Topband: FT1000MP click reduction

[Tom Rauch]

The FT1000MP is entirely different than the 1000D or 1000. It keys 
the carrier after the filters, not before them.

The problem with the MP is entirely due to the waveshape of the 
envelope near the maximum amplitude spot. Yaesu uses a single 
pole R/C filter, which does not provide an optimum-bandwidth 
waveshape for a given rise and fall time. The rise and fall is too fast, 
besides having the poor one-pole R/C filter shape.

While there are much better methods, such as something 
producing a raised sine waveform, adding one capacitor will help 
until someone finds a better solution. Either a multi-pole filter or a 
more complicated system that produces a raised sine waveshape 
would be much better.

With a single pole filter we are limited in BW improvement, but it 
can still be much better than stock.       

This mod should only take a couple of minutes.

Locate T2018 at the right front area of the IF Unit. Remove the IF 
Unit by unplugging the ribbon cables in that area and flip it over.

C2128 is a .0047uF chip capacitor on the center pin of the 
secondary of T2018. Either change C2148 to .15uF, or add a .1 uF 
capacitor in parallel with C2148 (from the CT of T2018 to ground). 

When I have more time I might try some form of active filtering, but 
at least this is much better than stock!

I noticed the radio I looked at had the high power limiting pot set to 
150 watts. 3rd order SSB IMD was only -21 dB with that power 
setting. IMD improved about 11 dB by reducing the power limiting 
control to 100 watts. (IMD referenced to one tone as in commercial 
standards, not PEP like the ARRL uses.)

About 1-1/2 dB more power at a cost of 11 dB IMD is NOT a good 
tradeoff.

73, Tom W8JI
W8JI@contesting.com 


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