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Cobra 25LTD - No Modulation but PA works

Discussion in 'General CB Services Discussion' started by Mikey D., Jun 9, 2018.

  1. Mikey D.

    Mikey D. New Member

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    My Cobra 25LTD, the back-up radio I used on my trip to SC and back has no modulation all of a sudden. The last time I was heard by somebody was in NC when I got a radio check; no problems with audio then. Once home, I connected to a Dosey meter to check the AVG and PEAK readings and then MOD % - I was only able to read the power output (3 watts dead key), the modulation meter didn't move at all. I tested the mic and the Dosey with 2 other radios and everything worked as it should. The PA circuit works OK too.



    The XMT light lights up and the meter moves on the 25 when I key up, but there is nothing heard when monitoring from another CB radio (the meter indicates a received carrier but there is no sound). I checked for loose connections/solder joints on both sides of the board and I tested the CB/ANL - PA switch with a VOM and all is OK. The MIC jack is OK - no loose/broken wires and the PA circuit works with the MIC GAIN as well.

    I am leaning towards a defective component at this point, but I need a direction to go in to find the culprit(s). As always, any/all help would be appreciated. Thanks.

    Mike
     

  2. OldTech03

    OldTech03 Active Member

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    Is this a 25LTD ST or just a standard 25ltd? it is or could be
    relevant I think.
     
  3. Mikey D.

    Mikey D. New Member

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    It's a standard 25 LTD.
     
  4. kaos513

    kaos513 Well-Known Member

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    Check the cb/pa switch maybe could be going bad,spray some deoxit if you have.
     
  5. jon666

    jon666 Well-Known Member

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    try another mic.
     
  6. Mikey D.

    Mikey D. New Member

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    OK, tried different mic with same results. Old mic works fine on 2 other radios that were tested when I found out that the 25 had no modulation. Also, tested CB/PA switch with a VOM for continuity and there were no problems found. Radio receives with no trouble.

    Can't say when the 25 stopped modulating because the last time I used the radio and got a reply was in NC (radio check), everything else was 'reading the mail'.

    Thanks.
     
  7. sunbulls

    sunbulls Active Member

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    It seems you already eliminated the PA switch. The fact that your receive audio and PA audio is OK, that eliminates many of the audio components. Mic, mic preamp and audio IC chip are good. I would first look at AMC adjustment pot VR5. It possible the pots wiper has bad continuity, especially if someone turned the pot up so far as to cause damage to the wiper. Make note of pots position(mark it with a fine sharpy)and adjust the pot in both directions a few times while transmitting. If you get no results, you can always adjust it back to its original position. I would also carefully look at all the solder connections in the area of TR13, TR14 and VR5. People have been doing various bogus modifications in this area to increase modulation. Cold solder joints and solder bridges are often found, especially involving someone with little experience doing those mods. Also while transmitting, try pushing on the print side with a plastic tool or pencil eraser. If your audio suddenly pops back in, at least you know it’s a bad connection somewhere on the board. Again, it’s probably around TR13,TR14 and VR5. You may prefer to resolder that entire area.
     
  8. kaos513

    kaos513 Well-Known Member

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    Is the radio recapped?
     
  9. Mikey D.

    Mikey D. New Member

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    No, no recapping done.

    Upon further testing I found TR 13 has voltage present on the COLLECTOR and BASE in the REC mode: 0.35V and 0.78V respectively, there should be 0 volts present. The EMITTER voltage measures 1.4 volts instead of 1.7 volts in REC mode.

    In the XMT mode the EMITTER, COLLECTOR and BASE leads have the following voltages present: 0.57V, 1.79V and 1.07V respectively - they should read 0.7; 2.1 and 1.4 volts respectively according to the 25 LTD VOLTAGES sheet I downloaded from http://www.cbtricks.com/radios/cobra/25ltd/index.htm I used both analog and digital VOM's for the test measurements.

    BTW, I also tested TR 14 and it had 0 voltage present on both XMT and REC as is indicated on the voltage sheet.

    Mike
     
  10. Handy Andy

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

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    Ok, this may be why...

    Remember the thread about the Cobra 29? Nomad as well as others were suggesting a cap on the voltage feed to the Mic Amp may have failed - in your 25 - it's C94. IT's 10uF to as high at 47uF but uses 16WVDC rating.

    IF replacing that brings back the voltages then the radio needs to be recapped.

    Good to see you found CB Tricks - it's a go-to site for a lot of this that we're trying to help you on.

    You'll also see on your caps issue, there are a LOT of marginal 16Volt ones in there. Watch out - they will have to be replaced soon.

    Hope this helps!
    :+> Andy <+:
     
  11. Mikey D.

    Mikey D. New Member

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    Thanks once again for the help and suggestions everybody!!!

    Question on the caps, can I replace a 16V cap with a higher voltage cap like 20V? Also, if a cap's voltage isn't listed on the schematic does that mean it's rated at a standard 16 volts? Is there a list of all the components for the 25LTD and their values anywhere on the net?

    Dumb question dept: should I replace TR13 and TR 14 as a precaution?

    Mike
     
  12. Dmans

    Dmans Member

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    Mikey D,
    I have recapped a few units and my standard practice has been to up the voltage on each cap by the next largest size. (I.e. I replace a 10v cap with a 16v, change a 16v cap to 25v etc. )I am sure that there are those that would argue with this logic of mine (if a little is good more is better!) but I stick with the same values (uf) with the exception of main power supply caps. I tend to put the largest that will physically fit in the space provided (powers supplies that is)upping both the Microfarad and voltage rating. I would never use a 10v cap at all. Since power supply voltage is typically 13.8vdc, it doesn't make sense to me. I am aware that there are circuit branches running less than 13.8vdc (as in 9.0vdc, 5vdc etc) but it seems that newer technology capacitors are much smaller than the capacitors "from the good old days" so more capacity (both in uf and voltage) will fit in the same space.

    If some of my $10.00 or less scores from flea bay were instead Nuclear Missile launchers or Life Support Systems, things would be different. I tinker with these cheap radios for the nostalgia and to keep my mind busy. After recapped, some have never seen voltage again.

    Just my random senior thoughts. Your mileage may vary. Good luck with your project.

    73's
    David
     
  13. sunbulls

    sunbulls Active Member

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    Ditto on the above post.

    Yes, to answer the first question. 25V is the common next step up in voltage from a 16V capacitor. The answer is NO to your second question. Nothing is “standard” in relation to working voltage on these capacitors. If it’s not marked in the manual, check the voltage rating on the removed capacitor. I usually go by that anyway. The common working DC voltage steps are 10V, 16V, 25V, 35V, 50V, 63V, 100V, 160V, 250V, 400V and 1000V. In general, I’m not in favor of selecting a working voltage any higher than one step up beyond the original, but I always upgrade the 10V and 16V ones with a 25V.

    No, at this point I wouldn’t replace any transistors. Let’s see how things go after your capacitors are replaced. Taking things one step at a time avoids unnecessary problems or confusion.
     
  14. Handy Andy

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

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    You're welcome, you don't realize how "refreshing" it is to find something we all seem to know a thing or two about how to fix...

    Yes. There are exceptions to any rule though - but they will try to give you a heads up...

    No, you replace by the VALUE listed (Like 50WVDC) on the caps' shrink-wrapped can for Electrolytic - or any working "V" like 50V or 100V on a disc cap - or even by letter, or if no letter - value, and if marked with a color on the top of the disc, it signifies a special NPO type that means it's frequency dependent and needs to be replaced exactly with value and color.

    BELIEVE IT OR DON'T:
    In the realm of radios - don't waste a TON of money on tolerance or temperature - the values used in that particular chassis - were AVAILABLE for CHEAP when they made this in the factory. So yes, they saved money by running the line on margins - meaning don't spend more than you have to.
    Same applies to your situation -
    If you can find a value - GREAT!.
    If you can find it in a greater power (working voltage) - FANTASTIC!
    If you can find it on sale? OUTSTANDING!
    Can you make it fit?
    A: Possibly - but not without some repositioning - OK
    B: Maybe - but requires heavy power tools - Hmm...
    C: WTF - I need to call a body shop. - OUCH - Sorry Try again...
    So again find the right values, in the package you need to make it fit - if it's a higher working voltage - go for it...
    Drawbacks?
    Some caps - like Tantalums are very specific in use and location - position and size - so they can remain viable and useable in the environment and provide the transformation needed to provide a function. (Cobra 25 uses these in their Audio Amp sections - as the Non-inverting working offset impedance for the Audio Amp chip not having to require excessive resistor and power supply network routings to handle single ended power supply feeds) - so you gain little audio benefit and perhaps even greater INSTABILITY if the radio gets put into an environment the original part would make it pass, but this new part makes it fail - in application.
    EXAMPLE - C59 by IC1 (your audio amp chip) it's 16V if you changed this part from the original voltage - it may obtain too much gain and make your radio SQUEAL when in TX mode - just because of the performance the original part did to the circuit, kept the squeal out. It has to be the EXACT part to replace the original one.
    Yes, Go here, by Clicking HERE and see if this works for you

    No, at least not yet.

    Why? You have PA that works, if you were able to adjust volume using the Mic gain circuit - then the audio is ok, but the LIMITER section may have a failed part - refer to the above link to help you locate a schematic and track it down for it.

    And... please remember this Memo:

    The only dumb questions are the ones never asked .

    :+> Andy <+:
     
    #14 Handy Andy, Jun 12, 2018
    Last edited: Jun 12, 2018
  15. Mikey D.

    Mikey D. New Member

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    I tried the radio while it was hooked up to the DOSEY (with the covers off) before and the thing had modulation; I could also measure some 'swing' on the PEP scale. I guess from my slightly moving components I hit a nerve. However, once the covers were back on the radio stopped modulating again...I pressed on the case and the radio started talking again. I'm gonna check the limiter section closely to see if anything is lose or looks marginal. I'll replace the caps in that area first.

    Andy, only the schematic has the component values, unlike the sheet I found for the PC133/155. Some listings are cryptic i.e., C90 shows 50V with a 1 listed directly below the voltage number - I take that to mean C90 is a 1uF 50V cap then, is that correct? I also see caps C127; C129; C130; and C131 listed as 0.01 with no voltage rating what so ever listed.

    Mike
     

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