First time writing. Read a lot of good info here so I thought I'd pick y'alls brain. Got this old 25 ltd made in Taiwan. Only way I know how to describe it is its like the squelch is turned way up. Rf gain is wide open. Also when I turn squelch up you can hear a pop and it's completely silent like the squelch is working. Where do I start checking? Forgot to add I don't have a scope yet.
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Cobra 25 ltd won't receive weaker signals.
- Thread starter Collin Barnett
- Start date
YouTube needs a video, "How to align the receiver of a Cobra 25 for first-timers".
Since the adjustments inside that radio are irresistible to someone trying to get more transmit power, sometimes the receiver adjustments get disrupted by someone focused on the wattmeter, randomly turning EVERY adjustment one by one while whistling into the mike. When the tweaker unkeys the mike, he discovers the radio won't receive like it used to. Only a few of those adjustments were for the transmit side. The rest of them are in the receiver side of the radio.
Checking the "alignment" as it's called is a diagnostic first step any time a receiver has lost its sensitivity. If you find tuning slugs that are set way off from their 'peak' settings, this might fix it right there.
But you need a steady source of a receiver signal that is just loud enough to hear. Keeping the test signal weak makes the peak setting of each adjustment easier to see on the receiver's meter.
So there's the rub. How do you check the alignment of a receiver without a steady weak signal feeding into it? Doesn't make economic sense to buy an expensive gadget like a signal generator to fix a radio many people would consider "disposable".
Could be that the loss of receiver sensitivity will be fixed just be restoring the receiver's adjustments to their proper peak settings. Only one way to find out.
Even if you don't get back full sensitivity of the receiver by checking the alignment, the behavior of each adjustment can help pinpoint other kinds of failure. A slug that won't "peak" properly when turned can tell you that the circuit around that adjustment has a fault. But without some experience of which adjustments normally have a "sharp" peak when turned, and which ones do not, this won't be as helpful.
Can't replace piston rings without a ring compressor. Don't know how to bring back a 'dead' receiver without some sort of steady signal loud enough for it to hear.
Finding someone sympathetic nearby who has the tools you don't have might be your best bet.
73
Since the adjustments inside that radio are irresistible to someone trying to get more transmit power, sometimes the receiver adjustments get disrupted by someone focused on the wattmeter, randomly turning EVERY adjustment one by one while whistling into the mike. When the tweaker unkeys the mike, he discovers the radio won't receive like it used to. Only a few of those adjustments were for the transmit side. The rest of them are in the receiver side of the radio.
Checking the "alignment" as it's called is a diagnostic first step any time a receiver has lost its sensitivity. If you find tuning slugs that are set way off from their 'peak' settings, this might fix it right there.
But you need a steady source of a receiver signal that is just loud enough to hear. Keeping the test signal weak makes the peak setting of each adjustment easier to see on the receiver's meter.
So there's the rub. How do you check the alignment of a receiver without a steady weak signal feeding into it? Doesn't make economic sense to buy an expensive gadget like a signal generator to fix a radio many people would consider "disposable".
Could be that the loss of receiver sensitivity will be fixed just be restoring the receiver's adjustments to their proper peak settings. Only one way to find out.
Even if you don't get back full sensitivity of the receiver by checking the alignment, the behavior of each adjustment can help pinpoint other kinds of failure. A slug that won't "peak" properly when turned can tell you that the circuit around that adjustment has a fault. But without some experience of which adjustments normally have a "sharp" peak when turned, and which ones do not, this won't be as helpful.
Can't replace piston rings without a ring compressor. Don't know how to bring back a 'dead' receiver without some sort of steady signal loud enough for it to hear.
Finding someone sympathetic nearby who has the tools you don't have might be your best bet.
73
Thanks for the reply nomad! I figured it would be difficult to test and try to align receive. Guess I need to start researching for some test equipment.
How much do i really want to spend to fix a 25 would be my first question. Second would be how often will I use this test equipment, is it worth buying for limited use? Decent test equipment isnt cheap. Just something to chew on.
Have you opened it up to take a look inside yet...
Taking a good set of high quality good focused pictures can help you in finding problems you will have to repair - just use your phone - keep them in a folder or your photos and review them, zoom in, pan around...
You say it's an old Cobra 25, well - how old - is old?
If you can, you can locate the approximate year it was made, they're printed inside - on one of the chips the Radio has.
The Big rectangle one with all the pins on it - that should be the PLL - it's over by the channel selector.
This is a similar but not exact board...
See that "black rectangle chip with all those pins?"
There's a name, Manufacturer, and type of chip - and then the Date Code
97 - refers to Year circa 1900's - 1997 - 9 month 2nd week. In the one above...
If the '25 is really old, it may need a lot of things needing to be done - replaced - like that big black round can right next to the one in the above photo, it's an electrolytic - it filters the power supply feed to this chip - you'll have similar in yours.
You can also look for similar board radios like the TRC-427 or the Uniden PC-66 or PC-68 - they use similar design - to help you get started.
Just use the Search Box in the upper right corner and look for say PC-66 no RX you'll find quite a few threads out there.
Good luck and be patient...
Taking a good set of high quality good focused pictures can help you in finding problems you will have to repair - just use your phone - keep them in a folder or your photos and review them, zoom in, pan around...
You say it's an old Cobra 25, well - how old - is old?
If you can, you can locate the approximate year it was made, they're printed inside - on one of the chips the Radio has.
The Big rectangle one with all the pins on it - that should be the PLL - it's over by the channel selector.
This is a similar but not exact board...
See that "black rectangle chip with all those pins?"
97 - refers to Year circa 1900's - 1997 - 9 month 2nd week. In the one above...
If the '25 is really old, it may need a lot of things needing to be done - replaced - like that big black round can right next to the one in the above photo, it's an electrolytic - it filters the power supply feed to this chip - you'll have similar in yours.
You can also look for similar board radios like the TRC-427 or the Uniden PC-66 or PC-68 - they use similar design - to help you get started.
Just use the Search Box in the upper right corner and look for say PC-66 no RX you'll find quite a few threads out there.
Good luck and be patient...
Absolutely, the zoom feature on most phones is a great magnifying tool but like Andy said take a set of pics not just one. Sometimes dirt and shadows can play tricks so its good to inspect from different angles. A lot can be diagnosed with a decent multi meter but as was previously mentioned if a golden screwdriver was inside you may need more test equipment to straighten it out.
Judging by its age it may be time for a recap also. Radios built in the 80's-mid 90's are coming due for a freshening up.
Judging by its age it may be time for a recap also. Radios built in the 80's-mid 90's are coming due for a freshening up.
I'm wanting to get into fixing my own stuff and others anyways. Just gonna buy little bit at a time.
Thanks for the reply! I looks like my pll says 8520H. I'm trying to figure out how to post pics. I found a spot that has leaked out some black stuff next to L5. I found it by just adjusting L1-7 listing to static change. I know that's not the proper way to do it but... Anyways I got to L5 and the noise never changed so I looked around and there it is. I'll try to post a pic as soon as I figure out how lol
It will need some TLC...
Go here to this link - it can provide you some help...
http://www.cbtricks.com/radios/cobra/25ltd/index.htm
Go here to this link - it can provide you some help...
http://www.cbtricks.com/radios/cobra/25ltd/index.htm
Andy thanks a million! I was trying to search and see what diode I could replace D18 with. I believe I have some. What should I use to clean that old glue off? Alcohol? Or?
Works like a charm now! Thanks for all the info! Now I'll have to wait for equipment to do proper alignment.
For me I just would have tried to gently remove it using alcohol and a Q-tip swab.
But that holds another part - the Black Box, is a Murata IF filter.
You can still find those filters, but their performance "Curve" and or pinout connections - may not fit that board. The OEM version - may be price prohibitive - at least in the light of how much effort and value is this radio worth to you it can be a tough choice.
If the radio works, you're far ahead not to mess more with the glue in trying to get it off.
May be safe to reheat some of the soldering joints that connect parts to the board. This may help refresh the connections as the heat the iron puts into the joint can help reflow some of the solder .
Why? for some parts it "touches" -when you remove the glue, it will damage the part that was originally done one way but as time moved on - they found different and sometimes (unfortunately) cheaper means and methods to do up parts with similar performance but the size or it's pinout (means to connect) is now different - that configuration - many are no longer around - so the adage of; "If it works - don't fix it" - applies.
A simple reheat can help in removing some of that glue, heat can help in softening - but don't get too aggressive in trying to remove all of it - for some if the glue needs to hold the part in position - if its' removed - now the part can shift and loses it's ability to keep in tune (or stay on the board) - now what? Well, then you're scrambling for a way to reposition the part correctly, like it was - to keep it working.
If it's like, some ones' radio from the family and they gave it to you - then by all means take the time to learn from it - you don't have the basic building blocks of communications equipment handed to you every day. It's a heirloom at the same time a lot of memories are there from the one that used that radio before you.
I think you've found a point where you can relax a little and take more time to study that radio.
It's a simple and neat way to look at something they scaled down, pared dawn - to the basic levels that when compared to the complex equipment available today - you're far better off keeping the simpler equipment working.
To try and fix the now nearly microscopic parts used in the newer ones that will fail if dropped or wired up wrong - it's easier to dispose of it and buy another.
You're holding a piece of history...
But that holds another part - the Black Box, is a Murata IF filter.
You can still find those filters, but their performance "Curve" and or pinout connections - may not fit that board. The OEM version - may be price prohibitive - at least in the light of how much effort and value is this radio worth to you it can be a tough choice.
If the radio works, you're far ahead not to mess more with the glue in trying to get it off.
May be safe to reheat some of the soldering joints that connect parts to the board. This may help refresh the connections as the heat the iron puts into the joint can help reflow some of the solder .
Why? for some parts it "touches" -when you remove the glue, it will damage the part that was originally done one way but as time moved on - they found different and sometimes (unfortunately) cheaper means and methods to do up parts with similar performance but the size or it's pinout (means to connect) is now different - that configuration - many are no longer around - so the adage of; "If it works - don't fix it" - applies.
A simple reheat can help in removing some of that glue, heat can help in softening - but don't get too aggressive in trying to remove all of it - for some if the glue needs to hold the part in position - if its' removed - now the part can shift and loses it's ability to keep in tune (or stay on the board) - now what? Well, then you're scrambling for a way to reposition the part correctly, like it was - to keep it working.
If it's like, some ones' radio from the family and they gave it to you - then by all means take the time to learn from it - you don't have the basic building blocks of communications equipment handed to you every day. It's a heirloom at the same time a lot of memories are there from the one that used that radio before you.
I think you've found a point where you can relax a little and take more time to study that radio.
It's a simple and neat way to look at something they scaled down, pared dawn - to the basic levels that when compared to the complex equipment available today - you're far better off keeping the simpler equipment working.
To try and fix the now nearly microscopic parts used in the newer ones that will fail if dropped or wired up wrong - it's easier to dispose of it and buy another.
You're holding a piece of history...
Thanks for the info! I'll just leave the old glue be.
The radio was give to me, so I'm just learning as I go. I've got several radios with various problems I'm trying to figure out. If I get stuck I know where to turn for some great info!
Thanks again Andy!
