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RX tune of Cobra 2000

Discussion in 'General CB Services Discussion' started by Brian G, Feb 23, 2021.

  1. Brian G

    Brian G Member

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    I had posted this question a while ago but since the thread has been dormant for awhile, i wanted to post it again.
    My 2000 seems to have weak receive. I have tried to peak all the coils. I seem to have trouble peaking the 455 kHz coils (L3,13,15 I believe). I use the Sam’s facts method with a signal generator inputting a 455 kHz tone to Gate 1 of FET1 and an AC VOM across the speaker coil. The issue is the voltage reading warlbles where I can’t get it to stay steady so I can tune the coils. The voltage wanders up and down where as I can get it to “lock” on the 7.8 coils and can easily peak those.

    I also tried to just use a walkie talkie with a shorted antenna jack to reduce radio gain and just tune it to peak read. I even replaced many of the diodes around L4.

    But even doing all that it just seems there is very little “ground noise” and or just seems to quiet making the recoded weak. Of course keying the walkie talkie certainly gives you a full in reading and squeal but seems the weak stuff isn’t there the way it usually does. Maybe it’s me?

    so.....any ideas on the 455 issue ? Is it ok to use the walkie or no? Any where else to check for why that background seems weak? I have tried FET 1 and 2. Doesn’t seem to be problem either.


     

  2. nomadradio

    nomadradio Analog Retentive

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    Does it hear normally in sideband?

    If it's weak in both AM and sideband, that's different than if AM alone has a problem.

    It's hard to see a proper peak on many of the receiver adjustments unless the test signal is weak enough, below a 2 or 3 on the meter. A strong signal triggers the radio's automatic gain, which will "flatten" the peak when you turn the slug.

    73
     
  3. Brian G

    Brian G Member

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    it does appear at least to be “louder” on SSB vs AM. But it’s been so quiet in the radio lately it’s tough to really compare. It assuming that is the case, what would I want to check?
    I will attempt to retune at a lower meter reading. That’s tough because of the signal generator or walkie
    Thanks.
     
  4. kaos513

    kaos513 Sr. Member

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    Check your recieve diodes also noise blanker sometimes needs some deoxit can help.
     
  5. Brian G

    Brian G Member

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    So here's the thing. What I feel it is lacking is "ground noise". I was just listening on it. There were 2 local guys chatting. One of them was booming in and putting the needle in the red (and splashing onto other channels like 5-10 channels away because he was definitely using power). The other guy was clear as a bell but not moving the needle at all. And when neither of them talk, it was dead quiet. And seems to be that way up and down all channels and on SSB. I did try to turn the slugs somewhat while they were talking and it appears the peaks were ok the way they were for the most part.

    Maybe it's just that the air is that quiet but just seems odd to me. I am going to hook up my other radio and see if I can tell any difference but it won't be exactly the same obviously.

    Any explanation on what (if anything) would cause strong stations to appear to be ok but ground noise to be (seemingly) so suppressed? Like I said, could be there's absolutely nothing wrong, but it does seem odd....
     
  6. nomadradio

    nomadradio Analog Retentive

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    Do any of the small black tuning slugs appear to peak with the end of the slug DEAD EVEN with the rim of the hole in the top of the can?

    And I do mean dead even.

    If you see this, it indicates the capacitor inside that can has failed. and this prevents the coil from being peaked properly. That 'flush with the rim' position is the physical end of travel. The coil's inductance is at its max in this position. What appears to be a peak when turning the slug is just the slug reaching its limit before achieving a true peak.

    Gotta make a video on that. Comes up more and more as the years pass.

    73
     
  7. Handy Andy

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

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    How good is your "Memory" of how the 2000's used to sound like, versus now?

    Here's why I ask,

    Put the Radio in SWR CAL mode. check SWR - if you have a spare wattmeter - might be a good idea to put it in line to validate the internal meter versus the external - this is for discrepancies in the Receive as well as TX side.

    What I want you to check for - if only by memory, is if the SWR FWD is working like it should. - should Calibrate to the same spot - but if it don't there may be a broken RF return trace that is part of a Chassis versus Foil Board grounding condition.

    Why TX?

    Well, if your injecting a bias voltage into the Collector - that same voltage is on the output tank cap.

    So?

    Well, are you tuning the Receive, or peaking out the TX when you tune this thing up?

    A small IF leakage can occur in both the TX strip and RX sides, which knowing this radio has two Local ~7.8 MHz sections - one can detune to fit the slot of the other and leak out a birdie - not in RF, but IF - and kill receive that way.

    • Especially if you have a broken or poor RF grounding return trace from the output network towards the Antenna jack - if any works' been done or even a corroded rivet - you might want to reflow some solder in and around those rear panel "lugs" and board mounting screws to reestablish an RF bond if only for a little while to see if the receive can return. upload_2021-2-25_11-36-7.png

    It's when you are losing SSB mode Receive, you should have a ton of noise in there - L14 "taps" off and goes directly to the De-conversion section - while the AM side still uses L14 - but it shoots it's signal thru a lot of other stuff including that "flat slug" IF can that is the 455kHz side of this same IF strip.

    upload_2021-2-25_11-58-50.png

    These are "Age related" oxide issues, so knowing environment that beast is used in, and what the age of it is - In decades, the radios been stuck in that case for a long time. That condition alone - may offer clues as to why the Receive is weak - poor RF grounding at certain key spots affecting the way the radio gets rid of it's IF signal before it leaks out into the real world thru the antennas connector.

    I call it the "Oxides Of March" seems to happen every spring - dead receiver - due to corroded or poor RF ground bonds from the radio to the antenna and all the places in between.
     

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