cobra 25 ltd classic st/mosfet

[Illahe]

I have read online that the IRF520 and the ERF-2030 are NOT the same transistor. When I got my IRF520's from mouser, they all have a gate capacitance of about 680pF. I am going to guess that the ERF-2030 had far less.

When I first modeled this, I used the values the schematic showed. The mod sheet did not talk about removing C149. It didn't drive the 520 at all. I made the assumption that the IRF520's capacitance was so high that C149 did not have to be there. When I removed it in the sim (turned it into 1pF so I didn't have to delete the component), I noticed I was now getting drive to the 520. However, it wasn't enough, so I then raised that C46 to 680pF to let more through and I was getting good results.

I don't think there is anything wrong with your parts or installation, I simply think the IRF520 requires far more drive than the 2030 due to its gate capacitance. This is, of course, pure speculation. If you are up for experimenting, I would completely remove C149 and slowly increase C46 until the negative peaks can't go any lower (which is not going to be exactly 0v, maybe closer to 1v). If my suggestion costs you a 520 or two, let me know. I have about 80 of them and would happily drop a couple in the mail to see if this works.

edit: the matching network may need adjusting too, but thats a different battle

I beat you to the punch. 5 of 6 520's left...

When I smoked the 520 last night I had had enough. The unit went out with the trash this morning. The lesson was worth the cost. I contacted the vendor of the bias kit, they stated that the kit works with anthing from a 2030 to a IFRN24...

I Agree that the 2166 driver was a stretch, I found the suggestion on the web. Oh well.

The reason I purchased the kit, was the claim of "instructions included." Nada.

Its been 30 years since I have been on the key, so be it.

I was messing around with a PC68LTX as well, before a cap and resistor it was swinging 4w dk 18+pep and now 7/8 w dk to 16 pep. It baffles me when I watch the Youtube and see 50 pep... Being green, it seems there isn't enough drive to slam the mosfet. (Lack of understating on my part).

 

[Cable Guy]

It baffles me when I watch the Youtube and see 50 pep...

Me too, baffled and burdened, but then a voice from the past whispers in my ear "Don't believe anything you hear, and only half of what you see." Then comes "If it sounds too good to be true, it probably is."