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Uniden pc68xl receive issue

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w4ffx

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I have a uniden pc68xl, transmits awesome. receive is great for about 3 minutes. it slowly fades down to nothing. as if i was slowly turning the rf gain knob down. then it goes to complete quite. I am thinking maybe a component is heating up and failing. maybe a cap or transistor. Anyone have any ideas, i believe this is the same chassis as cobra 25 and uniden pc66xl. here is a youtube video of the issue.

https://youtu.be/aLUAr5QQ15Q

please help. i like this little radio.
scott
w4ffx
 

KC4RAF

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Saw your vid at youtube. Using skip won't do for signal checks. If you had a friend or some body that had a CB transceiver about a mile from you would be the best way to check. I take it that you do not have a signal generator.
It could be as you suggested that a component is over heating. Do you have the schematic for that transceiver? If not, look at the CBTricks (or something like that) website and see if they have one you can download.
Let us know how it goes.
(if you had another CB, you could put a dummy load on it and see how the signal comes into your 68xl. if you need more info on that setup, let us know.)


edit: just looked on cbtricks and they don't show that model. If some one could pipe in and let you know what other transceiver is basically same as the 68, might be able to find it at ...tricks.
 
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w4ffx

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i tested with another cb

i did what you said. it has about 1 s unit of signal with my radio transmitting 6ft from the radio with the issue. so seems like receive is very low. like rf gain is all the way down. not sure what in this circuit would cause this. i don't really understand the circuit. this chasis is the same basically as the cobra 25 and uniden 66xl
 

KC4RAF

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I will look at the schematic for the cobra 25 in a little while. Got to go to town here shortly.
My way of thinking, it's probably in the RF section because that's normally where it picks up voltage for the "S" meter. Do you have the necessary tools to check this with? VOM mainly. Of course there are other tools that should be used, but if you don't have them....well that's life some times! lol
But I'll give you a post in a bit.....
 

KC4RAF

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Looking at the Uniden pc66 schematic, (which hopefully is the same as the 68xl), check the 3 transistors, Q7, Q8, Q9 and the caps and resistors located in that area, (C143, C39, R19, ETC.). The coils are more than likely ok. The aforementioned transistors (Q from here on) are NPN and common if need replacement.
The IC1 could be at fault, but that would be the last thing I'd check. Just me on that.
If you could check those Qs and determine they are good, let us know.
 

JayMojave

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Hello RAF and all: Not having the P68 schematic, but having the CBTricks.com of the P66 which is supposed to be close, see:
http://www.cbtricks.com/radios/uniden/pc_66/graphics/uniden_pc66_sch.jpg

Did you mean Q1, Q2, Q3, and Q24 and Q25????? Q7,Q8, and Q9 are the transmitter transistors and are pretty close to the receiver front end transistors on the diagram. Or em I out to lunch on this?

Of course having the right service manual is a big plus.

With just a VOM you might be able to monitor the receiver voltages and find one that decreases or increases that correlate with the receiver going dead. Then you need to change parts in that section trying to fix it. Even with a signal generator and signal tracer which is the way I have done it a few times, you will have to change some components to fix the stage that has stopped working. Good luck.

Please let us know what all you find out.

Jay in the Great Mojave Desert
 

KC4RAF

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Yeah Jay, that was my fault. I was trying to do too many things at once.
The Qs that you mention are the correct one to check for reception.
The testing you suggest will help locate if possible the fault.
I also saw that L2 has a cap inside of the can which could be a problem, but generally when a cap goes bad it's a fast acting fault, not slow as he is describing.
Hopefully he posts what he is finding.
And thanks again Jay for catching my mistake, much appreciated.
 

JayMojave

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Hello RAF: OK good I wasn't sure, I don't mind being corrected, thought I would ask.

The real way to detect and isolate the lost and fading signal is of course with a signal generator and tracing the signal thru the receiver with a signal tracer or scope. I think he mentioned using a another CB Radio on a dummy load as the signal generator, I like that keeps it simpler. And of course to follow the signal thru the receiver you need a service manual to show the test points and such. Hope he gets it fixed.

Jay in the Mojave
 

KC4RAF

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If I'm wrong, I like being corrected. That way I learn from my mistakes!
Yeah, I mentioned to use a radio with a dummy load and try to trace the signal in the receive circuit.
And you are right about using a signal generator and tracing that way. Much more accurate, faster and less frustrating! But I figured he didn't have such test equipment and that's why I suggested the dummy load.
Gotta go to town again, so see you guys later.
73
 
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