by C151 in the TR-296 schematic?

    73’s
    David
     
  6. Handy Andy

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

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    It's the way it arrives.

    Coupled with the preceding stage via that cap, but part of the circuit I'm talking about the resistor and cap combo C151 and R181 AND C200 and R209 - several mods can be done there - including "SHORTING OUT" R181 to obtain more power - doesn't hurt it per-se, but many radios with the 1969 / 1307 final - it blew the final in ways that are two fold.

    First, the preceding cap in the TR radio it's 390pF - in the 148's or 2000's is was a 470pF - power levels were stronger in the older days...

    When Final quit, I'd also check and also see C200 and R209 would also have been removed. Too much power from the previous stage got onto the Base with R181 shorted too - POOF...

    It was a local that played those games.

    I now solder a 390pF on the bottom of the board ACROSS the L40/R181 junction to ground - saves the final and no one seems to care about the power levels dropping too far. Another aspect I see as Parallel caps - C151 was 0.0047uF or 472 - so it does impact the power level that arrives from the Driver - and most of the time - harmonics and all, usually goes thru R181 because of C151 being a cap - there is also the load sharing that isn't equal in this section - people scoffed at the idea and I've been poked fun at over this - but Gosh Darn It - you don't have to do anything special - I'm just trying to protect your investment - don't take my word for it... in 148's and Grants even, it's a 1/6W or 1/8W resistor and you have a part ahead - upstream - that has replaced 2078's and gives us 4 watts. So it's why I was sending up the flag - make sure you don't send the current into the part that is not there - usually when I see it wont go below 70mA I see someone blobbed R181 a short across it. The 1969 is turning on a lot faster, longer and heating up ready to fail if not repaired - I see your point...trying to protect you - and why replacing the R181 with a 5.6 ohm 1/4W and replacing L40 with a 10 ohm resistor with the ferrite on the open lead puts a lot less stress on the part especially when they go back to the driver and yank out those other two parts. At least Bias won't climb...you've got that buffered and choked - using the idea above.

    And it's true that when someone popped the R181 blown open SSB doesn't work well, nor does AM seem to work right...so open condition is one thing - shorted out part is another...

    It was in a; name I won't mention here (Cu), catalog that always posted these silly mods for people to get more out of something that used something that now, we wish we still had a drawer full of...

    To prevent R181 failure and popping the Final - I usually install the 10ohm on the ferrite bead - let the R181 thingy stand, the kids getting it back are going to do it anyways...so I'm passing this along to help you for if they shorted out R181 - look for the short, and fix the problem with the ferrite and resistor combo where the ferrite feeds into the mess...they want the power curve - just protect you from losing a lot of finals to someone that would otherwise destroy the radio and blame you for their mess - been there - done that...

    Glad to see not all the radios had the shorted R181 thingy...
     
    Dmans and Shadetree Mechanic like this.
  7. Dmans

    Dmans Well-Known Member

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    Thanks for the explanation Andy.

    I can tell that the 1969 has been replaced previously. I will look longer and harder at R181 and the L40 Ferrite. I replaced about 30 caps in it yesterday and found the plastic screws holding the Audio IC and the Balanced Modulator IC broken. I cleaned and re-gooped the insulators and replaced the screws. Not sure if that was the cause for the 1969 replacement but it had to be done anyway.

    Strange thing this radio (aside from the UGLY ASS Blue meter LED-new stock of Brite Whites inbound) it still has the clarifier unmodded, no PLL mods for extra channels either. It did have a resistor between the PA jack and EXT speaker jack (Talk back I suppose) that I removed but still has talk back when an external speaker is used. Works fine with the stock speaker. More to do there.

    Not sure this is the chassis I want to install into the Regency Range Gain case but we'll see! (I'm kinda partial to the "Charlie Brown" {think Charlie Brown's Christmas Tree} radios so it may work out!

    Thanks again for the help!

    73's
    David
     
  8. Handy Andy

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

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    Had a guy at CB Tricks did up a rack for a Galaxy chassis his own way too.

    I wish you all the best on that project - it's certainly is a labor of love - keep us posted on your progress!
     

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