I have a 25 ltd that has no receive or transmit. I compared the voltage on a working 25 ltd to the non working one. The voltage to the pll and the mixer on the nonworking one jumped all over the place. According to my frequency counter I have a signal from my 10.240 but the frequencies from at the pll are off.Does anyone here no what controls the voltage to the pll?
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cobra 25 ltd
- Thread starter eric brune
- Start date
No PA? Audio IC most likely. Seems like you have voltage coming in. Look for stuff that looks not right.
Good chance that the PLL is unlocked. Could be a bad solder joint or a bad PLL. Time to check the PLL with the voltage chart on CBTricks and see if it is working for starters. Got a bit of a road ahead of you on this one.
You could try the "heat and beat". Not my term. Essentially you reflow all the solder around the PLL one at a time. Flow and cool,, key the mic and gently tap the side of the case of the radio(beat). You may find that you will find one(or a few)pins on the PLL has a cold solder joint. I would certainly follow Robs recommendation first.
Slow down - nothing like the thrill of fixing your first radio...
But let's make sure the patient is stable first...
Can you at least put it back up on the table?
Ok, you have PA, then more than likely it's something more like a broken wire or a failed part or two that kills the PLL's ability to stay stable...
Ok you did check for and got 10.240 that's good, but there is a power feed that uses a Zener and a cap or two for filtering.
It also means you have access to a "scope" - correct?
So if the PLL isn't working - let's make sure it has a power supply to make it work...
To help me help you...
Do you have any markings or numbers you can read on the PLL chip - like 9106 or 25-LTD or 2816 - anything?
Does it have a working Channel display - do the LED's show what channel it is supposed to be on when you turn the channel knob?
I'm asking these questions to help determine if the PLL can be salvaged or you need another one.
Ok I'll stop here - catch your breath and gather up your instruments (of torture or otherwise) and lets' see about getting that thing to work...
The pll says 25 ltd 0350 on it. I do have a scope but I just bought it so I'm not very good with it. The voltages at the pll and the mixer keep changing. The channel display works but when I change the channel the frequency doesn't change.
Ok, that's is a big help....and also tells me about the "age" that radio is.
In my previous post I mentioned several different PLL "numbers" and you gave me one that tends to date the year to after 2000 but before 2014 - now they just have "Cobra 25 or KEIC 25 " on them which means another supplier is making the PLL chips.
So you're in luck, we may have a schematic or two for it...at CB Tricks...
Go here...
http://www.cbtricks.com/radios/cobra/index.htm
Look thru that list - there are several "variants" of the 25, one I call "hodgepodge" which is because of the way it looks to me... they lay out the board with things not just row and column - but diagonal and turnsy (no flames please) meaning helter-skelter. They got better at it in revisions and then another came in that has KEPC -XXX silkscreening on the board and uses more organized as row and column design and the foil side of the board is more "squared" and right-angled versus hand drawn in the etching.
So there are several different LTD's models out there - so to narrow down even further - look for the audio chip there are two types one like a flat SIPP - is it a long flat package of 10 pins bolted to the side with all the pins soldered along the edge of the board - or is It a small tabbed "looks like a TO-220" device bolted to the side with 5 pins soldered into the board extending out from it.
In any case, its' the best we can do to get you started in the right direction - find a schematic and layout that most matches' the silk-screening parts and their values - then work with that to help you get it fixed.
We'll be here to help you sort it all out...look thru all the 25's at the site, there may be a voltage chart or two for you to use from these - even in GTL models - that are pretty much the same except for the front panel looks and their GOD-forsaken ST NW WX LTD GTL II VI - which when you look at how Cobra modified 51 different models, 6 different platforms and seems to me that they make these in batches of 500 or so and putting it all together makes me LIVID about this - in the first place...if you lived in Rome - you'd be HOME...
In my previous post I mentioned several different PLL "numbers" and you gave me one that tends to date the year to after 2000 but before 2014 - now they just have "Cobra 25 or KEIC 25 " on them which means another supplier is making the PLL chips.
So you're in luck, we may have a schematic or two for it...at CB Tricks...
Go here...
http://www.cbtricks.com/radios/cobra/index.htm
Look thru that list - there are several "variants" of the 25, one I call "hodgepodge" which is because of the way it looks to me... they lay out the board with things not just row and column - but diagonal and turnsy (no flames please) meaning helter-skelter. They got better at it in revisions and then another came in that has KEPC -XXX silkscreening on the board and uses more organized as row and column design and the foil side of the board is more "squared" and right-angled versus hand drawn in the etching.
So there are several different LTD's models out there - so to narrow down even further - look for the audio chip there are two types one like a flat SIPP - is it a long flat package of 10 pins bolted to the side with all the pins soldered along the edge of the board - or is It a small tabbed "looks like a TO-220" device bolted to the side with 5 pins soldered into the board extending out from it.
In any case, its' the best we can do to get you started in the right direction - find a schematic and layout that most matches' the silk-screening parts and their values - then work with that to help you get it fixed.
We'll be here to help you sort it all out...look thru all the 25's at the site, there may be a voltage chart or two for you to use from these - even in GTL models - that are pretty much the same except for the front panel looks and their GOD-forsaken ST NW WX LTD GTL II VI - which when you look at how Cobra modified 51 different models, 6 different platforms and seems to me that they make these in batches of 500 or so and putting it all together makes me LIVID about this - in the first place...if you lived in Rome - you'd be HOME...
It's a 25 ltd classic that I bought brand new in 2007. The manufacturer date is 10/2006. It has the flat 10 pin audio chip.I have the schematic and voltage chart.
Then PLL don't "lock" - it means they can sync their own clocks to the system. They use a loop to look at another oscillator that makes up part of their loop...
That other section uses a "varactor" and the PLL uses a set of comparators to look at - or "tap into" that loop - then using it's own clock (the 10.240MHz one it has) is "adjusts" the other loop by sending a small correction voltage to that "varactor" and what that does is change the frequency of that loop.
Comparing the two loops is the main purpose of the PLL - you select a channel - the PLL tells that loop to "go here" in frequency. IF it can follow - you have a channel, if it don't you have a - what you consider a "out of Lock" condition. That is a truth - and is correct - but to presume out of lock because of .... hmmm - well, I'm glad you have that other radio because you'll need it to compare to.
So there are spots on the radio where you can safely look at how or what the radio is doing - they're call Test Points.
Some Test Points are DC related, so they can be checked with a simple DVM and if not right - you can do this to change that. If doing this doesn't help - then more service is needed - and the stupid manual STOPS right there.
Other Test Points generate a waveform - you can use a frequency counter on them, but it's better to use a 'Scope on the signal to see the waveforms' shape. It can be quite intuitive - for quality, degradation and strength of signal generated. Like TP 4 or Mixers or the IF - they can help you see what the radio is trying to do.
So you told me the radio was made in 2006 you might have to start replacing caps, but to know that we need to know if it can generate any RF - meaning it uses two types of RF to make your channel - the PLL issued one for the loop (~17MHz) - a tap off the 10.240 MHz Xtal for the other - and between the two you can also have a 3rd signal or RF as a DIFFERENCE between the two (455kHz).
The radio uses the two main ones (17MHz and 10.240MHz) to make up the TX frequency (27MHz) but when in RX it generates three ~17MHz + 10.690 - 455 - and your have RX. (note the + and - symbols) for the radio adds and subtracts thru heterodyning - and images are created as "sampled" wave energy from the original RF signal modified down to eventually the audio you hear thru the speaker.
I'll stop here, you're going to have to take a look at the voltage off of TP2 - and try channel 1 - read, channel 40 and read that - if they are not stable then your issue may be with the loop generator and detector - those are C101 and C102 2.2uF Electrolytics on the above schematic
I'm using the above "graphic" to start - which schematic are you using - the one from CB Tricks or something else - I'll need a link if you need more help. Because I'm gonna' need to know what page you're on to get you thru this.
That other section uses a "varactor" and the PLL uses a set of comparators to look at - or "tap into" that loop - then using it's own clock (the 10.240MHz one it has) is "adjusts" the other loop by sending a small correction voltage to that "varactor" and what that does is change the frequency of that loop.
Comparing the two loops is the main purpose of the PLL - you select a channel - the PLL tells that loop to "go here" in frequency. IF it can follow - you have a channel, if it don't you have a - what you consider a "out of Lock" condition. That is a truth - and is correct - but to presume out of lock because of .... hmmm - well, I'm glad you have that other radio because you'll need it to compare to.
So there are spots on the radio where you can safely look at how or what the radio is doing - they're call Test Points.
Some Test Points are DC related, so they can be checked with a simple DVM and if not right - you can do this to change that. If doing this doesn't help - then more service is needed - and the stupid manual STOPS right there.
Other Test Points generate a waveform - you can use a frequency counter on them, but it's better to use a 'Scope on the signal to see the waveforms' shape. It can be quite intuitive - for quality, degradation and strength of signal generated. Like TP 4 or Mixers or the IF - they can help you see what the radio is trying to do.
So you told me the radio was made in 2006 you might have to start replacing caps, but to know that we need to know if it can generate any RF - meaning it uses two types of RF to make your channel - the PLL issued one for the loop (~17MHz) - a tap off the 10.240 MHz Xtal for the other - and between the two you can also have a 3rd signal or RF as a DIFFERENCE between the two (455kHz).
The radio uses the two main ones (17MHz and 10.240MHz) to make up the TX frequency (27MHz) but when in RX it generates three ~17MHz + 10.690 - 455 - and your have RX. (note the + and - symbols) for the radio adds and subtracts thru heterodyning - and images are created as "sampled" wave energy from the original RF signal modified down to eventually the audio you hear thru the speaker.
I'll stop here, you're going to have to take a look at the voltage off of TP2 - and try channel 1 - read, channel 40 and read that - if they are not stable then your issue may be with the loop generator and detector - those are C101 and C102 2.2uF Electrolytics on the above schematic
I'm using the above "graphic" to start - which schematic are you using - the one from CB Tricks or something else - I'll need a link if you need more help. Because I'm gonna' need to know what page you're on to get you thru this.
- No one is chatting at the moment.