Hi Markus,

Oh, you described it exactly! Just before the 'pop' of the fuse
blowing I could smell something burning. When I opened the PA I
could immediately see the nylon screw holding down the FET was
partly melted! There is no doubt this released pressure on the
Sil-Pad. I had quite some fun getting the remains of that screw out!
Nylon is very brittle after being melted and hardening again!

I admit it was foolish to use a nylon screw instead of a metal one
with insulating washer. But the heat sink was drilled for a thread
size which will just pass through the hole in the FET tab without a
washer. I thought about drilling or filing out the hole in the FET
tab to make room for a washer, but decided not to try it.

I was very lucky it worked for some months last winter and spring. I
do learn from foolish things (sometimes), and this is one mistake I
will not make again. :)

I have a growing pile of dead FETs. I don't want to intrude on Chris
2E0ILY but I think I could be a candidate for dead.fets.NA :-)

For now I will do something "quick and dirty" about this heat
sinking of the FET in the little PA. After I get the big PA repaired
again, I want to look at redesigning this one significantly.

73,
Paul



On 11/24/18 8:41 AM, Markus Vester wrote:
> Hi Paul,
> 
>>> It's a physically small FET and maybe was not screwed down
>>> tight enough with the Sil-Pad interface,
> 
> Same thing happened here a few years ago during an attempt to
> melt off a thick layer of hoar frost from the antenna. It almost
> worked and I could see the resonance creeping up but then the PA
> died. At the time, the four BUZ111 devices in my 300 W class-D
> were bolted to sil-pads using M3 nylon screws. Due to increased
> dissipation with reactive load, the plastic became soft enough to
> release thermal contact. After FET replacement, I went to metal
> bolts and insulation washers, with no such problem since. This
> was actually the one and only FET that I lost in my LF career
> (knock on wood).
> 
> As capacitance is not a big issue at LF you could also solder
> your FET to a copper heatspreader plate and use a larger pad to a
> grounded heatsink. Or use a configuration with grounded drains
> and negative supply and take off the output from the source pin.
> You'll need to use an input transformer with a gate-source
> secondary but that's a good idea anyway.
> 
> Regarding dummy loads for LF: I've been using a coffee maker or a
> toaster, with 1 kW at 230 V being close to 50 ohms, and
> negligible reactance. Just make sure to use an older one
> containing no stupid and unnecessary microprocessor. Albeit
> American 120 V devices may have less favourable impedances.
> 
> Best 73, Markus
> 
> -----Ursprüngliche Mitteilung----- Von: N1BUG <[email protected]> 
> An: rsgb_lf_group <[email protected]> Verschickt: Sa,
> 24. Nov. 2018 12:48 Betreff: 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