Topband: Re: dedicated topband transceiver

[Tom Rauch]

Hi Steve,

> I had a pretty poor opinion of the IF gain distribution of my off-the
> shelf FT1000MP - which was fixed up by adding the INRAD board.  Now my
> RF training is only technician level and that was over 25 years ago...

Be careful with that mod.

It is almost the same as adding an RF preamplifier! The only 
selectivity in front of the added gain is the roofing filter, and so 
Blocking and IM dynamic range decrease almost exactly by the 
amount of added gain, unless you put the front end attenuator back 
in!

The IF hissing noise in the FT1000 series, most noticeable in the 
narrow filters, occurs because they used a cheap 3SK74L FET in 
the 455kHz amplifier following the last narrow filters, and the 
several stages after that are all broad bandwidth. Because the last 
stages are broadband, there not only is a wide unfiltered noise 
response there also is reciprocal mixing of the noise. That noise is 
mostly an annoyance since it is such a broad (but low level) noise.

(That same type of device, the 3SK74L, is also a major source of 
transmitter composite noise in the FT1000's. In that case it is the 
ALC control FET in the transmitter section.) 

I replaced the problem FET with very quiet GaSFET, moving the 
gate tap up on the last IF transformer so the gate was better 
matched.  That single change was worth about 5 dB or better in 
noise reduction.

The real cure would be to add perhaps 10dB gain just before the 
last narrow filters, and get rid of that 455kHz amp completely. 

Rather than mess with gain, just add an audio filter for CW! It will 
have the same effect of eliminating noise on CW, and not hurt 
dynamic range a bit. As a matter of fact, this method is commonly 
used in communications receivers. 
 
> contesting.com reflectors like this one, etc) and it could be sent as
> a petition to the managing directors and technical directors of Yaesu,
> Kenwood, ICOM and Ten-Tec.

I'm probably getting on a you-know-what list for complaining so 
much, but I feel obligated to point out what I observe because we 
are ALL stuck with these radios for years. So I wouldn't mention 
me!

Individual complaints generally have a larger effect that one 
complaint as a chain letter. 
73, Tom W8JI
W8JI@contesting.com