Cobra 29LTD/GTL 10 KC mod????????

[brandon7861]

I did not have stability issues as I removed from 1 radio and tried it in 2 other cobra 29s and still worked well right on frequency.

It was my intention to have this done and back in the mail by tomorrow, but I think I want to tinker with it throughout the work week and try to get it more reliable, if you don't mind. I'm having a lot of fun with this and I'm learning a lot.

I achieved my goal making it run with one crystal, but it turned it into a motion detector. Useless. Two crystals seem to work good, but I think the lack of resistive load with the high impedance jfet is making it more sensitive to the surroundings.

If you are not in a hurry, I'd like to see if I can do better. Maybe higher gain BJT is best here so I can have a little load resistance (or maybe I can add it to the JFET). Just like an audio input made quiet by a 10k shunt resistor, I think its something I need to explore here.

 

[dss56]

It was my intention to have this done and back in the mail by tomorrow, but I think I want to tinker with it throughout the work week and try to get it more reliable, if you don't mind. I'm having a lot of fun with this and I'm learning a lot.

I achieved my goal making it run with one crystal, but it turned it into a motion detector. Useless. Two crystals seem to work good, but I think the lack of resistive load with the high impedance jfet is making it more sensitive to the surroundings.

If you are not in a hurry, I'd like to see if I can do better. Maybe higher gain BJT is best here so I can have a little load resistance. Just like an audio input made quiet by a 10k shunt resistor, I think its something I need to explore here.

Brandon I did not expect you to do this and im in no hurry whatsoever. Im a tinkerer as you are, and when I start I like to find out what makes things tick.
I was reading a forum on this 10kc jump and seems even though its old subject I said let me try for this 10kc jump. Many posted different ways with no success. So I tried and got hooked. LOL

 

[dss56]

I added a second crystal to this JFET oscillator and it raised the trim capacitance to something around 1.75pF. I then discovered that putting my hand anywhere enar it changed the frequency, so I grounded the crystal cases. That really helped with stability, but it once again reduced the required trim capacitance down to about 1pF.

The best trimmer I have only goes down to 3.5, so putting it in series with 1pF got me in the ballpark, but the instability came back. So I once again replaced the trim cap and series 1pF with another gimmick cap and I had it running at 5.123 333 for the entire night nice and solid with only couple tens of Hz shift getting my finger on the gimmick.

So today, I have plans to make a coaxial trim capacitor (to shield it) using a piece of telescopic antenna and borosilicate pipette as the dielectric, but instead of moving the metal, I plan to move the glass pipette to vary the frequency. Going from glass to air dielectric in my prototype achieved 1.3 to 4.8pF. I need to cut about 10% off its length so it fits on the board, then I will cut slots in the outer sleeve to further lower the capacitance. I figured we can wax the glass tuning rod in place once it is set.

I am curious, did you have stability issues using two crystals in your BJT oscillator? I tried the same 2 crystals and gimmick cap in my expo clone and the frequency dropped about 1kHz, couldn't get it quite high enough, so I am really surprised it is working for you. These little crystals are putting up a good fight.

Heres the cap, I plan to install it north of the crystals after I take the gimmick off.
View attachment 72533
I also need to add a buffer since changing my probe from 10x to 1x shifted the frequency a little. I was almost tempted to take advantage of that to trim it on freq.

As is, with the gimmick over the crystals, raising and lowering the gimmick from the crystal cases changes the frequency enough to not have to play with the twists, I just don't think it would be happy in a mobile environment.

I did not have stability issues as I removed from 1 radio and tried it in 2 other cobra 29s and still worked well right on frequency.

 

[brandon7861]

Cool. I've been working longer hours this week and haven't had much time to play with it, but this weekend just opened up. Family was coming but they cancelled due to the weather.

I would like to add shielding, so I was thinking about starting over on FR4. If I photoresist and etch, I can copy/paste and make a few at the same time. Only problem is that I am out of SMD capacitors and trimmers, so I need to place a mouser order.

Question: Do you want me to improvise some shielding on the perf board version and get it in the mail, or would you be ok waiting for my mouser order to arrive? If you wait, I can send a couple extra oscillator boards for you to play with.

 

[dss56]

like i said im in no hurry and did not expect your input to this project.

Im in no hurry.

Thanks