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Cobra 2000 with problems....needs a new p/s transformer

Discussion in 'CB and Export Equipment and Accessories' started by toader32, Sep 1, 2005.

  1. toader32

    toader32 Member

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    Hello all.

    If anyone knows where I can get a replacement transformer for my Cobra 2000 power supply I will appreciate it. It says TF-164 on the top.
    Does anyone know what the voltage should be on the secondary output. Mine is 0.0 .

    My Cobra 2000 is not well after 15 years of service. It started to squeal on keyup but would clear up upon warm up. Meters would slam to the right.

    Now, after a lightning strike to a tree outside, I have no receive. The receive is the same with or without the antenna plugged in. I have checked the 2 diode pairs near the antenna input but they look good on my diode tester. This was suggested by a local tech.
    I also swapped the final transistors out from a Cobra 148 I had sitting around. No difference.

    I may have 2 separate problems here asside from the transformer problem.



    Any assistance will be greatly appreciated.

    Thanks,
    Toad
    Mayer, AZ
     

  2. nomadradio

    nomadradio Analog Retentive

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    Wow. Three days and nothing but a thundering silence?

    To start, the TF-164 was discontinued years ago. Cobra ran out of them 8 or 9 years ago. Like any other discontinued product, they just get more rare (and expensive) every year.

    At the same time, external DC power supplies just keep getting cheaper year by year.

    Getting the internal power supply fully fixed up will no doubt cost more than a 5-Amp DC power supply. Don't try to use one with a rating lower than that. The Cobra 2000GTL has a higher current demand than other 40-channel SSB radios. The counter/clock is a power hog. For that reason, the TF-164 has a higher current/power rating than all the other SSB base radios made by Uniden. This is the ONLY model that uses that transformer. Robbing (recycling) one from another Uniden-made SSB base like a Cobra 142, President Washington/Madison or such just won't cut the mustard. Not big enough.

    Every sub we've found that had a large enough rating was too tall to put the internal shield cover back.

    Makes a "junk" Cobra 2000GTL worth more than some radios are working, if you can recover the power transformer.

    Oh yeah, theTF-164's output voltage is 16 Volts AC at 4.5 Amps. A slightly higher voltage may work, but too high and the big filter capacitor will get more than the rated 25 Volts placed on it. Not supposed to put more than 90% of that rating onto a filter cap anyway. Runs between 22 and 23 Volts DC with the stock transformer. The higher the winding voltage, the hotter the big regulator transistor on the heat sink underneath will get. Too high and it will fail too often. Don't assume that the transformer is the only failure. Leaving it plugged into the wall during a storm is all it takes to blow the transformer. Or, using a fuse that's too large can be hard on it when something breaks down and overloads the power supply.

    The rectifier, filter and regulator may be okay, but don't hook up a replacement transformer until you're sure.

    The receiver problem. Diodes D12,13,14 and D16 are a likely failure, but if you didn't check the old ones, important diagnostic data has been lost. If they were hammered, this was worth knowing.

    C31, L4 and TR14 are also likely places to look for failure. Just touching the tip of a screwdriver to the collector (center) pin of TR14 can be a quick check. If you hear any noise in the receiver when you lay a finger on the scredriver shaft, then try it against the emitter leg, the one connected to L4. Should be louder on the emitter than on the collector, whatever stray noises get picked up. If there is a computer and especially a CRT monitor running in the room, you're very likely to pick up enough 'stray' RF noise to hear this way.

    If the noise is louder on the output side (collector) of TR14, that stage is not amplifying what comes in from the antenna. Most of the other things besides C31, L4, the diodes and TR14 that can do this would also cripple the transmit. If it shows normal RF power output, the problem is either at or downstream from C31, or both.

    73
     
  3. toader32

    toader32 Member

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    Cobra 2000 No receive and squeals

    I checked D12, D13, D14, and D16. They all looked good with my DVM with the diode tester. They are the original diodes. None have been replace. TR 14 looked good as per your method. L4 looks good as far as I can tell . I took ohm measurements and compared to a Cobra 148 I have. C31 looked open on both my 2000 and 148 even after removal. Not sure about this cap. I did notice measuring from the leg of C31 that goes to L4 and chassis ground is open on the 148 and 3.3k on the 2000.

    Also a random measurement was takin across the large 1000uF cap...I think its C189 or C190. It measured 20 ohms on the 2000 and on the 148 it charges up to about 1.5k and discarges to a lower value and then back up. I removed the electrolytic cap but still measured 20 ohms at the solder pads with no cap.

    The RF meter normally goes to the 4 watt position....It goes to the 30db mark on the meter since this problem.

    This is about all I can tell you. If you have any more suggestions I will appreciate it. All I have is a DVM to work with.

    Do you think this unit is worth shipping out to a tech and having it repaired. How much do you think it would cost?

    Thanks for all your help,
     
  4. nomadradio

    nomadradio Analog Retentive

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    Hi Toad,
    If you don't have an inline RF wattmeter and a dummy load, then you are really flying blind here. A DVM is okay to troubleshoot a lot of common breakdowns, but if you can't verify what's coming out of the radio on transmit, that's a problem. That would be VERY helpful to know.

    As to your use of the "Diode Test" on the DVM.. Did you check each diode twice? Once with the banded end to the black probe, and again with the banded end to the red probe?

    A normal reading with the banded end to the black probe would be between 0.600 and 0.700. The reading with the diode pointed the other way, with the band to the red probe, should be an open circuit, as if the probes weren't touching anything. The diode must turn ON pointed one way, but must ALSO be OFF, when pointed the other way. Unless you checked EACH one BOTH ways, you can't say that any of them are good, yet. This is the basic character of a diode, that it conducts in one direction, but NOT in the other. You gotta see both sides of this characteristic, to know that it's good.

    One clue in your last post, about the RF meter slamming on transmit (I think that's what you meant), suggests to me that the connection from the antenna socket has been clobbered. It would be very helpful to know if any transmit power is coming out of the radio. If not, it's time to find out where the connection from the antenna jack has been blown apart.

    L4 has a very tiny inductance, not much more than one microhenry on one winding, less on the other one. Hard to measure with a proper L/C meter, but hopeless on a DVM. Just because it shows continuity does not mean that the windings are okay. A lightning pluse can fry the enamel insulation on the tiny wire windings inside. The DVM will still show continuity, but fried windings don't deliver RF from the antenna to TR14. If your DVM had said that the windings on L4 were an open circuit, that would be a clear failure. Just because the continuity checks okay, you can't be sure L4 will tune properly.

    C31 has only 12 pf of capacitance. A DVM can't measure that, unless it has a "C", or capacitance scale. Only the really high-dollar meters include that scale. A correct DVM reading on C31 is an open circuit, as if you had not touched the probes to it at all.

    The one 1000 uf cap I know is C172. The reading in the 2000 may be from the failed AC power supply, or more likely from all the lights on the front panel. The 148 probably has a blown light bulb in the S-meter, making this reading higher. Several of the front-panel lamps in the 2000 are wired in parallel with that cap. Checking it while soldered in the circuit will not tell you a lot about that cap. The DVM will be reading the resistance of those bulbs, in parallel with the cap.

    One other detail. You can't use the radio's chassis to ground your DVM's negative lead. The chassis is 'floating', connected to the circuit-board's ground circuit through a dozen or so disc capacitors. They make the chassis into a ground for RF currents, but NOT for a DC ohmmeter. The reason for this goes back 30 years, to a time when some cars had the POSITIVE side of the battery grounded to the vehicle frame. Mostly this would be MACK brand tractors 30 or 40 years ago. This "floating ground" arrangement allowed you to use the radio in either type vehicle, positive-ground OR negative-ground. Since the DC power socket's negative side is NOT hooked directly to the chassis, the radio will work in a 1972 White or 1974 Mack OTR tractor. Pretty sure the truck industry went totally negative ground years ago, but the CB manufacturers still use the floating-ground hookup, even now. To make a measurement like the one you describe, the DVM's negative clip should go to the negative pin on the DC power socket, or to the negative lug on the big filter cap underneath, the one held down by the metal clamp.

    I can't escape the suspicion that this radio has gone beyond the "DVM only" limit. Lightning damage can be a real pain. Once the radio comes back to life, hidden damage can cause the receiver or transmitter to still be "weak". Alive, but not nearly as strong as before. A DVM won't help much in that situation.

    A reputable shop will not quote a price for this kind of job without a chance to see it and do the troubleshooting first. Especially with a lightning job. The best way (I think) is to charge a fixed "bench" or "estimate" fee. This way, the shop gets paid for the labor they invested in adding up a total price for the job. The customer knows how much he risks spending for "just" bad news, itemized in dollars and cents. Nobody wants to write a blank check and say "fix whatever's bad". Nobody with a clue, anyway.

    One (of several) reasons we don't solicit work shipped in is what it does to the price of "bad news only". If I charge 25 bucks to diagnose a 2000GTL, and the estimate totals 200 bucks, the customer is (rightly) going to say "No thanks". All he owes is the estimate fee, and takes it home not repaired, no hard feelings.

    Add to this the cost of packing, insuring and shipping the radio twice, and that 25 bucks jumps to 70 or more. Pretty high price for "just" bad news, if that's what we find. Yes, I have other (good) reasons for not soliciting work shipped in, but this is a good reason all by itself. Seventy bucks (or more) for just bad news? Gotta figure people would gripe.

    And about that "worth" part. What it's worth is pretty much what your other choices would cost you. If you can replace the radio for 150 bucks, this one can't be 'worth' any more than that to you. Mostly I can sidestep the "worth" question. Most of my customers seem to know what another radio will cost them, before they bring one in to get fixed. Once I present them with an estimated job price, they have a way to compare the price of that option to the price of chucking the radio and starting over with another one.

    My gut intuition is that even a pro isn't going to get this one going with just a DVM.

    73
     
  5. toader32

    toader32 Member

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    Cobra 2000 No receive and squeals

    Thanks for all your assistance during this ordeal. You are an asset to the CB world. Especially to us who dont have access to knowledgable technicians.

    I will be putting this radio on ebay in 2 or 3 weeks for parts. Probably sell the speaker separately.

    It was a good rig for 15 years.

    Thanks again
    Toad
     
  6. nomadradio

    nomadradio Analog Retentive

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    Smart choice. 15 years is a good run. Lightning is hard on them, whether it comes in the antenna cable, or up the power cord from the outlet.

    Haven't been watching what "junk" Cobra 2000 GTL radios fetch on Ebay, but I sure don't get many offers when I ask around. Thought the "Wanted Dead NOT alive" ads would get a chuckle or two. Just silence.

    Gotta figure they're getting bought as soon as they appear.

    Hmmm. Never know, the speaker may bring more than the radio. Especially if it looks good.

    Good luck with the auction and 73
     
  7. Arj51k

    Arj51k Member

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    Hello all from Arj51k. I have a cobra 2000 made in Taiwan. Have a very low volume going to the speaker. What solution could correct this problem. appricate the help.
     
  8. TonyV225

    TonyV225 W9WDX Amateur Radio Club Member

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    You mean the parts Cobra 2000 I have around here somewhere will eventually be worth something?? LOL!!
     
  9. 9C1Driver

    9C1Driver Sr. Member

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    To be honest I would box it up and send it to Doug at Custom CB Radio. He is real good with Cobra 2000 radio's.

    Custom CB radios

    Both you guys. Get it done right by a pro that does great work and can be trusted. You could also get with DTB who is a forum member, he is also a good guy.
     
  10. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Does the radio have RX meter movement, indicating that it's actually receiving, but no audio? Or is there no needle movement, either?

    If there's needle movement, but no audio, most likely the TA7222AP audio IC went south, which is common with close lightning strikes. Easy fix.


    I'm curious to know about the RX issue.


    ~Cheers~
     
  11. QROdx

    QROdx W9WDX Amateur Radio Club Member

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    ummmm...... you guys do realize the original post was made 7 years ago lol
     
  12. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Yep, digging up old threads are fun!



    ~Cheers~
     
    wavrider likes this.
  13. El Kabong

    El Kabong New Member

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    Has anyone located a modern QUAlITY transformer that puts out a few more amps than stock trans but will fit in the stock transformer space with the RF shield top on?

    I hope I hope.
     
  14. Joseph Riffle

    Joseph Riffle New Member

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    Thinking bout using one out of a 7 amp astron supply
     
  15. loosecannon

    loosecannon Sr. Member

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    EDIT: the power supply i linked to won't fit in a 2000GTL. it will fit in any of the other uniden bases, but not the 2000 because of the freq counter.

    here is an 8 amp supply that might fit, but i would need to pull a 2000 out of a storage box in order to measure and be sure. not a bad gamble for 10 bucks though. if it didn't fit, im sure someone would come up with a use for it: https://www.ebay.com/itm/AC-To-DC-1...843851&hash=item2cc9605c0c:g:cRYAAOSwYVlaAWza


    Just having a transformer that has more amps available won't be enough.
    There are other parts of the power supply downstream from the transformer that might not be rated to handle the extra draw.

    I replace the old style power supplies with these: https://www.ebay.com/itm/12V-10A-120W-Regulated-Switching-Power-Supply-for-LED-Strip-light-CCTV-Camera/183257685274?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

    I have not encountered noise issues from using switching supplies in any of the radios i've used them in.

    the only thing about these supplies is that they are made for a 12v output, and we want 13.8 volts for our radios.
    there is a 1/4 watt resistor that connects to the voltage adjustment trimmer that needs to be lowered slightly in value in order to be able to bring the adjustment up to 13.8 volts.
    (most that i've seen won't go above 13.5 volts without the resistor change)

    the resistor value i see most is 1.5K, and i replace them with a 1K resistor. then set the trimmer for 13.8 volts. this will lower the current capacity a little bit, but not enough to notice unless you were drawing the full 10 amps available.

    WARNING!!!

    DO NOT lower the value of this resistor too much, (like, 500 ohms would be bad) or you will have a supply putting out 19 or 20 volts, and that probably won't last too long without smoking something.

    the added advantage of doing this is that the radio is now significantly lighter!
    LC
     
    #15 loosecannon, Dec 1, 2018
    Last edited: Dec 1, 2018
    bobl9355 likes this.

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