Help needed Cobra 29 IRF 520

[btrapr]

*First tried a kit that pretty much does the DTBradio mod with a irf520____this one put a 3.3K in 53 and put a 470R and a 1n914 positive facing forward in 56____tuned/peaked----- 8-10w peak power (mostly around 5-7w)___ Thought might have a bad 520____TOOK this kit OUT! My frined put the same kit in his Cobra 29 with the same results

I built the homemade companion part with a 470, 3.3k and 1n914... pulled R55, R56, and C61 following the CBtricks page except swapped for IRF520 (thinkning I had a bad 520 I put in a RadioShack irf510) and a homemade ef369FN

I added a 68pf on the back of c59_______turned VR4 to max (turned CC) adjusted L14, L17/L20 to peak ____only peaking about 10w's?

I checked the voltage on the GATE of both Mosfet's and both times only about .032v's>>> I read somewhere else to grab power on the +side of C116 running it through a 10K pot and adding it to the left (gate) of the MOSFET and adjusting it to 2.8-3.2 volts with 3.0 volts being about right to keep the Mosfet from running to hot. I have no idea why Im only getting .032v's at the gate with either approach.

Any ideas?

I am not an electronic genius but can follow direction. I did look on the data sheet for the irf520 and it looked like 3v gate with an acceptable capacitance of 330-450pf.

I ran the tests on an antenna with a swr 1.5
*I bought a DUMMY LOAD max60w, cont's 20w and a 5w max 50ohm impedance_____Will one of these give better ture resutls OR am I just missing something that is keeping me from getting at least 25watts out of my efforts.

 

[Robb]

This is a great read; lotsa info too:

http://www.worldwidedx.com/cb-band-cb-radios/73873-radio-shack-irf-510-cobra-29-impressions.html

 

[btrapr]

Thanks but I read that prior to doing my mod.
Per his post

- Changed C71 (20 uF 10VDC) electrolytic with a 33 uF 35 VDC
-*did not do but i'll try it___ Installed a 1K resistor in series with D11
- Removed VR4 (a 680k? pot) *Did not do this either BUT I will___and replaced with a 1K ohm fixed carbon resistor
- Removed C42 (1000 uF) and replaced with a 2200 uF 16VDC cap *NOT available locally- can I put TWO 1000's together to get close until my 2200's come in the mail). (Moleculo calls for a 3300 uF cap, but my voice is quite bassy and the 2200 worked best for me)... I'll tell ya, this and the next monolythics at C37 and C68 really gave the best audio frequency response for my voice. Talk about 'broadcast quality'..... HOLY COW!
- Changed C37 (0.33 uF monolithic) to a 2.2 uF mono
- Changed C68 (.1 uF mono) to a 2.2 uF. (Note, in another Cobra 29 LTD I merely moved C68 to C37, and added a 2.2 to the C68 spot. Seemed to work equally well) *These just came in the mail today!
- Adjusted L17, 23 & 24 for tonal quality under audio and an RF Freq counter.
- After all, I DID indeed wind up adding 68 pF of total capacitance to the rear of C59 (270 pF), for a total capacitance of 338 pF *Is he saying he added a 68pf Cap or just what was added after 59 brought up the total capacitance? I am no expert but he added TWO 2.2 Mono's DOES the 2200 change capacitance too?
- Finally, I added a ground jumper from R142(what is or how do you determine the + side of a resistor? and I assuming that the other end goes to the strap across the housing or at least I would think it was a grounding source) to Audio Transformer to minimize any "squeal" when using a power mike with the dynamike all the way up. (This was essential due the the mods at VR4).

The other TWO attempts did not bring up my watts at least the way they should have according to the people who done them. I thought perhaps I needed to take power off of+ side of C116 and run it through a 10K pot to the gate side of the Mosfet. My volts were really low .032v and the data sheet said a 3v gate, so I thought that it needed at least close to 3v's to run properly

I also read the one by Moleculo and ExitThirteen___________No pro here by any means and I welcome input