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Post by 238 on May 5, 2019 23:53:23 GMT -5
Radio looks new, but has no receive. Has transmit. Keys 5 watts swing 9, but does not show any incoming signal. 238
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Post by 321 treeclimber on May 6, 2019 7:29:09 GMT -5
Great radio. Run one in the mobile. No activity on the s-meter?
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Post by 238 on May 12, 2019 20:50:57 GMT -5
Nothing on the meter. The usual mods you see done to these radio are not done. But they have put a 220 ohm resister across L11. They soldered a #39 cap across two of the legs on the final. If you are looking at the radio from the rear it is the left & center. There are a couple of more caps added. 238
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Post by 238 on Jul 6, 2019 22:29:26 GMT -5
In all the mods for this radio, i have not seen these mods referred too. Still no receive. Audio chip is good. 238
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Post by 321 treeclimber on Jul 7, 2019 5:04:58 GMT -5
Thats my mobile radio too. I've run one for the passed year. Sounds like someone was told a cap could do magic things, so they went home brew mod crazy. Pull them all and start over. 321
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Post by 2600 on Jul 7, 2019 22:19:58 GMT -5
Do you have a meter?
A schematic?
Hard to offer a specific suggestion about troubleshooting if you don't have a way to zero in on a particular circuit or part.
If you can find radio's crystal filter, turn it on with the volume up and turn right-side up with the circuit board's solder side facing up. Lay one finger along the shaft of a small screwdriver. Touch the tip of the screwdriver to the input solder pad for the crystal filter. You should hear some noise. Now move to the output terminal of the filter. Touching it with the screwdriver should get you a loud riot of noise.
If you get nothing, the trouble is downstream from that filter.
And if you get noise, the problem is upstream, somewhere between that filter and the antenna socket.
Gotta have a place to start.
73
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Post by 238 on Jul 9, 2019 16:30:54 GMT -5
What is the # of the crystal filter in the Uniden? I have a sch for a Cobra 25 but not a Uniden PC68. Have not been able to turn up one for the Uniden. I know they are alike but the #'s are different on some of the parts. 238
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Post by 2600 on Jul 9, 2019 22:18:46 GMT -5
Closest I can come is the PC66LX schemo from CB Tricks The 10.695 filter FT1 is probably a ceramic type, not a quartz filter that looks like a channel crystal. Those are smaller, usually covered in a brown ceramic layer and have three legs. The 455 kHz filter FT2 is a ceramic type, in a small plastic block frequently but not always black. Has three pins, the middle one is ground. I'll try to remember to shoot a pic or two next time I'm at the shop. 73
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Post by 238 on Jul 12, 2019 7:32:06 GMT -5
Thanks, that helps a lot. Looking from the top it looks like a small disc cap. Have not had time to look at the radio, but with that sch maybe i can make some progress. I need to buy me a cheep scope, if i new what to buy. Thanks again. 238
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Post by 238 on Jul 17, 2019 15:02:52 GMT -5
I tried the screwdriver trick, but did not get any noise on either side. When looking from the front which side is the input for FET1? If i put my finger on FET2 i get noise & on the audio chip. 238
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Post by 238 on Jul 24, 2019 0:44:57 GMT -5
Got no noise on either side of FET1. I have a Hickok 386 tester & a RS frq counter. Put a probe on counter & i get a 10.240 reading on both sides of it. There is a small diode off to the right of FET1 close to the edge of the broad. I get a 16.965 reading on it. I believe I was getting a 10.240 reading on FET2, not sure, getting late i will recheck it tomorrow. 238
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Post by 2600 on Jul 24, 2019 22:29:07 GMT -5
The 16.965 MHz reading would suggest the radio is trying to operate at 27.660 MHz.
Sounds as if the PLL is not locked onto the selected channel. This radio will shut down both the receiver and the transmit side when this happens.
If you'll find pin 4 on the PLL chip, you should have around 5 Volts DC if the PLL is properly locked onto the channel.
A lower voltage means the PLL has a problem and can't lock onto the selected channel.
Never learned how to troubleshoot a PLL without a 'scope. Somebody out there sharper than me may have figured out the drill for that.
73
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Post by 238 on Jul 24, 2019 22:59:57 GMT -5
I know it keys & swing & the pa works. But tomorrow i will recheck the output fq with it keyed. An check the pll pin #4. Thanks 238
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Post by 238 on Jul 25, 2019 20:53:46 GMT -5
Checked the output keyed up. It is 27.205 ch selector on ch 20. The pin still shows 16.965, but will change to 16.510. Pin #4 on the PLL shows 4.32v when checking on the solder side, but if check #4 leg at the PLL it shows 7.42v. 238
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Post by bobcat4109 on Jul 25, 2019 21:21:47 GMT -5
16.965 - 16.510 = 455 KHz. They are shifting the VCO one way in TX to generate the 27 MHz for transmit and the other way to feed in to the receiver strip to create your 455 KHz IF. I'm not looking at the schematic at the moment but I'm sure that is what it is doing. If you follow the output of the 16MHz VCO towrds the transmit there will be another crystal oscillator that mixes with it to give you your 27 MHz channel frequencies.
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Post by 238 on Jul 28, 2019 19:57:51 GMT -5
I rechecked the voltage on pin #4 & figured out i was checking the wrong pin. The real #4 checks 7.92 volts. 238
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Post by 2600 on Jul 28, 2019 21:59:04 GMT -5
Clearly I was wrong and the PLL is not unlocked, and must be okay if you can transmit.
Still looking for a PC68 schematic. Clearly they have changed the schematic component-callout numbers from the PC66 model. Makes it tougher to offer a specific suggestion if I can't tell you what a component is marked.
73
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Post by 238 on Jul 29, 2019 18:52:16 GMT -5
The pin mistake was made by me. I was counting from the outside end of the PLL, should have been from the inside end. Should not work on radio when tired. When i looked at it Sunday afternoon it jumped out at me. I swapped out a Cobra 25 for a working 68 that was in the big truck. So now i have a working PC68 to go by. 238
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Post by 238 on Jul 30, 2019 19:25:03 GMT -5
My Hickok test center only transmits on ch 20. Using my good radio i got a 16.510 on the center leg of Q1. The radio i am working on had nothing. I could see someone had solder on 1 leg of Q1 so i solder it & now i have 16.510 on he center leg. Still no receive. I reckon it could be bad, but have no way to check it. I will have to go pick up some transistors. 238
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Post by 238 on Aug 3, 2019 0:46:50 GMT -5
Changed the Q1 transistor still no receive. I am picking up more noise than before. If i put my fq counter probe on the solder joints around FET 2 or Q2 i pick up some radio broadcast. Not sure what it is i am listen too.. Jim
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