13n10 mosfet into cobra 29wx issues

[Jim Dickey]

I pulled out the wimpy 2078 along with r55, r56. C61 did not appear to have ever been installed in this radio. I built a en1230 from through hole components and placed it with the positive side towards the front of the radio in r56's holes. Replaced original final with 13n10 mosfet. I powered up the radio with the covers off (speaker disconnected-ext speaker plugged in) and was getting a 10w carrier with about a 45w peak. Sounded great on my scanner set to channel 21.

I hooked the speaker back up and put the covers on. Powered up the radio, tested receive which sounded good. The first time I keyed the Mic I got about 6w fading to zero, then came the smoke. Disconnected everything, pulled covers off and the finals heat sink was pretty warm, along with both power transformers that were extremely hot.

What did I cook and why?

 

[sonoma]

you did not say any thing about what the voltage to the gate was. it should be around 3.5 volts to be sure the 13n10 does not blow out. max is 4.0 volts. did you get 13n10 parts or did you get the 13n10L parts. the L suffix will only take 2.0 volts on the gate.

if the 2078 was working I would have left it in there. if you get the mosfet working no one will know you are putting out more watts other than you.

 

[Jim Dickey]

It's the straight 13n10 from Fairchild. I did not check gate voltage. Would the cooked final cause the transformers to heat up? At what level of modulation do I check voltage? Deadkey or full Mod?

 

[sonoma]

dead key and no modulation.

 

[Jim Dickey]

So the fried final could cause the transformers to heat up?

 

[sonoma]

yes because it is shorted. unsolder the legs of the final to see if it still heats up. if so you will need to check other parts in the final area and make sure you did not slop some solder across a trace.

 

[Jim Dickey]

Right on! I'll check her out in the morning. Thanks much!

 

[Jim Dickey]

I removed the final, the transformers are no longer heating up when powered on. I'm probing the far right solder pad when looking at the solder side of the board for the final. When I key the Mic it shows zero volts, then jumps to 4.5 volts for just a moment when the Mic is unkeyed. This should be a steady reading correct?

 

[Robb]

The intermittent voltage reading is either a bad solder joint, or it could also be the mic switch being intermittent.

4.5v on the gate feed is waaay too much for the 13n10. In fact, it is even too much for a IRF520. I suspect that you check your en1230 build and try again.

 

[Jim Dickey]

Hmmmm. Put a new final in and replaced what appeared to be a cooked r123 10ohm with a 22 ohm. Got good receive but now I'm not getting any modulation, dk @ 2w. The gate voltage is now a steady 4 volts when keyed. Which resistor do I bump up to lower the gate? One of the ones in the 1230? I'm not really trying to add a variable power option if it's not completely necessary.

 

[Jim Dickey]

Changed r123 back to a 10 ohm and now gate voltage is about 3.78 which is a little too close to MAX. Would dropping this down to a 5 ohm lower the gate a little bit more?.

Also, the modulation is back but weak. 10w dk swing to about 16w. Turning vr4 didn't seem to make any difference

 

[Robb]

~3.8v is fine. for a IRF520.
3v is more like it for a 13n10.

 

[Jim Dickey]

I have a few 520s, I thought I had read somewhere that the 13n10 was A stronger mosfet which is why I bought 50 of them. I'm still learning a lot about electronic theory and practice, so forgive any stupid questions or assumptions. Is the easiest way to adjust the gate to do the tip120 hooked up to the rf gain pot? It seems like I should be able to replace a resistor somewhere to accomplish this.

 

[Robb]

Just a hobbyist too.

Getting more power out a to-220/type case than a IRF520 is already pushing it - IMO.

I also would lower the dead key down to 4w; it is going to bang the crap out of the S/RF meter anyway since it will still swing some ~30/40w. Would also keep that MOSFET from popping, since they aren't as reliable as a bipolar-type transistor.

 

[Jim Dickey]

Well the meter is already bRoken so no worries there. I think my lack of swing is due to the high dead key. I'm only swinging to about 16w from the 8ish dead key.

Should I go ahead and do the variable power? Do I really need a transistor or can I just route through a trimmer or pot?