Paul
Any chance one (or more) of the 3 high stack of cores in the transformer might
have cracked? Might be interesting to hook a 50 ohm dummy load to the secondary
of transformer, adjust turns ratio to 1:1, and have a look. At least this test
would absolve anything up to and including transformer from being the problem.
Jay W1VD
----- Original Message -----
From: N1BUG <[email protected]>
Reply-To: <[email protected]>
To: <[email protected]>
Sent: 11/24/2018 6:47:45 AM
Subject: Re: LF: No 2200m TX tonight
________________________________________________________________________________
Hi Markus,
That is very interesting. It changes about 25 % during the first 2-3
minutes, then it seems to settle down and not change any more. It
could be moisture somewhere, but any moisture here is solid ice or
frost now. I did not find any ice or frost in the transformer box or
the variometer. Could be insulators or something with the antenna
itself. I don't see any "fuzz" on the scomematch voltage trace so I
think (hope) nothing is arcing.
What worried me is this did not happen last winter so something has
changed. Everything accumulates some dirt here because of blowing
dust, smoke, etc. I wonder if a small amount of dirt on insulators
plus moisture can combine to make funny things happen.
Anyway I examined the little PA and it seems to have died due to
poor thermal interface between FET and heatsink. It's a physically
small FET and maybe was not screwed down tight enough with the
Sil-Pad interface, which was also a previously used one.
Normally I do not like to put drain voltage on the heatsink but as
an experiment for this little PA (which is totally an experiment
itself, but served me very well last winter) I will isolate the heat
sink from the chassis/PCB and mount the little FET directly to it.
The thermal resistance would be much lower! I think this is fine so
long as nothing shorts the heat sink to ground. In that case some
fuses die. ;-) This would add some pf of capacitance between drain
and ground but it would be in parallel with the quite large C of the
Class E tank, probably not much difference at 137 kHz!
Parts to repair the big PA should arrive Wednesday.
73,
Paul
On 11/23/18 4:29 PM, Markus Vester wrote:
> Hi Paul,
>
> sorry to read that. Have pity on the poor FETs!
>
> You mentioned that the antenna resistance is gradually decreasing
> (i.e. improving) during longer transmissions. I often see that
> effect here, with the current rising by say 20 % during the first
> few minutes. I've put it down to moisture or dew around the coil
> and insulators (tiny little polycarbomnnate pencil tubes), which
> evaporates as things warm up. It is more prominent during cold
> damp weather, and much more so with the very high antenna
> impedance at VLF than at LF.
>
> Good luck, Markus
>
>
> -----Ursprüngliche Mitteilung----- Von: N1BUG <[email protected]>
> An: [email protected] <[email protected]>
> Verschickt: Fr, 23. Nov. 2018 22:12 Betreff: LF: No 2200m TX
> tonight
>
> No transmissions from me this night. The little amplifier has
> died. I think it may be related to this resistance change in the
> antenna which is getting worse and worse.
>
> I'm going back to MF for this night, sorry!
>
> 73, Paul
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