Hi Jay,
I wouldn't blame them for cracking in Maine. Sometimes I feel on the
verge of cracking. ;-)
I need to get an amp repaired and do some dummy load tests outside
as you suggest. First just the feedline, then feedline and
transformer. My biggest dummy load is a 250W flange resistor on a
heat sink that usually won't take 200W for long periods. But sitting
outside now, it will be supercooled. :-) I need to get a 1 kW load
if I'm going to play with LF stuff.
I'm trying to remember exactly when I added the new taps very near
the bottom of the loading coil. It may have been after most of my
transmitting in the spring. I've got the very bottom of the coil
connected to the transformer secondary, then a tap from the very
bottom to one turn up. Could this shorted turn be a problem?
73,
Paul N1BUG
On 11/24/18 7:21 AM, [email protected] wrote:
> 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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