Adding an rfx75 to Cobra 29 XLR

[nomadradio]

Seems to me the capacitor in question was added in parallel with the cap between the driver's collector circuit and the final's base circuit. Seems to me the magic capacitance value was 68pf. We ended up with thousands of ceramic disc caps that value after buying out parts inventories from closed shops.

Basically served to boost the RF drive level to the final transistor. Only helps if the final is capable of the additional output.

73

 

[Harley3315]

Seems to me the capacitor in question was added in parallel with the cap between the driver's collector circuit and the final's base circuit. Seems to me the magic capacitance value was 68pf. We ended up with thousands of ceramic disc caps that value after buying out parts inventories from closed shops.

Basically served to boost the RF drive level to the final transistor. Only helps if the final is capable of the additional output.

73

Where would I tie in on that 21 xlr to add a variable ?

 

[nomadradio]

Uh, "tie in"?

As in breaking the power feed to the driver transistor to insert a NPN transistor for carrier control?

Don't have a picture of the circuit board handy. I don't think there is a handy wire jumper that you can remove like the Cobra 29LTD radios. I'll see if I can find Sams CB121 next time I get to work. It has some pics. In a nutshell you would break the connection between L15 and the modulated B+ that also feeds the final transistor. You would need to make sure you don't separate L15 from its bypass disc capacitor C44. Easier to say than to do without some layout guidance.

73

 

[Harley3315]

Uh, "tie in"?

As in breaking the power feed to the driver transistor to insert a NPN transistor for carrier control?

Don't have a picture of the circuit board handy. I don't think there is a handy wire jumper that you can remove like the Cobra 29LTD radios. I'll see if I can find Sams CB121 next time I get to work. It has some pics. In a nutshell you would break the connection between L15 and the modulated B+ that also feeds the final transistor. You would need to make sure you don't separate L15 from its bypass disc capacitor C44. Easier to say than to do without some layout guidance.

73

Yeah I know on the 29xlr you break it between L15 and L13 I think it was there is even a couple blank traces I think to solder too you just cut between them but the 21xlr is laid out slightly different from what I was looking at last night. Similar but not exactly the same.

 

[Uncle Ronnie 336]

when i get back from work, i'll take a pic where I made the trace cut on a zach T (same PCB) upstream from the audio transformer to make it swing. i'd say get a piece of perfboard and a TIP142 darlington and hit up the pinned thread here on the more advanced swing mod. it beats the tip120/cap and the resistor/cap method hands down

the jumper pull method only works ok on newer cobras. older radios tend to make boxcars and pinched carrier wicked easily on the scope. thats why I moved away from that method.