Hello all,
Back at it again.
There seems to be little talk about this modification. This is a strange mod in which you place a variable resistor from L21 to PIN 10 of the MB8719 to achieve a 128 upward channel shift which is amazing compared to the usual 64 upward shift you get from simply grounding pin 10. I have encountered this mod in many of the MB8719 radios I've fixed and I usually scuff at it and remove it without really looking into it and seeing if it's a legitimate mod with good stability. I have encountered the also obscure pin 17 mod in the past which unimpressed me with it's poor stability and tendency to heat the PLL despite producing probably the same 128 upward shift. It is because of this PIN 17 mod and it's terrible stability that I also discriminated the L21 mod. The last radio I encountered with the L21 mod however I did bother to check and besides having that huge upward shift, what impressed me the most was the stability. It seemed to be as good as stock or as good as a pin 10 to ground mod. In this case if memory serves me correctly it was using it's 11.1125 mixer crystal and was achieving a frequency range of 26.175Mhz to 28.045Mhz. With a 11.325mhz crystal it should achieve from 26.815Mhz to 28.685Mhz. In both of these cases we are also using the PIN 11 to ground mod to get a little more range.
I think I'm going to bite the bullet and install this on a radio. I usually go for a crystal switch mod to achieve this kind of range but I realize it has it's own stability issues which in the end may turn out to be worse than the L21 mod. I've managed to make the crystal switch mod fairly stable by standardizing how I lay out my connections. I always use the front mounted switches and hook everything up with solid core wire which I then neatly hot glue to the board. All wire that hangs over the edge of the board towards the switches gets a zip tie. This is to prevent all the leads from moving which is the main cause of drift with this modification. It works quite well but if this L21 mod turns out to be stable I will say bye bye to the crystal switch mod and stick with this new mod that up until now I've been scuffing at. The reason for my reconsideration is my latest attempt at a crystal switch mod which I can't seem to get quite right even though I'm relying on my usual techniques. It's really a clumsy way of doing it now that I think of it and you can only trim the synthesizer for one of the two crystals which in my case is usually the 11.325 because it will be used for upper channels where there is side band. When switching over to the 11.1125 crystal you are on lowers where side band is never heard of and a trimmer cap on the crystal is enough to get you within the ball park.
I'm going to undo the work I had done up to now on this Cobra 148 and go from a crystal switch mod to an L21 mod just to see what it does. From what I can see in the little discussion that can be found of it online the injection of an RF frequency into PIN 10 is somehow tricking one of the logic gates inside.
Here is a youtube video showing the mod.
Back at it again.
There seems to be little talk about this modification. This is a strange mod in which you place a variable resistor from L21 to PIN 10 of the MB8719 to achieve a 128 upward channel shift which is amazing compared to the usual 64 upward shift you get from simply grounding pin 10. I have encountered this mod in many of the MB8719 radios I've fixed and I usually scuff at it and remove it without really looking into it and seeing if it's a legitimate mod with good stability. I have encountered the also obscure pin 17 mod in the past which unimpressed me with it's poor stability and tendency to heat the PLL despite producing probably the same 128 upward shift. It is because of this PIN 17 mod and it's terrible stability that I also discriminated the L21 mod. The last radio I encountered with the L21 mod however I did bother to check and besides having that huge upward shift, what impressed me the most was the stability. It seemed to be as good as stock or as good as a pin 10 to ground mod. In this case if memory serves me correctly it was using it's 11.1125 mixer crystal and was achieving a frequency range of 26.175Mhz to 28.045Mhz. With a 11.325mhz crystal it should achieve from 26.815Mhz to 28.685Mhz. In both of these cases we are also using the PIN 11 to ground mod to get a little more range.
I think I'm going to bite the bullet and install this on a radio. I usually go for a crystal switch mod to achieve this kind of range but I realize it has it's own stability issues which in the end may turn out to be worse than the L21 mod. I've managed to make the crystal switch mod fairly stable by standardizing how I lay out my connections. I always use the front mounted switches and hook everything up with solid core wire which I then neatly hot glue to the board. All wire that hangs over the edge of the board towards the switches gets a zip tie. This is to prevent all the leads from moving which is the main cause of drift with this modification. It works quite well but if this L21 mod turns out to be stable I will say bye bye to the crystal switch mod and stick with this new mod that up until now I've been scuffing at. The reason for my reconsideration is my latest attempt at a crystal switch mod which I can't seem to get quite right even though I'm relying on my usual techniques. It's really a clumsy way of doing it now that I think of it and you can only trim the synthesizer for one of the two crystals which in my case is usually the 11.325 because it will be used for upper channels where there is side band. When switching over to the 11.1125 crystal you are on lowers where side band is never heard of and a trimmer cap on the crystal is enough to get you within the ball park.
I'm going to undo the work I had done up to now on this Cobra 148 and go from a crystal switch mod to an L21 mod just to see what it does. From what I can see in the little discussion that can be found of it online the injection of an RF frequency into PIN 10 is somehow tricking one of the logic gates inside.
Here is a youtube video showing the mod.
