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.>>
>>
>>
>>
>>
>>
>>
>
--
Paul
N1BUG 2200m-2m DXCC Honor Roll
FN55mf ME Piscataquis County
http://www.n1bug.com
http://www.aurorasentry.com
> I do see a mismatch s things warm up and the vario needs a tweak after
> a cold start. The loading coil becomes tepid after a while.
>
> I was very pleased to be heard by W3SZ three times last night! I have
> discovered a limitation in only being able to have one call sign with
> the U3S which makes divorcing WSP2 and WSPR15 TX's on the reporter map
> impossible unless there's another option?
>
> Tuesday, November 20, 2018, 10:54:15 AM, you wrote:
>
>> Hi Stefan, LF
>
>> I see some very interesting reporters on that list. :-)
>
>> I would like to transmit as well, but the PA is not repaired. I must
>> look into it to see if anything in addition to the FETs is dead,
>> then order FETs and whatever other parts...
>
>> When the PA is repaired I need to do some more testing before I try
>> WSPR-15. During every WSPR-2 TX period, the scopematch shows the
>> resistance of the antenna decreasing during the transmission. I am
>> worried this is caused by something heating. Maybe the ferrite in
>> the antenna matching transformer, maybe the the big coil? Or maybe
>> it is in the scopematch detector itself but that would be a little
>> difficult to believe. I would like to find the cause of this before
>> transmitting the long WSPOR-15 periods.
>
>> I just woke up and sent the rest of my overnight spots to WSPRnet:
>> DK7FC and DF6NM.
>
>> 73,
>> Paul
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