CB radio Cobra 29 wx nt wont power on - Page 1
Forum section: electronics / repair

author: Dkblueyes240
author_rank: Newbie
timestamp: April 16, 2019, 08:23:56 pm
content: Hi, I am not very technical when it comes to electronics, however I learn quickly. So my cobra was working when i had it hard wired to my battery. I connected the antenna and the radio blew. I could smell it. The online fuse did not blow. I took the top off and started looking for something burnt, I checked D8 and D23 which both appear fine. The part that looks blown is labeled L10. I cant find any info on this part or any idea of that would cause lack of power. Again I am new to this so if you could explain in lameins terms that would be great. I'll attach a picture of the area. I appreciate any assistance!

author: LateLesley
author_rank: Frequent Contributor
timestamp: April 16, 2019, 09:54:03 pm
content: It looks like that part may just be a ferrite bead, with a couple of wires running through it. I couldn't find a circuit diagram for your model, the closest I think I can find is the cobra 29ltd.http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd_sm_sch.pdfIt's hard to tell if this fits for your model though, as we can't see all the board. But it might be helpful in getting an idea of the circuit. I find it concerning that it blew when you connected the antenna though, that shouldn't happen, and suggests something seriously wrong with it. Were you in receive mode, or trying to transmit? how is the antenna wired? what type of antenna is it? lots of questions need answering to figure out WHY it blew.That then may help trace where the fault on the board is.

author: floobydust
author_rank: Super Contributor
timestamp: April 16, 2019, 10:22:44 pm
content: L10 is a ferrite-bead transformer for the SWR meter. Not labelled on the schematic.The antenna feed passes through it (yel wire) and the blk wire is for the SWR meter.Normally L10 would not overheat unless you did mods to increase TX power, or if your antenna touched +12V power on the battery. You could check for shorted C60 which would also pass DC through L10. But I think it would only cook with too much TX power.The ferrite core looks discoloured and damaged from heat, it does not look promising. Take out L10 and pull out the two wires, check the insulation and see how it is.But something else has failed so the radio is dead? Should have a 2A fuse to stop drama...

author: TheDefpom
author_rank: Frequent Contributor
timestamp: April 23, 2019, 06:30:27 am
content: Your vehicle isn't a + chassis type is it?Very odd to have that L10 fail, I have never seen it before, it should only need to pass RF from the final to the antenna jack as it is only for the SWR meter function.Check for burnt out traces from the power connections before and after the on/off switch, the reverse polarity protection diode will only protect the DC input from polarity issues, not if the antenna line has a DC feed coming down it.FYI I am the guy that created and owns the RadioMods.co.nz site, you may have seen it.

author: M6YRU
author_rank: Contributor
timestamp: April 24, 2019, 12:32:40 pm
content: that is IMO the least chance thing to burn out on a board, it is just 2 wires with a ferrite core ...instead of winding up the power of the radio, get a small amplifier, even a 25watt unit, these are designed to run more power than what the radios are meant forquestion, what voltage is your car/van/truck? was you using a voltage converter to power the radio?its very odd that connecting the antenna would do this?if everything is a 12v baseline, however if your mode of transport was a 24v system and if you used a buck-style voltage converter, the output of the converter may not be the same refrence to ground and maybe it allowed high voltages to be passed through when you touched the antenna to ground...connect to 13.8v measure the pins at the voltage regulator, should be input voltage, gnd and voltage out, this will give the first place to try and fault find...

author: CJay
author_rank: Super Contributor
timestamp: April 24, 2019, 02:51:27 pm
content: Quote from: Dkblueyes240 on April 16, 2019, 08:23:56 pmThat part won't cause the radio to be dead.Take a good pic or a series of pics of the solder side of the board so we can give some tips on where to test??

author: Pattymorr
author_rank: Newbie
timestamp: December 30, 2020, 12:02:35 am
content: Some CB acts really erratic when PLL circuitry is compromised (i.e Not RF Power at output or not RX signal at all or combination of both).Just repaired a Realistic TRC-429 One Handler (means all controls are in handset) from late 1970s, cousin of the following list : Cobra 19GTL, 19GTL AM/FM, 19LTD, 78X; Craig L103 ,G.E, 3-5804G, 5816, 5900A; President VEEP, AR-14, AX-14; Realistic TRC210. TRC411. TRC416; SBE LCM5; Sears 663,3802,38009;Teaberry Stalker IV, Statker VIII and Uniden PC 14.Not much info for service manuals available for the TRC429 model and of course this presented a nice challenge as all functions seems to work and I was able to turn it ON and OFF from volume knob, view the channel number and choose for desired channel 1 to 40 on rotary selector, but Not sound, not transmitting not Squelch . Opened up shield cans showed a PLL MB8733 kind of similar to TC9109.There is not much documentation on the MB8733 PLL other that is completely compatible with TC9109. Found some schematic for the realistic mobile TRC-411 which uses same MB8733 PLL. There is a lot of differences with TC9109; the Xtal circuitry is not external for the MB8733 and is internally controlled by pin 2 and 3, Pin 2 acting as source voltage for the oscillation and pin 3 the actual reference oscillation and to feed other circuits. Pin 14(Bin data) is not connected to "d" 7-segment, instead is connected to "b" segment and it cause all tables from TC9109 data to be a lot different (which almost get me going to reverse wires on Rotary Control). The R/T pin logic levels are inverted as well Rx being H and Tx being L. On some sort of frequency protection by internal ROM the PLL will not lock if any Data Line is missing or erroneous data or if VCO is out of frequency.There I was trying to troubleshoot the PLL. Found Not oscillation on XTAL pin 2 and 3 (10.240Mhz), traced for shorting in capacitors or Variable cap and found nothing. One might think "BINGO" the Xtal is bad ( had never seen a dead Xtal). So, because I will CHANGE the xtal I need access to the soldering pads which was just underneath of another metal shielding covering the PLL and VCO circuitry on the soldering layer.Then the culprit was found , the RF pickup transformer for the VCO had mostly invisible breaking cold joints caused by some accumulation of old soldering and little knocks and of course little vibration here and there. By re-soldering cold soldering pads the anomaly is gone and then Xtal begin to oscillate and the whole thing is fully operational on RX and there is 3 watts on TX.Hope it helps for others and here the source of info I usedhttps://www.manuals.cornpone.net/Realistic/,http://www.cbtricks.com/radios/teaberry/stalker_iv/graphics/teaberry_stalker_iv_viii_sm_pg10_22.pdf,https://www.fichier-pdf.fr/2017/07/06/pll-hb-1/PattyPS. Sorry if I am on wrong post as newbie.