Need to unlock delta tune on Cobra 29

[TNT1450]

Hi all,

I'm currently modifying an old Cobra 29 23 channel radio for use as a 10 meter amateur CW rig. I've already replaced the 11.275 crystal with a 10.240 crystal which gives me frequency coverage from 28.000 to 28.290, which covers the CW portion and beacon band. I will build and install a BFO so I will be able to demodulate the CW, and will key the PTT to use the empty AM carrier as a CW signal.

Even without unlocking the delta tune I will still be able to operate on several frequencies, and the delta tune would work as an RIT. However I'd like to be able to unlock it and use it as a VFO. It would also be nice to expand the coverage of the delta tune to about 5 KHZ or so, then I could cover the frequencies in between the 10 KHZ steps. I was able to find and download a schematic, however I'm unfamiliar with how a clarifier or delta tune is unlocked to turn it into a VFO, and I haven't found anything online for this particular radio. Any help would be appreciated.

[MDYoungblood]

I don't have a schematic in front of me for the Cobra, there is a potentiometer that adjust the voltage to your now 10.240 crystal, if you place the delta tune pot after it but before the crystal you could change the voltage to it and in turn change the frequency. 2 things first, put the radio on frequency and measure the resistance at the delta tune, install a resistor close to the value across the leads to it, that should keep the radio on frequency, than wire the delta tune pot to the above. This is an idea since I have never tried it on a radio so it will be trail and error.

3's

Greg

[TNT1450]

Thanks for the information. I found and downloaded a schematic online, but apparently I'm not allowed to say where, probably because I'm still new here. The crystal I replaced was X12, the 11.275 crystal near TR16 at the bottom center right of the schematic. I don't see a pot near it, but R75 the 15K resistor must be what provides voltage to the crystal, since the other connections are the base of TR16 and R76 which goes to ground. I've been looking at the schematic until I've gone cross eyed, but can't figure it out yet. I'll mess around with it when I get time.

[TNT1450]

I found the schematic here: [Please login or register to view this link]. I won't be too worried if I can't unlock the delta tune, since it will still act like an RIT. But failing unlocking the delta tune, I may be able to build a VFO which would tune each channel, and the delta tune would still act as an RIT even if it's not unlocked.

[TNT1450]

OK, I think I know what I need to do. After looking at some VCXO circuits, I just need to isolate the wires to the delta tune pot and add approximate resistors like was pointed out above, then add a varactor diode between the 10.240 crystal and ground, then connect the pot (for tuning voltage) between the crystal and varactor. That should tune the crystal just enough to get a bit of variation on the frequency on each channel. If it doesn't work, then I'll just put everything back the way it was. Worse comes to worst, I'll have screwed up a free radio. But I think this ought to work.

[TNT1450]

OK, talked to a friend who's a college electronics instructor, and he says all I need to do is connect a wire to the wiper lead on the delta tune pot to another 470K resistor to a diode (reverse biased) between the crystal and ground, and while it likely won't give me 5 KHZ tuning range, I can likely get about 2. If I'm not busy I'll work on it this weekend.

[TNT1450]

Oh and he also clued me in as to where to inject the signal from the 455 khz BFO in order to hear CW, so it was a productive evening.

[TNT1450]

Well this weekend I tried the unlock mod. I soldered a 1n4004 diode between the TX crystal and ground and connected that through a 470K resistor to the wiper lug on the delta tune pot. While it did work in tuning the tx signal up & down about 1.5 KHZ, it put the channels off frequency from where I wanted them, and the worst part was that the receive frequencies were about 5 KHZ away from the TX frequencies. Not good for CW or anything, really. So I undid the mod and will just use the delta tune as an RIT control. I suppose if I had replaced D8 (near the RX crystal) with another 1n4004 it might have made the RX frequency closer to the TX frequency, but I think I'm just going to leave it alone.

[TNT1450]

Revisiting this old topic to post how I finally managed to construct a slider (tune) pot for this radio. I was too focused on trying to bend the frequency of the TX crystal when what I really needed to do was take a step back and look at the bigger picture. I had a friend look at it, and he suggested connecting a varactor to the four crystals X7-X10. It totally made sense, especially after I once again looked at the crystal mixing chart ([Please login or register to view this link]). So that's what I did, used an MVAM209 veractor and built a tuning circuit on a small piece of PC board, isolated X7-X10 from ground, and then connected the board between the crystals and ground. Then I connected the board to the squelch pot to use it as a tuning control. And voila, it worked! And the good news is that I still have the delta tune pot available to use as an RIT control.

I just recently obtained some 455 khz resonators, with which I'll be able to construct a BFO so I can hear Morse signals with the AM receiver. I've already done this with a signal generator, but need to build one to stick inside the radio. Hopefully I can finally get this project done soon.

[KP68]

Very cool! It's great to see folks out there using their knowledge, willing to learn more, research and experiment with radio. It is what this is all about. Thanks for sharing!