Question about Uniden PRO510XL 40-Channels Base CB Radio

2 Answers

Uniden PRO510XL low output and "motorboating"

My Uniden Pro510 XL CB radio has reduced rf output. As the transmitter is held on the output starts out at about half power and slowly decreases. Also it will go into a "motorboating" condition when modulated or when the power level decreases and it is keyed on again. I have replaced several of the electrolytic capacitors, but the problem persists. It is transmitting and modulating on the proper channels, and the receiver seems to be normal. Hope one of you techs have had experience in working on this model. Thanks. Bob W.

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  • rdw0yoc Jan 19, 2008

    Thanks for your input. I should have stated that as far as I can tell this radio is virgin. It does have the original Uniden mic, and I have heard of Rife/bare systems, but I have no idea what that is. The audio amplifier is the original TDA1905 IC --- and something I also neglected to mention is that I have removed the 470MFD modulation output capacitor to help isolate the audio amp from the rf stages. (It didn't help). Audio output from the amplifier looks normal to me -- and clips at 5 volts in both directions at the amplifier output pin (1). The voltage on mic pin 3 is about 8vdc, and the unit continues to motorboat with the mic disconnected and pin 3 grounded. Again --- the RF output is very low and fading with time --- maybe a half watt, as read on my BIRD Thruline watt meter. Oh yes -- you mention VR2 -- isn't that just the output power level indicator calibration? There are just two segments showing on the front panel rf out indicator. What is the function of Q6? Is there a good schematic or service manual anywhere for this radio? I will continue replacing electrolytic capacitors at this time. Thanks again for the assistance!

  • rdw0yoc Jan 19, 2008

    Good thought --- but I have not been able to detect any obvious bad solder joints --- but I will continue to be on the lookout for them. Thanks for replying!! Bob W.

  • rdw0yoc Jan 19, 2008

    Thank you for the advice on solder joint examination. The CB radio belongs to my son, who bought it on eBay I believe. As to it's age, the circuit board says Copyright 1987 -- PA-375AA, and the serial no. of the radio is 55095536.

  • rdw0yoc Jan 19, 2008

    Thank you VERY much for the schematic! I will proceed to check the voltages that you list, etc. The reason I started replacing electrolytics is because I suspected a "motorboating" issue, and that is generally caused by insufficient powersupply bypassing. I find that old electrolytics have often dryed up and can cause such a problem ---- However, maybe my description is faulty -- what I am calling "motorboating" is pulsing RF output -- it starts out with clean CW, albeit at low RF output, then decays and starts rapid bursting --- a few short bursts, a longer burst, and then a repetition of that --- etc. On a receiver it sounds like "motorboating". I will see if I can figure out how it works now that I have a diagram --- Thank you for the support!!! Bob W.

  • rdw0yoc Jan 19, 2008

    YEA!! --- IT'S FIXED --- And do I feel stupid!! Would you believe it that all that was wrong was a bad fuse holder??? The 13vdc supply was falling to below 8v when the transmitter was keyed on due to a corroded connection in the fuse holder! Now why I didn't check the primary power supply AT THE radio I cannot answer! My heartfelt thanks go out to the great support you gave me toward trouble-shooting this CB radio! Thank you again!! Bob W.

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  • Master
  • 6,966 Answers

Hi,

Based on your post/description, I am assuming that:
1. The standard Uniden microphone that came with the radio has not been replaced with a power microphone;
2. The amplitude modulation method (transformer) is not modified;
3. The Power Output has not been modified;
4. The radio is not being used in a Rife/bare system.

Your posted problem may be caused by:
1. Mic. You can verify if your mic is causing the motorboating by grounding pin#3 to transmit without any input audio/mic connected; with pin#3 grounded and no audio in, determine if you still have the motorboating problem. If no then chances are its the mic, if yes, then its internal to the radio;
2. Audio amplified IC and/or its associated components. Check the audio amp is TDA1905, capacitors C50 to C58;
3. AMC (amplitude modulation control). VR2, Q6 likewise check the Limiter Q5.

In some versions, a microphone with an Electret Condenser Microphone Element is used. This type of mic uses 2.8~3.3 volts DC from the radio. These versions may use the audio line (pin#2 in a wire pin mic plug) or a separate +3V mic power (in a 5 or 6 pin mic plug). Motorboating then may be further caused by the dropping resistor that has gone up in value or by the capacitor after the resistor that has lost its capacitance along the +3V line. Indication low or no +3 in pin#2. In some instances, the +3 goes down when talking into the mic.

Hope this be of help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards.

Posted on Jan 19, 2008

  • 2 more comments 
  • Louie  Role
    Louie Role Jan 19, 2008

    for Cerfer39,

    It would seem that you have posted while I was composing my reply. Sorry to have stepped in.



  • Louie  Role
    Louie Role Jan 19, 2008

    Hi again,

    Appreciate the quick reply/post. By removing C51 you really have elimated the audio (RX/modulation) as the possible source of your problem. My bad on the VR2, you're right it being the RF meter adj, was thinking of something else. Q6 is modulation audio limiter but as earlier stated should not be considered.

    Concentrating on the lo power, pls verify that:
    1. the collectors of Q501 & Q2 should have +13VDC, Q3 11.2 & Q4 6.2 on TX;
    2. the emitters of Q501 & Q2 should read zero, Q3 2.5 & Q4 2.4 on TX;
    3. the emitters of Q 3 & 4 are not directly grounded but passes through resisors R54 & R57;
    4. the +6.2 on the collector of Q4 is limited by Q5 who in turn is supplied by 8V voltage regulator IC6;

    I would presume that the reason you are replacing the capacitors is that you suspect anyone of them to be defective. But other than capacitors, have you considered also the regulator L7808CV? If the +8V is low or under current, then you would have fluctuating +8 supply in the early stage of the RF amp (Q4) and RF source (Qs 701 & 702). There is also a zener tied to the 8V line, D18 (?).

    I have a schematic but postal service from my country would take 22 days to reach US, FedEx can do in 48-72 hours but kinda expensive (30USD). I will try find a downloadable schematic.

    Cheers.

  • Louie  Role
    Louie Role Jan 19, 2008

    and the link to a schematic of a Uniden PRO 510-XL..

  • Louie  Role
    Louie Role Jan 19, 2008

    That's what dads are for. LOL Glad that it's now working.

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  • Master
  • 709 Answers

Hi

It sounds like your circuit board may have a bad solder joint. Sometimes during manufacture of the ciruit board, the joint could not be perfectly made. This will limit conduction and create heat which will worsen the effect over time. If you are familiar with opening and working with electronics then I suggest you remove the circuit board and inspect it for cold solder joints. When you find one, you can use a Cigar Torch type lighter to help reflow the solder easily. Out side of that, you mayu want to send it to a Electronics repair shop to do this for you. Let me know more and I will help. Good luck

Posted on Jan 19, 2008

  • Greg Stokes
    Greg Stokes Jan 19, 2008

    Benimur,



    Thats ok, your input is good and thank you for the comment.



    rdwOyoc,



    When looking for Cold Solder joints, I use a 30 X jewelers loupe and bright Flash light to get a really good look.



    How old is this CB?

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