I need a proper cobra 148gtl variable power mod

[brandon7861]

The KB262 is a voltage stabilizer diode that contains 2 junctions (the KB362 has 3 junctions). This means the forward voltage drop of a single KB262 will be higher than a single 1N4148. For each KB262, you would need roughly two 1N4148's.

The choice of diode (voltage drop) there depends on the capacitance/volt on the varactors, and the capacitance needed depends on the crystal and tuning requirements. Not every radio is the same there.

The best thing to do when a diode is missing in that spot is to try to figure out what was there originally (voltage drop wise) and find a replacement based on that requirement while trying to stick with a part having similar If (forward current) vs Vf (forward voltage drop).

Mouser has surface mount stabilizers in stock, and what I would recommend is the CMXSTB400. It is a 6 pin dip and contains 4 silicon diodes in series with each junction tapped so you could choose what voltage drop you want for that application.

 

[1iwilly]

when i did my 148 i ran 3 wires from vr10 to swr pot then to get the swing when the radio is dead keying 1 watt a member from here shockwave told me to lift the unbanded leg from D54 then run a jumper wire from that empty spot to pin 9 of audio chip all limiters are still intact and working good loud audio reports

 

[nomadradio]

I have seen data books that list those tiny diodes as temperature compensating parts. And that's what I think they're meant to do here. The original design sets the transmit frequency from an internal trimmer pot. The front-panel control is wired to be active only in receive mode.

Each diode junction contributes so many millivolts of change to its forward voltage drop per degree of temperature change. Putting two of them in the same package doubles the per-degree compensation you'll get. Just why the transmit side of the factory clarifier circuit needs more temp compensation than the receive side does I don't know. Never have figured that out, except that you would seem to want the same total number of diode junctions in each side. A glance at the clarifier circuit serves to contradict that thought. Only D51 is in line with the clarifier, but two of the temp compensating diodes are in the transmit side.

An ancient mystery.

73

 

[brandon7861]

I don't think D51 is there for it's voltage drop, its there to prevent the TX voltage that goes to the varactors (during TX) from going back into the RX supply via the voltage divider at the clarifier. Having the RX chain go active during TX, even a little bit, would be a bad thing. D51 stops that from happening.

IC4 already regulates the voltage to the varactors, so adding a "stable" voltage drop in series with that regulator voltage will still pass any variation with temperature from the regulator to the varactors. The voltage would only be stable across the diodes, but since they are not in a zener configuration, it can still pass any variation from the supply (while only remaining stable across the diodes themselves). Thats just my take on it anyhow.

Edit: i initially misunderstood nomad and took it as voltage stabilization, not voltage compensation to offset a variation from a different part. I'd love to understand how that configuration achieves it.

 

[brandon7861]

Here is (what I think is) the reason for the two diodes... Lets say the varactors need 4v. Without the diodes, there would need to be 2150Ω to the right of the red line. The filter capacitor will take a lifetime to charge (in RF terms). With the 2.8v drop from those two double junction diodes, there only needs to be 645Ω on the right side of the red line. That gets you on frequency RIGHT NOW on TX, the FCC doesnt care if your receive is off for a split second because you wont even hear that.

 

[Braddah Vic]

C

The Cb tricks web site is not available....Account Suspended.
Again.

https://www.cbtricks.com/cgi-sys/suspendedpage.cgi

73
Jeff

Here's the link to cbtricks if anyone is still instressted it's free
cbtricks.org