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Cobra 148GTL This issue has me stumped

Discussion in 'General CB Services Discussion' started by Tolteca, Apr 27, 2018.

  1. Tolteca

    Tolteca New Member

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    Hey guys,

    I have an interesting one for you. I've never gotten a radio with this fault before and frankly it's driving me nuts. I have a Cobra 148 that after aligning everything, side band is always still 1.5khz off frequency. When I put it on LSB I have to turn the clarifier about 15% low and when I have it on USB I have to turn it about 15% high. I measured the 34Mhz oscillator to see how much I was compensating and it is indeeda bout 1.5khz.

    It's almost as if the shift in the 7.8Mhz oscillator isn't getting out. I can see the 7.8Mhz oscillator changing when I switch to side band and I can also hear the side band change when I tweak the 7.8Mhz while receiving a signal, so it's not like the oscillator isn't working or it's signal isn't going anywhere. It definately has an effect on the radio and also properly shifts. Regardless the amount I'm off frequency on side band corresponds exactly with the amount of shift that this oscillator is supposed to provide when it changes from 7.8Mhz to 7.985 to 7.8010.

    This radio had a high frequency mod where PIN 10 was being sent about 4V through a 3.3k resistor sourced from L21, this mod is unfamiliar to me and it seemed to cause a lot of drift so I undid it. Instead I replaced the mixer crystal with an 11.325 and did the usual logic switch on pin 10 and 8. This fixed the stability but not the bizarre issue with the shift.

    Sideband reception also seems to be low as I was not able to properly calibrate the side band receive meter.



    Any ideas? Can't seem to figure this one out.
     

  2. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Try checking voltages at the mode switch. In AM mode, you should get roughly 8V at the mode switch, but 0V on the LSB and USB legs. Same thing with the other modes. If you have 8V on any more modes than the one that is selected at the switch, either the mode switch has gone fault, or there is a switching diode in the circuit that has gone short. Let us know how you get on.

    ~Cheers~
     
  3. Handy Andy

    Handy Andy Active Member

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    Greetings!

    And check the clarifier/Voice Lock adjust wire heading to the board - were you converting back to stock?

    When you switch between modes, the wires' voltage will drift a little but settle down - so as you re-check the 1.5kHz difference - see how much different the Voice Lock output wire is - as in how many tenths of a volt do you have shift that knob to get it to the right "center slot" in meet the other two modes?

    If they are the same level of shift that's one thing - if you require 3X the shift between the three modes may be something deeper. Lets move on for now...

    It' might be that AM mode may have the fault in it due to tracking in AM is a lot broader than in SSB - at least you don't have the sharpness...

    How's the "toggle" function for that AN612? If a prior owner was an AM only'er - and didn't mess with the other modes, the AN612 itself may be ok, just the shift cap may be goofy...C98 and C207 - if they go bad or got shorted out during a conversion - or the "grey cable" run got re-tied elsewhere it may have affected the tuning - these little details can have an impact on the 1.5kHz shift - it tells me the radio wants to go to that gear - it's clutch is keeping it from fully engaging...Even Carrier Balance can be affected...VR4

    Wonder if their expansion mod made them have to change a Varactor - there are two one D49 by C212 and C213 and the other one D35 by the 11MHz Xtal you just replaced...

    Lots of little things...

    Regards!
    :+> Andy <+:
     
    711 likes this.
  4. 711

    711 Active Member

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    Nice seeing you here Andy from CBT
     
  5. Handy Andy

    Handy Andy Active Member

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    Greetings!

    Going back over the PIN 10 part of the post, - not sure if ANY programming was possible on that 7th line. Usually the MB8719 is a lot like the 8734 (older) ones that use 6-pin and it may explain the drift.

    At least it would have no VALID programming, so it's making the PLL go to a frequency or generate an error where it left the loop open and in light of that - look around pin 6 and see if any work was done to it.

    Thinking this; it (Pin 10) forced the Chip to go into this thinking is was a valid frequency and if some way they thought fooling the PLL by forcing PIN 6 and Pin 10 would make it track or at least they'd have an undocumented way of going to a frequency - just couldn't lock it there.

    Now the AN612 - if the 7.8MHz shifts, but what is the PLL doing in these modes? That was the "voltage reference" - because of the "loading" - if a diode blew shorted - it would change the output of the clarifier - that diode would "load" the clarifier by adding or taking power from that line - a lot of people would say that don't make sense.

    They would be correct,
    It's not what I meant...

    I'm asking is "How much of the clarifier to you have to "turn" per your 15% - to offset the 1.5kHz skid?" I'm looking at this by how much and which direction you need to turn the clarifier to obtain the "center slot" - IF the SSB modes need you to turn LEFT on Upper and turn RIGHT on LOWER means the power direction is not an issue as far as SELECTOR goes...it's a SHIFT issue and either the PLL's not allowed, forced - or not even enabled....

    Now, well if UPPER SIDE of D32, D33 and D34 mess was goofy, that's possibly your answer - one of those lines is not isolated CAPACITIVELY from the your 11MHz Xtal - meaning it's always in circuit (L59) usually.

    USB has ONLY a diode on it's line and if it's blown open USB doesn't do anything BUT IT HAS AN INTERACTION WITH THE OTHER TWO COILS. So, it would appear you would seem to need a lot of tuning to get the other two modes to "Center slot" - and depending on which way the power from the clarifier went - up or down; where you had measured 3 different voltages to OBTAIN center, e.g. - two go up and one stays down, or two down one up kinda thng meant the switching side of the clarifier the 2nd pole or third pole usually - was loaded down and the 3756 is trying to keep up. (Shorted diode in mode)

    I've had messes where someone threw 12 volts into the AM mode and AM - ALL of the AM section of the radio was powered by that 12 volts - a lost cause...

    - I hope it's only something as simple as a misplaced wire and not the kind of digging I had to do like above.

    Regards!
    :+> Andy <+:
     
  6. nomadradio

    nomadradio Analog Retentive

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    Um, okay.

    Occam's razor is looming large here. Have the adjustment slugs in L22, L23 and L59 been set on frequency since the crystal was disturbed?

    If they have not been set with the clarifier at 12 o'clock to put each mode on frequency, that would explain the symptom.

    Any time the 11.325 crystal gets changed this will disturb the settings of those three trimmer coils.

    Your "aligned everything" phrase leaves me wondering how you went about setting those three adjustments.

    73
     
    loosecannon likes this.
  7. Tolteca

    Tolteca New Member

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    Hey guys, sorry my internet has been a complete piece of crap lately. Let me go over your posts and try out your suggestions and I will report back. I tried checking the selector switch and the problem isnt there. All voltages appear when and where they are supposed to.

    Nomad: When I mean everything was aligned I mean the radio was put through the entire alignment process incuding 34mhz and 7.8mhz adjustments. Same procedure I always use except I was surprised to find sideband reception was low and out of frequency.

    The clarifier is modded however it does not appear to be the issue since the 8v is sourced from the filter cap next to the PLL and the whiper voltage does not change when changing mode.
     
  8. nomadradio

    nomadradio Analog Retentive

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    Long story short, we set L22,L23 and L59 to put the radio on the correct channel frequency with the clarifier centered. Now, for some radios with a stretched clarifier "center" may be 3 o'clock, not 12 o'clock.

    Whether I use a receiver signal, or watch the radio's output frequency with a counter, I don't like to connect a probe inside a radio to measure a frequency.

    Just the same, hooking the probe of a frequency counter to a circuit can throw it off frequency when you unhook the probe. Not all circuits, just some.

    We quit using the alignment method in the Sams' book when I got our first decent service monitor decades ago. Makes getting the receiver on frequency no big deal. So long as the clarifier is working properly, transmit will now be dead on the money.

    We also use a 1 kHz tone box for setting SSB tansmit, if the clarifier is not modified. That provides a carrier from the radio that a counter can display in a stable way. Also provides a quick check that I didn't get the receiver frequency wrong. Using an AM signal from the signal generator modulated with a 1 kHz tone can put you exactly 1 kHz off frequency. There are two receiver frequencies that get you this tone from the receiver's speaker. Choosing the right one is best. Only works if you can trust the frequency that comes out of the signal generator.

    Did you use a frequency counter with a probe tapped into the radio's oscillator circuits?

    That's my best guess, anyway.

    73
     
  9. Handy Andy

    Handy Andy Active Member

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    Greetings!

    This is an interesting one...

    TR31 and TR32 - that little section...hmmm...

    D36 - LSB, D37 - USB, D38 - TR31 (TX AM MODE), D39 - USB (CT2), D40 - LSB and D41 - AM Mode (L31)

    There are three diodes that "route" or steer power thru TR32 and offset 7.8MHz. So are we looking at it the wrong way? As in - WE need to KILL AM?

    Well, in a way...read this thru and see if it applies...(SIGH)

    TR21 - RE-Routes audio to bypass AN612 in AM mode - unless the transistor has failed and is not even working (thinking shorted) and letting Audio thru in both AM and SSB modes into the AN612. A quiky check on the Base lead and C99s' closest to TR21 on that 2.2uF Tantalum (at least in mine...) should tell you if the Base is even getting power from AM mode (R113 or R112 one side)

    And if mods have been done - does R113 (a transposition if someone did this) still send TR21 anything? You'd see that in TR21 check in AM versus SSB modes...

    So with the above, the only other area I can think of that would PREVENT this shift is with TR32 (1675) is the TR31 (945) that re-steers power to the 7.8MHz OUTPUT thru D36, D37 and D38. They work a lot like an OR gate, If USB OR LSB OR TX is on - do this...

    Otherwise TR10 and TR11 are "on" all the time fighting your efforts...and upset the bridge that helps with the "deconversion" of the SSB stuff...it really screws up AM reception but, if you don't see these colors on your TV set...

    This is why I asked earlier about that 1.5kHz shift, for in a way, your modes are being selected properly (depending on which direction you needed to tune the clarifier) but if the thing was left in AM mode and SSB modes never turned on the bridge at TR10, TR11 and FET2 (yes way over there) you lose the ability to "trim off" the carrier the AN612 is throwing at you too.

    BEFORE THE FLAMES ERUPT!: ( See LAVA, VOLCANO, HAWAII ) (Tr10, Tr11 and FET2 have nothing to do with the TX part, just reception is off and the AM IF coils are on, fighting the conversion process)​

    So a quick check, again, is to locate L30 and L31 and CT2 - the 7.8MHz stuff and verify TR31 is "toggling" that L31 coil, on and off, as you select your modes - Else this section is what throws the carrier off and never truly shifts over the 2.5KHz it needs to be because it's all Coil trim is effectively summed together without CT2..

    You'd even notice this when you trimmed while in SSB mode - L30 or L31 - meaning they're both on at the same time. That means even the D39, D40 and D41 steering diodes may need to be checked - inclusively - in the list...

    The above comment is in no way meant to be harsh - so no SHTF stuff please!
    From my own experience, when TR31 has gone bad - it allows 8V AM into 7.8MHz all the time and skew any readings because the PLL does not care about the 7.8MHz - it's not set up to look for it. IF a Diode blew, that mode and AM would be the only two modes UN-able to lock in right. R113 SHOULD BE OFF (LOW) IN SSB MODE SENDING Audio through - into AN612 in USB or LSB TX - TR31 gets power from AM mode - but only ON when TXing! So ONLY TR31 works in AM TX modes, Think AND Gate (AM Mode, RX no, TX YES!) R113 sees a voltage D38 and TR31 supply but only only operate in AM mode and in TX - not RX - X1 is used for AM RX IF and it already "Shifts" 455kHz into the next section. The Image was already selected from PLL 1st IF back at TR15. Essentially L31 only is used for 1 mode AM TX - but TR31 or D38 is bad (short) - it's on all the time.
    I would verify TR31 in either USB or LSB modes, is not powering the TR32 or R113 (as a method to verify) up UNLESS TX Light is on. Else 7.8MHz from this section would be on even in RX mode in ANY MODE - throwing off readings.

    And depending on why the Which Way(?) you had to turn it, I strongly urge you to verify TR31 (945) and the L31 trick and see if AM mode "Stays on" all the time fighting you...

    I'm approaching this as a "undo" redo - approach, and see if someone ahead of you got something undone and lost their place in the radio and bailed out on it...

    Regards!
    :+> Andy <+:

    From the: "Not To Sound Dumb But I Have To Ask Files"
    The Unlock clarifier - they did break the TX connection right? D52, D78 stuff?
     
    #9 Handy Andy, May 11, 2018
    Last edited: May 11, 2018
    Robalo likes this.
  10. Robalo

    Robalo Active Member

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    i have cobra 2000 gtl(PC-497AB crystal 11.1125) open voice lock -18kc +6 kc on the bench right now it has channel mods from 26.175 to 28.045 every thing is right on but only on upper 40/40 27.605 to 28.045 no rx/tx on am/usb and frequency counter drift .lsb is work ok.so i goin to check tr31 and tr32 plus diodes thank you Andy
     
  11. Handy Andy

    Handy Andy Active Member

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    Good luck, I'm hoping it's not because the TX and RX side of AM are tied to the two Xtals - and one coil L31.

    Hint: Diode power routings to keep all the channels accessible, but not shorting out the modes - in one particular mode. Causing the AM TX coil L31 to stay active and fight your efforts to tune.

    :+> Andy <+:
     
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  12. Robalo

    Robalo Active Member

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    ok I pulled TR31 and tested hfe=209 uf=691mv TR32 hfe=97 uf=695mv good now remove and test D36,D37,D38, D39 ,D40 and D41 all test good uf=682mv to 700mv the only part that is not right is R158(10K) the schematic from Rich said 3k9 so what is right? i still dont have AM/USB. The only thing i haven't checked is the mode switch, I don't know the color codes for usb/lsb/am, I'll check the schematic tomorrow.
     
  13. Handy Andy

    Handy Andy Active Member

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    It's got 4 poles, with 3 of them used. One has 12 V constant.

    Be careful - the other two work off the MB3756.

    I never did hear back from you on which way you had to move shift rotate the clarifier (having to slide in any particular direction0 Because, if one of the SSB modes won't go off, that adds to this too - so if LSB seems to be the only one that works correctly, it may be the one mode affecting all three - so those components and diode isolation (power steering diodes) one may have failed - adding to your problem. You'd know it right away when you tune in USB mode, and tweak the LSB coils one at a time would also affect it at the same time. Depending on which coil affected that mode from the opposite side - can determine which diode or section is still active. So tune USB, then check LSB coils in USB mode - same for AM. Heck, even a metal tipped tool can help find the leaky part.

    The only thing I really wanted you to check was if L31 affects tuning of the RX - only much more "slowly" - meaning it has power applied and is affecting all the other IF efforts as a mixing problem.

    No isolation.

    TR33 ok? It works off the AM mode and powers the 7.1MHz AM IF - NPN (945) part directly. That would be an option to see if that IF is not getting knocked down for SSB.

    R158 is to narrow down the power level applied for the tuning cans off the Xtal, it's on the ground side. - keeping it at 10K can help with the tuning range - so if it's already 10K leave it alone until you got this mixing problem fixed first.

    There are several revisions of that board - so if you have NW or ST - they use the 3.9K to narrow the range to tune.

    Older used 10K and gave you better slide, but as a compromise - you also had drift.

    :+> Andy <+:
     
    #13 Handy Andy, May 23, 2018 at 1:00 PM
    Last edited: May 23, 2018 at 1:43 PM
  14. 357magnum

    357magnum Well-Known Member

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    I must say the Tech info on here as of late is astounding ! Although @ my age I'm still learning . But Wow ! Too bad you guy's don't get paid for this info . This forum just gets better an better JMO (y):)
     
  15. 357magnum

    357magnum Well-Known Member

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    And Handy Andy , I don't think I ever told you but Welcome to the Forum .(y)
     

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