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Cobra 148 GTL 7.8000 MHz issue

Discussion in 'General CB Services Discussion' started by Unit 75, Sep 7, 2019.

  1. Unit 75

    Unit 75 Sweet Home Alabama

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    I'm a little confused on 7.8000 MHz entering the mixer IC5. Especially as it goes thru the coil before it enters the mixer. Reference schematics do not look correct to me. Here is Rick Jackson's re-draw of the Cobra 148GTL. He has the 7.8000 MHz entering IC5 on pins 11 and 13 (where the 34.XXX MHz is entering) and has 34.XXX MHz entering IC5 at pins 7 and 8 where the 7.8000MHz is entering. Either way, I would like to know how that 7.8000 MHz is pushed thru the coil. Is it the AM TX +8v on one side of R191 that pushes the 7.8000 MHz thru the coil into pins 7 and 8? Can someone explain it to me? IMG_4774.JPG


     
    #1 Unit 75, Sep 7, 2019
    Last edited: Sep 7, 2019

  2. Handy Andy

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

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    The 7.8MHz signal is what is subtracted from the 34~35MHz signal to regenerate the 27MHz.

    In SSB mode, FT3 plays a role as the resonant Xtal the 7.8MHz uses.

    Here's a schematic from the old CB Tricks days..

    R191 also plays role in this too - to Wash off RF not wanted on the 7.8MHz line...

    COBRA_-4.JPG

    The coil - if you ever take it apart, is just so the 34~35MHz has something to use to remove or subtract from - so it doesn't need a lot - you don't want a lot in this amplifier section

    The above is what I once posted to a Newsgroup about possibly getting it as a Mural for a wall in our house...the XYL had different tastes than mine but nonetheless - the schematic above - I feel, is more accurate to the design layout of the 148 of the Philippines' era...
     
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  3. Low_Boy

    Low_Boy Well-Known Member

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    There are a few errors in Rick Jacksons schematic.
     
  4. blasphemy000

    blasphemy000 Active Member

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    Both of those schematics show the signals entering on the same pins, the author of the first one just drew them in opposite places on the triangle. 11 & 13 are on the back in the official schematic, the redraw has them on the side for some reason.
    R191 is there to provide a current limited 8V-TX source to D53 in order to forward bias it and turn it on so that the 7.8 MHz signal can pass through it. D53 is being used as a switch to only allow the signal into the TX mixer when the 8V-TX voltage is present. Since RX and TX both share the same crystal filter, you don't want L48 and the TX mixer sinking signal off of the filter in RX mode possibly decreasing receiver sensitivity. D53 blocks that from happening when in RX mode, as the IF signals are too small to turn on the diode by themselves.
    As for how L48 works, it's just a tunable transformer. The capacitor on the secondary side is inside the metal can with the windings. Screwing the core in or out changes the inductance of the windings and thus changes the resonant frequency due to the L-C resonance on the secondary side. Maximum signal is passed from primary to secondary when it's adjusted to resonate at the same frequency that is being passed through it, 7.8MHz in this case.
    I wasn't completely sure what you were asking for, so I tried to explain as thorough as possible.
    73s
     
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  5. Unit 75

    Unit 75 Sweet Home Alabama

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    Thank you guy's for the explanation. It sure clears a lot of the functionality up. The 7.8MHz enters pins 7 and 8 of the mixer chip and the 34+MHz signal enters pins 11 and 13. It doesn't matter where he has the transformer placement on the schematic. What matters is he has the wrong frequencies going to the wrong pins of IC5 in the schematic. (look at the actual input pin numbers of IC5) This is evident on all the radio's that I have measured You can check for your self. IC5 has 34+ MHz at pins 11 and 13 at all times. The 7.8 MHz should only be present at pins 7 and 8 (differential input) during AM transmit only, I have the 7.8MHz at the anode of D53 but I'm not getting it through the transformer can. I'm guessing that internal cap is defective and I will need to replace that peaking coil and try again. Thank you all.
    -75
     
  6. blasphemy000

    blasphemy000 Active Member

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    I don't have a 148 here at my house to look at, but it looks like the service manual schematic is wrong then also. I guess the 7.8 MHz does indeed enter at pins 7 & 8, with the ~34 MHz LO entering at pins 11 & 13. It's weird that both paper schematics that I have in my filing cabinet have this backwards just as in both schematics posted in this thread.

    The 7.8 MHz is present in SSB modes also, but ONLY when audio modulation is provided. Since SSB doesn't have a carrier, that signal is only there when modulating in SSB mode and it will be offset from 7.8MHz based on USB/LSB and the audio frequency. It may also be quite smaller in amplitude than in AM. In AM mode, it is just a constant amplitude 7.8 MHz carrier, since the modulation is provided at the final amplifier stage.

    Now, you say that you have the 7.8 MHz signal on D53's anode when in AM-TX. Is the signal also on the cathode of D53? This is where D53 connects to the metal can transformer. If the signal isn't present there then you need to check if you have a positive DC voltage of around 0.6v at the anode (junction of D53 & R191) when in TX. If the DC voltage is there, but no signal on the cathode, then the diode may be bad. If you do have the 7.8MHz signal on both sides of the diode, then the metal can transformer may be bad as you suspect.
     
  7. Handy Andy

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

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    WILD!

    GrantXL.jpg

    Grant XL's are affected too!


    Note R265 and R190 - R190 is the 100 ohm between Pins 10 and 12 - but R265 is the one that acts as a divider - it is connected to ground.

    So if you have a signal "fail" check for those divider resistor networks - the above is from a Grant XL - but may help you in sorting out that S042P mess...
     
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  8. Handy Andy

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

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    To avoid confusion...

    COBRA 148GTL (Philippines' Origin)

    Cobra 148GTLa.jpg


    Again, note the use of R265 along with R190 - used a signal attenuator - divider.

    ALSO - the LOCATION of R265 as the other arm of that divider...

    R190 sources to Pin 10 - divides at Pin 12...

    But, on the GRANT XL and later Cobra boards...

    S042Pvs6130.jpg

    R265 is the ground arm of the DIVIDER AT PIN10 (Divides at pin 10) while R190 then feeds PIN12.
    (see post just above this one)

    Also - R265 is also a 1K resistor on these boards - see Grant XL above....

    Makes a difference as signal level...
     
    #8 Handy Andy, Sep 8, 2019
    Last edited: Sep 8, 2019
    Shadetree Mechanic and Unit 75 like this.
  9. Unit 75

    Unit 75 Sweet Home Alabama

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    Very nice information, fellas! Blasphemy000, I do have that 7.8MHz on both sides of D53...just not at pins 7 and 8 of IC5. I'm going to replace that can and re-tune it and go from there. Maybe I will have a carrier :)
     
  10. blasphemy000

    blasphemy000 Active Member

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    Andy, your explanation of those 2 resistors and the image of the IC internals is 100% completely wrong. Please read the datasheets of these devices.

    https://www.cbtricks.com/radios/galaxy/datasheets/ic/pdf/s042p.pdf

    The S042P and the TDA6130 (in the DIP package, SMD has a different pin out) are both Gilbert cell double balanced mixers and are functionally identical in this application.
    Pin 5 is VCC and is there to provide the biasing for the transistors inside. These are pretty versatile devices and the left and right halves of the bottom long-tailed-pair aren't connected. For the mixer to function properly, there needs to be a connection between pins 10 & 12, this is what R190 is for. R265 can be connected to either pin 10 or 12 and serves the same purpose in either case, to INCREASE the conversion gain of the mixer. R265 to ground increases the tail current through the long-tailed-pair and increases the gain. Pins 10 & 12 are the emitters of the transistors, neither is an input nor an output and those resistors aren't dividers nor are they attenuating any signal.

    Good luck with your radio 75. I hope you get it working again.
     
  11. Unit 75

    Unit 75 Sweet Home Alabama

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    Thanks, I should have that new can in tomorrow morning and let you know.
     
  12. Unit 75

    Unit 75 Sweet Home Alabama

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    installed another peaking coil and had no change. When I looked, I had lost the 7.8000 MHz on both sides of D53.
    Here is what I have. The 8V TX from IC4 (MB3756) I have coming in on R212 (1.5k).
    On the other side of R212 (at R191 junction) I have .392V RX (AM) and .680V TX (AM)...which is good because I need at least 0.6V there on TX to bias that D53 diode to allow the 7.8000MHz in thru that peaking coil and into pins 7 and 8 of IC5 (mixer).
    Yesterday, I had it coming out of the crystal filter. I could measure it on the C52 side of R61. This morning..... I have no 7.8000 MHz coming out of the crystal filter!!! I'm going to unsolder it from the board and hook it in my test unit. I have alligator clips soldered in my 148 GTL test unit that I can just clip up to the filter for a quick functionality test. I'm off thinking what else is going to go wrong....lol
     
  13. blasphemy000

    blasphemy000 Active Member

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    I don't see an R212 on my schematic. It's an old one, but it shows R191 directly between pin 8 of IC4 and the anode of D53. I'm not familiar with all of the differences in the different 148s. If you look at the schematic though, the 7.8MHz signal comes from pin 7 of the balanced modulator (just a carrier in AM) IC3. From there it goes up through the crystal filter FT3, then back down to D53 and into the metal can. Start at pin 7 of IC3 and probe along the path until you find where the signal disappears. If you don't have any signal at pin 7, try removing your newly installed can and check again, just in case something got shorted during your install.
     
  14. Handy Andy

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

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    And you tell me I have it all wrong...

    That seems to be the norm here...

    I'll let you two stumble along on this one...

    You didn't notice the reference so I'll post it here to help explain the problem you say I got wrong.

    The two 6130 and S042 are different ok? We know that...so to use the one part on the other you just need to pay attention of the resistor layout if issues do come up.

    Enjoy...
     
  15. blasphemy000

    blasphemy000 Active Member

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    Of course they are different chips with different specs. They are still both Gilbert cell double balanced mixers and when it comes to the DIP14 package, they have the same pin out as far as I can tell, since I can only find the SMD data for the 6130. Your picture you posted of the bottom of the 148 circuit board is mirrored left to right so it's a bit confusing to look at, at first, but they're using the two ICs in exactly the same fashion. Either way pins 10 & 12 must be tied together in order for the bottom long-tailed-pair to function in the mixer cell (these emitters aren't connected internally). Some radios tie them directly without a 100 ohm resistor (Galaxy 88 for instance then a 1k to ground to increase gain). Still, a resistor from either pin 10 or 12, to ground, increases current through the LTP and increases conversion gain. It's right in the datasheets of both components how that works. Those images of the IC internals you posted are not accurate.
    Now, since I don't physically have a 148 here to look at myself, the only thing that I have is the schematics. So any assumptions made could be wrong if there are errors or omissions in the diagrams, such as the one we found at the start with the pin numbers flip-flopped.
    I'm not trying to be ignorant here, and I'm far from knowing everything, but I'm pretty good with different varieties of Gilbert cells.

    73s
     

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