Hello, I am working on a Cobra 2000 GTL. I am starting to align it or check that it is aligned. The first step is to check (TP13) pin 8 on MB8734. The service manual says to check for 10.240 MHz. I get a reading of 10.238 MHz . Is this ok or does it need to be exact ? What would cause this ? Two of my frequency counters get the same reading of 10.238 MHz. Do 10.240 MHz crystals go bad ? The service manual says there is no adjustment for that step.
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Cobra 2000 GTL PLL MB8734 off frequency
- Thread starter Beamer
- Start date
Needs to be exact on 10.240, i have seen them go bad, but it might just be an allignment you need too do.
THe 10.240 signal is used on several parts in the radio so it neds to be exact.
Depending on the accuracy of your counters of course.
THe 10.240 signal is used on several parts in the radio so it neds to be exact.
Depending on the accuracy of your counters of course.
you can get away with the 10.240 crystal being a bit off, and your readings are pretty typical.
the effect of this crystal being off frequency is that the further you get away from the frequency you aligned everything on, the more off frequency you will get, but its not that much.
for example, if your radio went from 26.175 to 28.085 mhz, you would notice that while channel 19 (27.185) is dead on the money, when you tune down to 26.175, you may be reading 26.1748 or 26.1752.
same thing up around 28mhz.
with a radio that only has 40 channels you'll probably never notice this.
you can make that crystal adjustable by changing out one of the caps next to it for a variable capacitor.
cant remember which one it is but i think its value is 18pF?
maybe someone can confirm that for me.
right now i would just move on with the alignment and not give it a second thought.
LC
the effect of this crystal being off frequency is that the further you get away from the frequency you aligned everything on, the more off frequency you will get, but its not that much.
for example, if your radio went from 26.175 to 28.085 mhz, you would notice that while channel 19 (27.185) is dead on the money, when you tune down to 26.175, you may be reading 26.1748 or 26.1752.
same thing up around 28mhz.
with a radio that only has 40 channels you'll probably never notice this.
you can make that crystal adjustable by changing out one of the caps next to it for a variable capacitor.
cant remember which one it is but i think its value is 18pF?
maybe someone can confirm that for me.
right now i would just move on with the alignment and not give it a second thought.
LC
Thanks I will check that out . At the very rate I will check them with my capacitor checker and see if the values are correct.
Well, you just need to pull one and see which "direction" the frequency goes.
I found this thru Cobra 19 at the CB Tricks - the 1st group of Service Manual pages covers the PLL theory...but I found this "blurb" useful. Because for a Service Manual to say or not to say - that it is not adjustable, is not a good manual to use.
There is a small blurb on one of the Cobra Radios where they talk about the shifting of frequency and the required Cap needs in picofarads to offset an Xtal that isn't precise or to meet the needs to fix the channel spread and it's drift...
Read it, it may help you in understanding how they seem to make these work...
Note this circuit and the location of all the parts and
their relation in connections to the Crystal.
The Factory has to deal with issues of supply chain and it's parodies of error, omissions and blatantly bad parts - so the engineers already incorporated a series of fail-safes to allow the production to continue using a specific part assignment, as a different value
It's an allowance - that; in using it, provides a small performance change, but corrects an error. It is a feedback principle they purposely incorporate to help meet their production deadlines.
Then, once determined which way you wish to go - add or remove the other one to help trim.
Does My Xtal Need Less capacitance or More capacitance? That is the Question...
I found this thru Cobra 19 at the CB Tricks - the 1st group of Service Manual pages covers the PLL theory...but I found this "blurb" useful. Because for a Service Manual to say or not to say - that it is not adjustable, is not a good manual to use.
There is a small blurb on one of the Cobra Radios where they talk about the shifting of frequency and the required Cap needs in picofarads to offset an Xtal that isn't precise or to meet the needs to fix the channel spread and it's drift...
Read it, it may help you in understanding how they seem to make these work...
Note this circuit and the location of all the parts and
their relation in connections to the Crystal.
The Factory has to deal with issues of supply chain and it's parodies of error, omissions and blatantly bad parts - so the engineers already incorporated a series of fail-safes to allow the production to continue using a specific part assignment, as a different value
It's an allowance - that; in using it, provides a small performance change, but corrects an error. It is a feedback principle they purposely incorporate to help meet their production deadlines.
Then, once determined which way you wish to go - add or remove the other one to help trim.
Does My Xtal Need Less capacitance or More capacitance? That is the Question...
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Consider that in this radio the 10.24 crystal's error is divided by 1024 before it feeds into the PLL circuit.
Let's assume for the moment that the counter is accurate. And if it wasn't calibrated in the last year, who knows?
But an error of 0.02 MHz, or 20 Hertz is not that bad by itself. Divide that by 1024 and the frequency error it creates in the radio's channel frequency is pretty dern small.
Other radios take that same 10.24 MHz crystal signal and use it differently. But in this radio that crystal can be off quite a bit before it can throw the channel frequency off enough to care.
73
Let's assume for the moment that the counter is accurate. And if it wasn't calibrated in the last year, who knows?
But an error of 0.02 MHz, or 20 Hertz is not that bad by itself. Divide that by 1024 and the frequency error it creates in the radio's channel frequency is pretty dern small.
Other radios take that same 10.24 MHz crystal signal and use it differently. But in this radio that crystal can be off quite a bit before it can throw the channel frequency off enough to care.
73
On a secondary note...
If the Xtal drifts, or was put together this way, and got thru Quality Assurance (READ:Control) then the error is not great, but...
Then too - looping back to the "Padding" they did to the Xtals output - those two caps, they (READ: Engineered this) put in a method in which to tweak an ENTIRE line of radios for the purposes of having them be "right" out of the box versus the OFF specifications even before they could even sold. They'd all would have been unable to even be sold due to off frequency specifications - due to FCC frequency requirements.
So - in light of this - it appears that "To H*ll with the FCC and the Customer - make it so it gets to the channel it's supposed to be" as a standing order of performance approach. No one wants to be a few bubbles off center to everyone else's bubble bath of roundtable talk.
Why Have That?
Versus a whole bunch of useless CARP that will rest on the bottom of the ocean in a shipping container approach, in the way that some countries do for their own recycling...SEE: Marianas Trench
There are tons of examples...
Just two...
So, nothing new...So don't be afraid to change it, you're well past it's warranty, lifetime and Service life - to resurrect this. So you don't have to worry about "Repo man" trying to take back that thing, it's paid for...
If the Xtal drifts, or was put together this way, and got thru Quality Assurance (READ:Control) then the error is not great, but...
Then too - looping back to the "Padding" they did to the Xtals output - those two caps, they (READ: Engineered this) put in a method in which to tweak an ENTIRE line of radios for the purposes of having them be "right" out of the box versus the OFF specifications even before they could even sold. They'd all would have been unable to even be sold due to off frequency specifications - due to FCC frequency requirements.
So - in light of this - it appears that "To H*ll with the FCC and the Customer - make it so it gets to the channel it's supposed to be" as a standing order of performance approach. No one wants to be a few bubbles off center to everyone else's bubble bath of roundtable talk.
Why Have That?
Versus a whole bunch of useless CARP that will rest on the bottom of the ocean in a shipping container approach, in the way that some countries do for their own recycling...SEE: Marianas Trench
There are tons of examples...
Just two...
So, nothing new...
Attachments
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Hello All, I just popped in a variable capacitor and with a slight tweek of it 10.240 MHz appeared on my frequency counter . Mission accomplished ! The original capacitor C86 18pF measured 16.2pF. I did put in a new crystal before this small modification . That brought the frequency up to 10.239.95. The variable capacitor is a 2pF to 18pF. I did drill a new hole for one of the legs of the variable capacitor so that it would fit on the circuit board . 
Here are a few pictures.
Here are a few pictures.
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