Question about Uniden Bearcat Pro PC68XL CB Radio

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How do you turn down the modulation on a uniden pro510xl?

My amp sqweels when you key up and my dead key is set at 1.5w. the modulation is the only thing that i could think of to try and turn down to get the sqweeling to stop....any ideas its a uniden pro 510xl with a texas star 550 and a wilson 5000.

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First things first. Make sure the antenna is properly matched to the radio and car, a bad match will most often cause squeal. Make sure you have a bonded ground direct to the battery of the vehicle and all must share the same ground. The amp and radio must be exactly the same ground potential. If you meter both the radio chassis and amp chassis back to the battery negative you should see the same reading of zero or .002 or less. Then meter the radio chassis to the amp chassis and make sure they have a zero ohms reading. This is the most common problem in radio and amp setups. Most often people get ground loops and this causes all sorts of issues like you describe. Rarely is modulation the issue unless you have an incorrectly installed MOD kit in the radio. Now if all checks out with a meter then meter the antenna shield to the outer part of the amphenol and make sure it is also at or darn close to zero ohms. The meter the center lead (at amphenol) of the antenna to the outer of the amphenol and make sure it reads 52 ohms or less. NOTE: if it is a magnet mount Wilson they tend to rust really bad between the magnet and the coil under the antenna and create a high resistance short. If you can't determine the antenna is good borrow another and test it. The Newer Wilsons since about the last ten years are made in China and fail often. I have the old one made in the USA and they still work if cared for. Hope this gets you on the right track, but never have I seen an issue with modulation causing squeal by itself unless someone went crazy turning it up way too high.

Posted on May 23, 2014

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Not getting enough voltage or amps will make them squeal,also. Run both the amp & radio directly to the battery using some heavy wire. I use #8,but I would like to have some #6. Also,those stock little mikes on the 510 are a lot to be desired. Get someone to let you try a GENUINE Turner Roadking 56 & see if it still squeals. Lots of good tweaks on CB Tricks site. Good luck.

Posted on May 19, 2014

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Posted on Jan 02, 2017

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SOURCE: Uniden PRO510XL low output and "motorboating"


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

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SOURCE: modification of frequency output of uniden pro 510xl

deweyp, hi, if you are wanting to turn the power up some on the radio, do this; tune L4, L5, L6, L9 for
maximum RF forward output. There are 2 versions of the radio, and both are listed as 510xl. If you are wanting to expand the freq's or add channels, then no, there is no mod for that radio. If your going to tune the radio for more power, make sure you have a power meter to watch the output. It is preferable to tune into a dummy load so your not broadcasting over the air. You tune the radio for peak output, using a steady awwww sound in to the mic. Just be careful as that radio has a fairly small driver and final transistor. Don't want to let the factory smoke out of it,,,
Here is what the little variable adjustments are for and what they control;
VR1 Squelch
VR2 TX Meter
VR3 RX Meter
VR4 RX IF Gain
Hope this helps you out, let us know,, Mechanic

Posted on Feb 19, 2008

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SOURCE: Uniden Pro 510xl can send but not receive

If you look on the solder side of board you shoud see a straight line of 5-7 solder joints, look on the other side the audio vhjp is a bmack thin retangular ic chip. Not to worry you wont hurt anything by doing this. Its an old cb tech trick to test audio chips.GOOD LUCK.

Oh one other thing does the radio have same same problem in either vehicle? A insulating washer on the antenna bracket can cause this also,
Happy New Year , Tom

Posted on Dec 31, 2008

  • 39 Answers

SOURCE: Uniden PRO510XL Outup

The Uniden 510 xl is quite possibly the easiest and one of the loudest radios I have ever pumped up .Now here's the problem it's been atleast ten years since I've had the cover off one so I can't tell you which diode # it is you clip for the modulation I know that as your looking at it (speaker side up ) it's about halfway on the right hand side buy the audio transformer.Then you change the resistor buy the final transistor , you go higher until you get where you want it , going higher is going to bring your output wattage down ; I usually tried to get it down around two watts . By doing just these two things and maybe a little adjusting you should be able to deadkey 2 watts and swing to 18 watts with just that cheap stock mic. if you change the final transistor I've seen them swing to 35watts.They are fun cheap radios that when you do this no one beieves that that's all your talking on . Good Luck !

Posted on Feb 04, 2009

  • 773 Answers

SOURCE: 1987 uniden pro 510 xl

Wow, from what you are saying, you should not be messing with the insides of the radio. Finals, are not resistors, that are TRANSISTORS, and they do not have resistance values at all.

The final is the little black transistor attached to the heat sink at the back of the radio. You can't just change the final. there are other parts that need to be changed if you want to do a high power mod. Don't even mess with it. you don't need too.

Next, D14 is not the uniden model. thats what you mess with in a cobra. Don't cut d14 in that radio or you will prolly blow it up. The diode you want to cut out is D203. DON'T CUT IT!!, Just un solder one leg of the diode. You won't be over modulated. That will bring you modulation up and your peak watts. it will swing to maybe 15-20 watts peak. That alone with make the radio louder.

You really don't need to raise the dead key. With a lower dead key you have higher swing. Its all about the audio. 2 watts for dead key is fine. Thats what I run.

You cannot do any other peaking with out a meter. If you have a meter you can spread the 2 coils behind the large transformer a tiny bit until you have peak wattage. L201 and L202. but from reading what you said, you should probably leave them alone, you will gain only a watt or 2, and its not worth it if you mess it up.

To be honest with you, all you really need to do is unsolder one leg of D203. Make sure you pull the leg out, and make sure it is not touching anything so it will not short.

Now if you do this, and it splatters, you can always solder it back in, and then turn RT201 all the way to the right, I think. If you turn it all the way to the right, and there is not modulation, you just turn it all the way to the left.

Please don't mess with anything else or you will screw the radio up.

Posted on Oct 03, 2009

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