1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice
Dismiss Notice
This forum does not allow a single user to have more than one username. If anyone wants to change their username contact an admin and it will be done. Multiple accounts belonging to the same member will be deleted without warning.

Cobra 29ltd issue

Discussion in 'General CB Services Discussion' started by kaos513, Jul 29, 2020.

  1. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    Hello guys cobra 29ltd regulator tr-23 getting super hot any help would be appreciated picture below of chassis .


     

    Attached Files:


  2. Handy Andy

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

    Joined:
    Apr 23, 2018
    Messages:
    3,280
    Likes Received:
    3,607
    Whoa!
    upload_2020-7-28_22-26-8.png
    Don't often see this - look for the Exclamation - and Red Circle...

    That area is a common tie off ground point for Front panel work. A lot of switches and even the Meter and it's light goes there...

    I see works been done to the harness - one of these it seems there is the RED wire that heads back to a grounding point - you may want to check that out!

    There is a point of note, this is a 1984 board, one of their most stable and best designed ones before Uniden headed for the hills, Jungles and other points of East...

    You also (you sly devil) have a 1969 Final - hope that works out for you.

    Some of the caps have been done, if the harness checks out - your bigger culprits are to fish out those filter caps yet to be swapped out and or check for reverse-installed ones - not likely but not impossible that one could have been inserted wrong - those heat up too - and you'll smell Vinegar and or Rotten egg odor as well when it idles or if you put in TX and then it develops the odor, then it's (Cap or short) is in the Transmit side.
     
    Shadetree Mechanic and Slowmover like this.
  3. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    That red wire is the ground to the meter light
     
  4. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    Noticed gets hotter on channel 40 than channel 1
     
  5. Handy Andy

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

    Joined:
    Apr 23, 2018
    Messages:
    3,280
    Likes Received:
    3,607
    Noticed the shrink wrap - but are all the front panel features working?

    When TR23 heats up - it's providing regulation - so only sections of the radio are sinking this power feed. You should be able to feel some heat in the area of the draw. It's my fear either a cap is in backwards or you have shorted a power line feed from TR 23 to ground.

    Might want to pull that TR23 - and do a simple check from Middle Leg - to Ground - see if you have a short - continuity check...Follow that main Power feed leg to each branch - follow that trace - see if you can backtrack to the caps or the short.

    Now, the METER light since you did notice it - it does NOT pull power from TR23. So when you do the "continuity" check - check the Dimmer circuit - that may be where you need to go - Switch Bright And Dim to see if problem persists but heat is less or more in the different modes...

    Again, check your Dimmer circuit section...takes a lot more current to display 40 than 1 - Go to Channel 28 or even 38 and see the same result.

    You may need to recheck the power feed to your Dimmer controller - TR22 - but again VERIFY the meter light is not shorting out hot to ground. They should not even affect the Dimmer TR22 because they use a separate Feed - only the CHANNEL and RX/TX light uses TR22 Meter is on a SEAPRATE circuit but TIED TO SAME SWITCH - again - I ask - please verify that the Front panel - especially Bright/Dim switch does not have any shorts ACROSS the twin pairs of switch lugs.
     
    Crossbow and kaos513 like this.
  6. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    Bolted a tip-29a to rear heat sink runs very smooth and no heat issues now
     
  7. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    Here ya go!
     

    Attached Files:

  8. Slowmover

    Slowmover Sr. Member

    Joined:
    Feb 17, 2015
    Messages:
    2,744
    Likes Received:
    3,965
    Gotta love it when Santa’s Chief Bench Elf shows up (that post read like a Xmas gift).

    .
     
  9. Lkaskel

    Lkaskel Active Member

    Joined:
    Aug 4, 2017
    Messages:
    114
    Likes Received:
    51
    Just to clarify the results..... Did you find any issues as Andy suggested could be in play or did you just put the TIP-29 in and call it repaired? Asking for a friend.
     
    Tokin likes this.
  10. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    I went over the schematic based on sams photofact areas where the components all meet including diodes,resistors and transistors all match voltages, as for tp-29a I did tons of research I let the radio burn in for 2 days before I made my findings.i don’t tend to sell this radio if it blows up it’s ok lol.
     
  11. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    Bright dim switch has no short yes all front panel works perfect.
     
    #11 kaos513, Jul 30, 2020
    Last edited: Jul 30, 2020
    Handy Andy likes this.
  12. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    Did find r118 with a different value after I change it to its normal value the meter light stays very dim.
     
  13. Handy Andy

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

    Joined:
    Apr 23, 2018
    Messages:
    3,280
    Likes Received:
    3,607
    That may be a clue.

    They used to offer "grain of wheat" bulbs in various voltages - that source seems to have dried up.

    I am lucky to find 14V bulbs with the 25mA rate - but remember, current flows thru the bulb different, the "dimmer to the bulb" correlation - means less voltage into the bulb - the current at the voltage going into it - shoots up. So if you use a bulb that is 14V but use a 9 volt circuit - it will take more current because the bulbs intrinsic resistive impedance effect - it looks a lot like a lower ohmic resistor at those voltages - so current shoots up because of the filament looks like a low-value resistor.
     
    kaos513 likes this.
  14. kaos513

    kaos513 Sr. Member

    Joined:
    Apr 10, 2014
    Messages:
    2,121
    Likes Received:
    595
    Sad news on the radio never turned back on but I do have 13vdc at the power switch
     
  15. Handy Andy

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

    Joined:
    Apr 23, 2018
    Messages:
    3,280
    Likes Received:
    3,607
    Ok, now we have to find what happened...

    As before - when things get hot, in the regulator - there are only several areas it feeds.

    Mostly PLL, Display (LED Channel) Dimmer, MIC amp, AWI and the main Xtal (10.240) and IF (where pure quiet filtered DC is a must).- lastly the PIN diode section needing a quiescent current to help with RX noise.

    This regulation is also sent to CB/PA switch - so when you switch to PA, TX switch and it's section turns off - If TR23 should cool down, then this is because it's output DISABLES TX but not the PLL - this PA mode may help isolate the draw heating up problem - if it gets (TR23 ) cooler, then the TX Switch area was at fault and needs investigation. (That has happened - hence the things to check are revered installed caps, they can stand some reverse voltage but hiss and leak vapors of sulfur smelling odors... along with drawing a lot of current in the process)

    Very little else goes to regulation - all the other sections including Squelch - it's power is arrived to thru filtering and a diode

    D21 - the Diode blocks the "backfeed" from cycling thru the regulator forcing it to drop current thru it trying to keep the radio working - it thinks it's a surge, the Zener would be cranked into conduction...so the part attempts to sink current thru itself - which will damage it..(D21 gives it Reverse Polarity protection too)

    So knowing this may happen, they use R103 - 200 ohm 2W resistor as a bypass - it provides some cushion so that TR23 doesn't have to work so hard at regulation. It parallels TR23 - so it will sink current thru it - so if you're having issues, you may need to check R103 and ensure it's value is 200 ohms. This is a tricky section, for too much ohmic resistance - TR23 works harder, if you use too little - the current drawn thru TR23 will not be enough to keep voltages down to regulation levels - extra noise and erratic IF / RX operation.

    • Some people I have done mods for, wanted Echo inside (amongst other) but needed an ~8+ Volt feed - which requires you to lower R103 to about 180 ohms, but keep the 2W rating - this keeps TR23 from blowing up - else if the Echo or other toy needs more current to operate, in comes the NTE-37 to it's rescue.
    IF you feel the necessary needs, you can always sub in a NTE-37 - Hilarious but I've seen it done - will need your very idea of heat sink panel to dissipate heat from this...

    Another aspect is the regulation circuit itself - Follow the current trail - Branches and all...

    Capacitors...​
    • C119 - 100uF 16 volt, Regulator buffer/filter output
    • C112 - 470uF 16 Volt, + 5V Regulator buffer/Filter output
    • C114 - 100uF 16 Volt, + 5V Regulator INPUT Buffer/Filter
    • C102 - 47uF 16 Volt, + 5 Volt INPUT Filter for PLL
    • C11 - 10uF 16 Volt , PIN Diode Power / BIAS for RF GAIN
    • C66 and C67 - MIC amp section past (R70) buffered thru a 330ohm resistor
    So you have some work ahead of you.

    This is why - when I saw the "red wire" looping back to ground - plus the harness work, I know that the meter lamps always need to be replaced, not everyone keeps the same wire coloring to help - but then too - I've had radios that used a meter lamp installed at the switch, but used so much heat they put a short across the switch. S205 t's a Dual Pole unit. One side feeds the Dimmer BASE circuit but gets power from the Regulator. The other side is 12V straight from the battery The lamp uses 12V thru a dropping resistor so you need only an 8Volt type for best illuviation results. But even with the best works done to it, it's still possible to "overheat" the switch and the contacts remelt together - this happens on old radio switches, they used Butyl-based plastics so they can show heat deformation (let alone stink). You had to move/slide the switch to the position of the contact underneath the tab so the heat would not deform it so quickly.

    Sorry to see you lost the radio but TR23 heating up means more work needs to be done, so you may have lost the Regulator but you also may have lost the power filter section too - again - Bad - Old caps or reverse installed ones, can do the same thing. IF someone tried to mod it - they you have to find those sections and undo that damage as well.

    I've given you a list of Caps in the feeder line from the Regulator - most are buffered thru a resistor - but if TR22 is bad, you can send full battery voltage back into the 8~9 volt regulator circuit (Short from Collector to Base) forcing TR23 to sink more current...

    Ok, you have all my Cliff notes on this...
     

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!