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Cobra 2000 S Meter pegged to the right

Discussion in 'General CB Services Discussion' started by Brian G, Feb 16, 2021.

  1. Brian G

    Brian G Member

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    Ok thanks for info. Well I was working on the receive strip due to weak Receive before the big pll and VCO blowup the last couple of weeks (which thankfully is resolved per the other thread). I’m not sure at what point that meter started pegging. I noticed it by accident so who knows. I will check the coil tuning and see if I can resolve. meantime, what IDSS rating would you recommend for this application? I was looking at an NTE133 as an equivalent and it seems to have a lower rating. See pic.

     

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  2. Handy Andy

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

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    Although you have the part, if the Original is what pegs the meter, then they chose is for it's gain. much like how a BJT (Bi polar) uses Hfe as a figure of Merit - the IDss is a figure of merit as to how the slope of Off Resistance to ON resistance (at a given set of voltages on the Gate and Drain to Ground source) - this mA measurement is a current the device passes
    • - knowing this, whatever made the Bridge unbalanced
    • - is this causing the AGC side to wake up?
      • - like it knows there is a carrier
      • - this can be from several sources,
      • L14 being the tunable one,
      • with FT3
      • and even cables that are unbound to the loom, if left in the wrong position can pick up the IF signals floating about just above the radio and inject them into spots it returns to.
    • I would keep the Original Part back in there for now, for the gain they chose - needs for it to be there and we haven't been given much incentive to change the part unless we are ABSOLUTELY sure we have exhausted all other possibilities.
    upload_2021-2-21_11-19-16.png
    The reason why I had you locate a specific part in a previous post is due to the very nature of what you're doing. You don't have to physically touch parts, but if they've been set - left alone in a specific position - then disturbed - can lose their point contact and show open, or worse, affect other regions that the parts takes current from, to balance out another.

    The board I posted to, is part of the "Squelch trim" this is prone to problems because of it's location - it's by the board you worked thru but didn't mess with, yet.

    The Squelch control and operation of it, is thru the lead wire because of the AGC, and S - meter - this circuit is also used for opening and KEEPING OPEN the squelch for a longer degree of S-meter signal capture - this is why they used the "bridge" so they know of the AGC, the IF involved their ratios of input, just not the capture of the RF signal and how interested the operator is in setting squelch (a Variable resistor on one side of this) to set a threshold, but not to full quieting, for so they can hear someone breaking in.

    It can be the open line that circuit sees, because of the contact arm is opened and now doesn't have a good ground reference - so the Drain has more, or less of - voltage on it than it should so the S meter goes crazy because of a wire that should be grounded, is, but the part that it grounds - is a physical trimmer pot that may have gotten dirt into itself from age, and now goofed up your efforts to tune the radio - upsetting the Bridge balance the circuit uses and the S-meter reflects this...

    Onto something like a failed Tantalum cap - or even a blown open resistor from a bump that most people would not have noticed.
     
    #17 Handy Andy, Feb 21, 2021
    Last edited: Feb 21, 2021
  3. Brian G

    Brian G Member

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    So when I remove the FET and just leave the holes open, the meter is still pegged. Shouldn’t that kill the circuit and stop it from pegging? Does that suggest anything or is that to be expected? I will put the old one back in but I am pretty sure I tried that already and didn’t matter. I also tried L14 and others but that made no difference. The only thing that remotely moves the needle is like I said putting my finger on that spot. Should I look upstream prior to the FET?
     
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  4. kaos513

    kaos513 Sr. Member

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    Try to ohm out the meter or try a different meter
     
  5. Handy Andy

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

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    Ok did you find that Squelch Trimmer pot? You haven't even answered to see if Squelch works when another radio keys up with a good strong signal - the Squelch should pop open.

    upload_2021-2-21_14-30-10.png
    It's on the Front panel - does squelch control work?
    VR3 is the Squelch Trim -- the effort I'm trying to help you with here is
    if you can't adjust squelch - then no matter what FET you put in there it isn't going to work
    The Squelch control needs to be fully working
    so all it's points need to have have power flowing thru them and have a place to go.​

    That is your big clue.

    Another would be here

    upload_2021-2-21_14-37-12.png

    TR11 - that is the S-meter amp - but it's getting signal from L14 to do this.

    TR13 supplies power to the FET2 from TR11 - and has several caps - including one that should be looked at and swapped out to make sure it's not adding to this. C29 0.47uF Tantalum - FET2 controls Tr10 and TR16 in the BASE bias of the IF - but the process of powering the S-meter starts at TR11 thru TR13.

    The reason the FET's being gone and yet, it's not killing the S-meter - is due to the imbalance. Not the FET - The FET sinks current but only when it's told to turn on - that is done by the Bridge - once it sees current flowing from the Tap at L14 - it then takes away from the the bridge and the trigger voltage from the Squelch trimmer - which allows the Squelch to stay open longer (during receive key up) In doing so, turns on AGC and start to pull down the incoming signal back at the Front End by the Pin diodes. This FET pulls the meter back from pegged Position because it works on TR16 (the IF Amp strip that has L14 in it) and Squelch - and TR10's ability to work AGC.

    So replace with the other part that gave you 1/2 the pegging - and look for that Squelch Trim pot make sure all three terminals have power or sending power - or pure ground. A dirty squelch control mounting or poor ground for it, ruins your day.

    Make sure all the Tantalums and Electrolytic caps are changed too - this whole mess could have been avoided if we find a bad cap that acted like a short (C29 - 0.47uF) and sent power thru when it really shouldn't.

    Another spot to check...
    Looking From Front panel to rear panel
    \Component side up...
    upload_2021-2-21_15-14-51.png
    Locate L14 - there are two caps by it. One in particular is C62/C63 one there C63 is a 1uF Electrolytic - replace it no matter what now...
     
    #20 Handy Andy, Feb 21, 2021
    Last edited: Feb 21, 2021
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  6. nomadradio

    nomadradio Analog Retentive

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    Quite the opposite. Shorting the solder pad for the Drain to the pad for the Source lead should provide max receiver sensitivity. The less current through the 2SK19, the more gain-reducing voltage goes to the rest of the receiver. Max sensitivity occurs when max current flows from Drain to Source. Just bridging the two adjacent pads with the tip of a small screwdriver should reveal a difference, whether the FET is in place or not.

    This is why the "Bl" suffix is important. All the other suffix types have a lower zero-bias current, and using one of those can cripple the receiver's sensitivity.

    73
     

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