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Strange audio issue on Cobra 40 Plus

Discussion in 'General CB Services Discussion' started by PoDuck, May 14, 2018.

  1. PoDuck

    PoDuck Active Member

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    I'm trying to track down a problem with a Cobra 40 Plus that I acquired for free. It's putting out a 1.62Khz tone on both transmit and receive. I am unable to detect the tone on pin 6 of the audio amplifier, which is supposed to be input, but I am getting a strong tone on pin 2, which according to the datasheet is a bootstrap pin. The big strangeness is that when I lift pin 6, I get no tone, but I detect no tone on pin 6 whether it is lifted or not. I also get no tone on pin 2, or anywhere else I can detect.

    I am having other problems with the radio as well, but I am not sure they are all related. The chanel display does not light up, the signal strength indicator lights don't light up, and the final is getting extremely hot, even when not in transmit.

    A wild guess would say the audio amplifier IC might be the cause of the sound problem, but I don't see how that could be causing the rest of my problems, and experience tells me that it is usually a single failure that either causes all the problems, or it causes a cascade of problems that cause all the problems.

    Any ideas of where this tone might be coming from based on the symptoms would be great.


     

  2. Handy Andy

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

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    First, welcome.

    Gotta' ask some questions though...can help direct me to help you...

    Is this a "smooth faced" brushed shiny steel - squared slide-buttoned radio with Push buttons (UP - DOWN) to change channel and some other stuff? Like NB/ANL PA/CB and had a LED S/RF/SWR signal meter?

    If you do, the thing is CONSIDERABLY OLD and will require a recap.

    The noise you hear, if only, is from the oscillator used for polling the display, it's a square wave on/off alternate switched solid state toggle function used in a lot of radios built when Dynascan and Uniden we more hand-in-hand and developed quite a bit of technology based platforms but used now outdated displays and LSI microprocessor stuff.

    IT was fun time yet a bane because of it's direct approach - all the display functions were handled as LED segmented - which is fine but the drivers and the TTL - approach was noisy.

    You have that same problem in any radio from that time - whether it's a Cobra 19 PLUS or Cobra 40 PLUS. Heck even the 18RV still has that "hash noise" problem...

    Many of the Electrolytic caps in these things dry out, get old and no longer filter/work like they're supposed to - so you'll have to roll up your sleeves and get to know this radio.

    Start here... See if it's a Cobra 40 Plus ... Click Here ...
    Or there was also a 40 X model with the BCD rotary channel knob that was a lot quieter... You can see that by clicking here

    Both of them used a 713X PLL chip - very straightforward but limited in ability - strictly 40 channels and some often bragged about getting uppers and lowers in it - but that required Crystal swaps to make that happen - so nothing new there.

    7131 - programmed logic PLL - but can use a BCD rotary
    7132 - BCD compatible logic PLL - had Channel Up Down - built in.

    Back then, they used "Computer grade parts" which - back then were the Cats' Meow - but they do age and that platform was essentially scrapped due to the noise - but during the building of those units - the parts they used to remove or attempt to isolate the noisy buss and clocking issues from the rest of the receiver - they were of high quality. Good crystal filters and used a bias design not seen since - but worked well for listening for quieter stations if the radio was "built and installed" correctly.

    Hence your bootstrap "shrill squeal" sound.

    So, a re-cap is in order but hope the links above can help you get started.

    Regards!
    :+> Andy <+:
     
    #2 Handy Andy, May 14, 2018
    Last edited: May 14, 2018
  3. nomadradio

    nomadradio Analog Retentive

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    Any chance that this radio was connected to the DC power source in reverse polarity some time in the past? If D306 is missing or appears to have been changed, it's possible someone connected the power in reverse with a fuse that was too large.


    A less-than-wild guess would be that a 35 year-old radio has a bunch of 35-plus year old electrolytic capacitors in it. Sometimes they will fail as a short circuit, and shut down some function in a radio.

    Other times they fail as an open circuit, allowing unwanted feedback between circuits that are supposed to be isolated from each other.

    Have a look at C228, a 33uf electrolytic connected from pin 3 of the audio power-amp chip to ground. Haven't checked the schematic for what's inside this chip, but if C228 goes bad you'll have audio trouble. Pin 2 should normally show a slightly-smaller copy of the chip's audio output waveform when the chip is operating normally. Pin 3 should have only DC voltage on it, no audio waveform should be visible.

    Unless C228 has failed open-circuit.

    You did say the final gets hot. Does anything happen when you key the mike.

    Anything at all?

    73
     
  4. PoDuck

    PoDuck Active Member

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    It is a 40 plus, with the flat brushed metal face, and just as strange, I found a bad voltage regulator, replaced it, and now all the problems except for the final getting extremely hot are fixed. It was IC205, which is an 8 volt regulator that was not putting out any voltage. I have no idea how it accounts for all those problems though.

    By the way, that 1.62Khz tone was a very nice looking sine wave, not a square wave.

    As for the final, it is putting out almost exactly 4 watts, but it is getting blazing hot, even when it's not transmitting. It modulating at about 97% with a 1khz tone applied. The only thing I see that is a potential problem is the overheating final. Otherwise, right now, it seems to be working just as it would have been from the factory.

    I'm not talking about it just getting warm, by the way. I found out it was getting so hot because I accidentally touched it with the back of my hand, and now I have a blister. I'm not used to that happening and it still working.

    As for recapping it, if it was a radio I wanted to keep, I would have done that already. Although this is a very nice looking radio, and it has the original box, power cord, microphone, owner's manual, and even the bracket, I don't think it's worth the time for me to recap it, considering I might get $20 or $30 out of it, if I'm lucky. I am hoping to just get it working well enough that whoever I sell it to can use it, and let them know that they can pay me to recap it if that's what they want to do. :)
     
  5. Handy Andy

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

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    I'm interested...lost one to a garage sale ...

    PM me here at this site - thanks!
    Regards!
    :+> Andy <+:
     
  6. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Flip over the radio to the solder side of the PCB, and desolder/lift the base leg of the final. It'll be the right leg of the final with the solder side of the PCB up, front of the radio facing you. If the final cools off and no longer gets hot, there's a biasing issue meaning there is too much voltage being applied to the base of the final. Check it out and let us know how you get on.


    ~Cheers~
     
  7. PoDuck

    PoDuck Active Member

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    Well, I'll check it, but I've had it on for a while now, since I fixed the voltage regulator, and while it gets somewhat warm without transmitting, it doesn't get near as hot as it did earlier. I may not have tested it enough after the change. I'm will still check that though.
     
  8. PoDuck

    PoDuck Active Member

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    Okay ExitThirteen, I checked the voltages before I pulled the leg up, and it was:
    RX: E=0.001V, B=0.593V, C=12.3V
    TX: E=0.000V, B=0.001V, C=10.77V

    After pulling the base leg up, I checked RX only:
    C = 8.357V, B = 0.005V, E = 0.009V

    For the base, I tested the pad where the base was, not the leg itself. After tacking it back down, I realized I should have tested voltage on the leg as well.

    When I pulled the base up though, the final got much hotter, much faster than before.
     
  9. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    The base voltage looks alright, so that's not the issue. What you can also do is unbolt the final, and reapply thermal compound on the final and the insulator. That will also help cool the final. Run the radio as is and see how the final does.

    ~Cheers~
     
  10. PoDuck

    PoDuck Active Member

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    Yeah, something is definitely wrong. Where it was putting out a hair over 4 watts at first, it is now only putting out about 2.6 watts, and it is still getting blazing hot, even in receive.

    As for those voltages, the service manual says that the base and emitter should be at zero in receive, but the base isn't as close as I would expect it to be, the collector should be at 13.6V, and it's lower than that. When I pull up the collector, I get 13.3 volts on the pad, and the radio pulls about 1/2 amp less power.

    It would seem that the final would just simply not work if it were bad, but something is pulling some power that probably shouldn't be.
     
  11. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Sounds like there could possibly be a leaky disc capacitor in the final area in the radio.


    ~Cheers~
     
  12. Handy Andy

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

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    I can vouch for that, had a PC122 pop the driver - due to the age and discs that seem to "absorb" water/flux crud and then one day - poof...

    :+> Andy <+:
     

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