The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

I just installed an irf520 in my cobra 29/NW/ST. It worked at first and only had a talk range comparable to my TRC-52. So I opened it back up and started expirimenting with some capacitance values. I was futzing around with capacitor values for C62 [I installed a 470pf cap here when I changed the final; I also removed the capacitor that was across it on the solder side]. I was going for a lower value so I paralleled two 220pf caps in place of the 470pf cap. I noticed my transmit audio increased, much louder with less clipping and distortion. I left the radio on, went and got something to drink, came back and noticed that when I keyed up the mic, the rf power meter on the radio barely even moved at all.

So I changed the 2 220pf caps back out with the 470pf cap. Nothing. The TX transmit lights up, and I could hear audio when I was transmitting to my nearby handheld unit. But the TX meter moves to around 1. What could have happened? Maybe I burnt up the driver?

I didn't use the en1230 part, instead I used a diode + resistor pair configured this way: GROUND <------>|------^^^^-----------> MOSFET GATE. The diode was an everyday 1n4148? and I used a 470ohm resistor with ferrite bead on the end.

I hope someone can help as this is the only 29 I got!

Okay, it was the driver. I had a 2sc2028 (I think... some of the print was worn off) laying around, so I put in and it seems to work now. Is this a suitable driver replacement? The original driver was a 2sc1957.

Also, anybody who has done this mod, what kind of distance have you gotten? I have a 54" center-loaded mobile antenna with open coil. I'm also using it in my car, so I guess the supply voltage is around 14vdc.

Yes, it will work fine. That is what some of the older versions used.

What kind of power output did you have when you first finished the mod?

timrim wrote:Yes, it will work fine. That is what some of the older versions used.

Thanks. Mine is one of the older ones that used the 1957, but the 2028 seems to work even better!

dizgolfer wrote:What kind of power output did you have when you first finished the mod?

When I first finished, before frying my driver , I measured around 2 watts dead key and 18 watts swing, not much at all. But after replacing the driver, fiddling with capacitance, resistances, and retuning her, I am seeing about 5 watts dead key and 39 watts swing, not bad for .90 cents worth of parts!

That sounds really good. I did this mod once using directions somewhere on this site and I got about the same thing maybe a little less.