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Shakymcquiver
New member
Username: Shakymcquiver
Post Number: 1
Registered: 4-2018
| | Posted on Friday, April 13, 2018 - 4:55 am: | 
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Picked this radio up for a few bucks... I figured if anything it's good for parts... but before I do, I thought I'd give a shot at fixing it.
Problem:
NO transmit or receive. All other lights, weather, channel LED's, etc.. all appear to work.
My power meter or frequency counter shows nothing when the mike is keyed, nor anything when whistle into mic.
I did notice a slight increase in amps on my BK bench power supply when the mike is keyed; not much, perhaps .1 or so... Enough for the power supply to notice.
I also cannot find much information on this rig, but from what I gather, it's a Cobra 25 more or less. So does that mean I can use (or follow) the information for a Cobra 25? Just curious. I have SAMS for the 25, so I'm hoping that will help if I have to do this the old fashioned way and dig out my old Textronix scope and go to town...
I'm hoping a guru on here might save me some time and point me in the right direction? It's been several years since I've cracked open a radio, but now that I'm retired, I get bored real easy. Not to mention if I don't prove to the Wife my old radios are still worth keeping around, she might just toss them out! YIKES1
Thanks in advance! |
Shakymcquiver
New member
Username: Shakymcquiver
Post Number: 2
Registered: 4-2018
| | Posted on Sunday, April 15, 2018 - 1:37 am: |
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Update:
The PA works fine as does the Weather function, but NO transmit or receive. I'm getting 10.240 Mhz on the reference crystal, so something is working but stops somewhere.
I'm using a Cobra 25GTL Sams and schematic.. several of the components and some test points appear to be the same, but the circuit board traces certainly aren't, so that makes it hard to find the right places to test. |
Tech237
Moderator
Username: Tech237
Post Number: 1916
Registered: 4-2004
| | Posted on Monday, April 16, 2018 - 1:03 pm: |
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The minimum increase of current when keying the mike shows that, at a minimum the RF Amp is not drawing power and I suspect a lot more than that is dead.
The fact that the PA works, shows that the audio stages are fine. Does the weather functions use the same antenna connectors the CB? Are you trying Transmit/Receive on AM or SSB (if on the radio - sorry I do not know the radio mentioned)? I know you say there is no receive, but is that just no audio or no meter movement when you know there is a signal there?
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Shakymcquiver
New member
Username: Shakymcquiver
Post Number: 3
Registered: 4-2018
| | Posted on Monday, April 16, 2018 - 4:55 pm: |
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Success! I found the problem... turned out to be a bad solder joint near the VCO.
The first thing I do to any problem CB is to simply flex the board. I've found several bad solder joints this way, but not on this PC68. These bad joints weren't easy to spot. They looked like a double-ring. In effect, it was cracked perfectly around the joint.
The way I found it was using a Cobra 25 SAMS and probe each test point. In this case, TP3 wasn't showing the frequency as it should, so tracing back and bingo... found it. The PLL checked out fine as did the finals.
My scope also showed SOME modulation and power out. My watt and frequency counter wasn't sensitive enough to see this, but my scope was, so I knew the radio wasn't completely dead.
So if you're getting strange results when testing - check those solder joints. |
Tech237
Moderator
Username: Tech237
Post Number: 1917
Registered: 4-2004
| | Posted on Tuesday, April 17, 2018 - 8:52 am: |
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Great job Shakymcquiver
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Shakymcquiver
New member
Username: Shakymcquiver
Post Number: 5
Registered: 4-2018
| | Posted on Wednesday, April 18, 2018 - 4:32 am: |
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Thanks for your reply... Once I found some activity on my scope, and the PLL was working as it should, but lost freq around the VCO, I figured something had to be broken or burned nearby... I got lucky I think before I had to start injecting signals and going at it the hard way. I guess I paid enough attention in my college electronics studies all those years ago... ! I haven't really messed with this stuff in 10-15 years.
CB's aren't used that much anymore (at least around my area), but they are still fun to tinker with, especially when the outside weather is too cold to do anything. Next up is modding an old 148 and Cobra 2000 I drug out of the garage... Time to dust them off and the Silver Eagle mikes... Ahhh the memories! |
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