Hello N1BUG,

Probably  teaching  granny  to suck eggs but if you take off the HV to
the  drains and just run the 12V supply and the exciter and `scope the
gates  you  will quickly find the dead FET / FET's and also see if the
driver IC is functional.  I think I have killed only one driver IC....
:)  My  IC's  are  socketed, makes changing one a snap. I did consider
socketing the FET's but I assume quite a bit of heat is disipated down
the  source  lead  to  the ground plane of the PCB so I chickened out.
Will  be  interested to see how the little FET's fair. I can still get
the big packages but supply is intermittent.

Tuesday, November 20, 2018, 2:48:10 PM, you wrote:

> Hello Chris,

> Thanks for the information.

> I will order FETs and other parts today. It will have to be 32N20
> this time, as you know the 34N20 in the large package is long
> discontinued. While I am at it, I will get a supply of diodes and
> resistors for present and future rebuild of that gate network when
> changing FETs. I'd best order a couple driver chips too, as I had
> problems with that before.

> I hope nothing else has been killed, but I haven't had time (or
> energy) to pull it apart and go through it yet. I believe this
> problem started with an oscillation in the exciter (converter) so
> anything is possible.

> My LPF is the same as yours and Jay's. During initial testing with
> the cover off, I found the center inductor was running warm. The two
> on the ends did not seem to show much if any warming. In the
> permanent form I put a fan on the center compartment, moving a good
> bit of air across that inductor. I didn't do much testing after
> this, but I think with the fan it should be running cool enough. I'm
> running my amp at less than 50% power. Of course I have a fan
> (rather larger than what Jay used) blowing air across the output
> transformer in the PA as well.

> Perhaps the resistance change as things warm up is nothing I need to
> worry about, but when I get the PA back working I will try to run
> some better tests to see if I can find what is going on. Even if
> it's not a problem, I would prefer to understand it. I'm odd that
> way. I don't like mysteries! :-)

> 73,
> Paul


> On 11/20/18 9:04 AM, Chris Wilson wrote:
>> 
>> 
>> Hello N1BUG,
>> 
>> When I have blown a FET on Jay's amp design and replaced it / them I
>> have sometimes seen less than optimal gate and drain waveforms from
>> the replaced FET's. A simple meter "diode test" on the parallelled
>> diodes / 10 Ohm resistors showed the diodes to be OK, but changing
>> them restored things to a good pattern. i now change them as a matter
>> of course if a FET blows. My matching transformer is a multi tapped
>> primary / secondary pairing on three individually Kapton taped 77
>> material 240 size cores stacked together. I see no heating t full
>> power WSPR15 or Opera but they are in a sealed plasic box of more than
>> generous size. I *HAVE* had issues with the ground terminals on the N
>> type connector screws arcing although they appeared tight. I now
>> solder direct to the socket flanges before mounting with the socket's
>> flanges on the inside of the box. Hope you get it sorted soon, a box
>> of spare FET's is a necessity, "things `appen" ;)
>> 
>> The only thing that gets warm is the LPF, it's in three divorced
>> compartments like Jay's and i think yours? I use a small PC processor
>> fan on the middle box with an exit hole meshed over opposite the fan.
>> Air flow is adequate to keep all three toroids near ambient with this.>>



-- 
Best regards,
 Chris                            mailto:[email protected]