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

 

[brandon7861]

I'll get it in the mail tomorrow on my way to work.

 

[dss56]

I'll get it in the mail tomorrow on my way to work.

Thanks very much for putting time into the 10kc board. I appreciate your time and effort. Looking forward to trying it out.

 

[brandon7861]

No problem, I had fun. I will include the dremel island oscillator from post 15 because I have too many of those laying around. I added a 8-40pF trimmer so it should work good for a crystal with the correct frequency printed on it. It was a fun project, I learned a bunch and I ordered parts that I've wanted for a while too so I'm happy I could be involved.

 

[brandon7861]

I just wanted to let everyone know that this little oscillator board failed in dss56's radio. Ran solid on the bench here all night, but he said the frequency was all over the place in his radio.

I can only imagine that, due to the small capacitance required to get the crystals pulled that far, it became susceptible to the RF inside the radio.

I might build two more to do a side by side comparison between shielded and unshielded for a variety of load capacitances and field strengths. Every radio has stray RF inside, and now I want to watch the output closely while the oscillator is subjected to various environments.

Anyone trying to parallel crystals and run a lower load capacitance to pull them up should check every crystal in an oscillator and pick the ones that are both closest in frequency to each other when run individually and closest to the target frequency. Randomly choosing crystals from a batch of unmatched ones isn't good enough. I think doing this raises the capacitance required and reduces instability.

 

[brandon7861]

The bigger HC-49U ones showed up on mouser so I ordered a few. The datasheet says the load capacitance is 30pF which I am pretty sure is a good thing for pulling it up further. Maybe I can keep it above 10pF this time.

 

[dss56]

The bigger HC-49U ones showed up on mouser so I ordered a few. The datasheet says the load capacitance is 30pF which I am pretty sure is a good thing for pulling it up further. Maybe I can keep it above 10pF this time.
View attachment 72845

Well hope the HC-49 will work. In past found out the bigger crystals work better.
The low profile not as good. Keep us updated.

 

[dss56]

Well hope the HC-49 will work. In past found out the bigger crystals work better.
The low profile not as good. Keep us updated.

Hi Brandon Bob here on the 10KC kit.
I saw your post of the xtals at mouser so I also ordered some also.
They came today and I tried one in the Expo 100 S kit and I added a 10pf cap between the trim cap and the xtal and works great enough up/down swing to zero in on center frequency. So you get all the RC channels and also ch 41 27.415 as everything moved up 1 channel. So when you get your xtals should work great.

Also you mentioned on the wire that you were getting low do you use #22 or #24 wire. I was able to score 4 full rolls of 22 and 24 if you need some I can real off some and mail to you. Let me know.
thanks Bob/dss56 here in Boston MA.

 

[brandon7861]

Cool! Good to know they will work. I had plans on sending you a few, but I guess you beat me to them. I am still waiting on my mouser order. Tracking says it should be here tomorrow. The casino I work at threw out a couple Patriot/Setec MK7-400 24v power supplies, so I brought them home to fix. I think some of those parts delayed my order.

I should be ok on the wire until I get back down to Ax Man, but thank you for offering! I don't use it too often and I think I have about 4 feet left of maybe 8 color pairs which should hold me over for a little while yet. It does go quick though, its good stuff. Id like to find it in teflon with the same color options. If they don't have any down at AxMan, I might check back with you and see which source you went with and order a roll.

Thanks!

 

[dss56]

Cool! Good to know they will work. I had plans on sending you a few, but I guess you beat me to them. I am still waiting on my mouser order. Tracking says it should be here tomorrow. The casino I work at threw out a couple Patriot/Setec MK7-400 24v power supplies, so I brought them home to fix. I think some of those parts delayed my order.

I should be ok on the wire until I get back down to Ax Man, but thank you for offering! I don't use it too often and I think I have about 4 feet left of maybe 8 color pairs which should hold me over for a little while yet. It does go quick though, its good stuff. Id like to find it in teflon with the same color options. If they don't have any down at AxMan, I might check back with you and see which source you went with and order a roll.

Thanks!

The wire I found on ebay someone was selling 4 full rolls so I bid and won.
Yes the xtals work great in a Expo S kit and and has plenty of adjustment both up/down.