100µH has reactance = 80 ohms, and 0.1µF has 12.5 ohm, so it isn't dramatic
extra attenuation for the 6 switches, possibly around 10-15 dB in a 50 ohm system, or did
I make wrong calculation?
Jan-Martin, LA8AK
-----Opprinnelig melding-----
Fra: [email protected]
[mailto:[email protected]]På; vegne av Joe Craig
Sendt: 11. januar 2005 13:45
Til: [email protected]
Emne: Re: LF: Re: Pre-amps on LF
Dear Jim and Alberto and LF group,
Thank-you for the info. I well remember reading Alberto's article.
The schematic of the FT-817 shows about 6 switching diodes
and some shunt 100 uH chokes and series 0.1 uF caps. Fortunately,
the internal preamp can be bypassed (by activiating the so called
AIP) which improves the LF gain significantly. It appears as if the main
culprit is indeed insufficient reactance of the transformer before the
mixer.
So we can replace the shunt chokes, pad the caps and replace the
transformer, but the frustration level of having to do this at the
SMT scale will be high. I think that this will also compromise the
VHF an UHF performance of the RX.
It is now starting to look easier to start from scratch by building
a new LF RX or simply to continue using the external preamp.
73
Joe VO1NA
On Mon, 10 Jan 2005, Alberto di Bene wrote:
james moritz wrote:
> I'm sure some people have described mods
>where this transformer is changed to improve LF performance, but I can't
>think who at the moment.
>
>
http://www.mods.dk/view.php?ArticleId=2711
73 Alberto I2PHD
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