From: Garey Barrell 
Subject: Re: [drakelist] Expensive R-4C


Garey Barrell  made an utterence to the drakelist gang
----------------------------------------------------------------------
Ron -

The main difference between the R-4C and the earlier models is the 
change in the second IF filters from L-C to Crystal Filters.  The 
difference comes in the skirt selectivity or how steep the "sides" of 
the filter passband are.  All this means is that as you listen to a 
particular signal, nearby signals drop faster down the steeper skirts.  
If you compare the filter curves in the R-4B manual with those in the 
R-4C manual, you'll see the difference.

You are correct in wanting the filters as near the front of the receiver 
as possible.  The roofing filter limits the amount of RF that reaches 
the second mixer.  The Preselector also helps to limit how much of the 
band the receiver first mixer is subjected to.  The R-4B has an 8 kHz 
wide "roofing" filter in the first IF, but it is only a 4 pole filter.  
The number of poles determines the steepness of the skirts, more poles = 
steeper skirts.  The R-4C has an 8 kHz wide 8 pole roofing filter, and 
so has steeper skirts.

The TR-7 is somewhat better, in that it's first mixer is passive, with a 
large LO signal, allowing it to handle larger signals without 
overloading and generating spurious garbage.  On the other hand, the 
first mixer is subjected to the entire band (no preselector).  The 
advantage is not so much the up conversion as it is being a passive 
mixer able to handle large signals without overloading.

Generally speaking all this isn't critical in day-to-day hamming.  Where 
it really becomes important is in contesting where you have wall-to-wall 
STRONG signals and you're trying to pick one weak one out of all that 
mess.  Personally, I prefer the gentler slope of the LC filters as I can 
hear other signals nearby, but at reduced levels.  If I get a really 
strong interfering signal, I can use the passband tuning to shove him 
further down the skirt.  I get claustrophobic with narrow, sharp filters!

I don't think adding the crystal filters to the R-4B is a particularly 
good idea.  Why tear up one of the best receivers Drake ever made?  You 
are altering the gain distribution of the receiver, and not really 
gaining all that much in skirt selectivity.  I admit to being prejudiced 
though, I much prefer to use this equipment the way it was designed.  If 
you want crystal filters, get an R-4C.   By the way, the T-4X filters 
are only 4 pole filters and aren't all that much steeper than the LC 
filters!  The T-4XB and C have 8 pole filters.

73, Garey - K4OAH
Atlanta


Ron Wagner wrote:

>
> Ron Wagner  made an utterence to the drakelist gang
> ----------------------------------------------------------------------
> This discussion makes me wonder.  How "bad" is the R4B and TR7 vs say 
> a modified up R4C?  I know that I have read about the R4C and the 
> close-in <5Kc signals with out a roofing filter.  What about the other 
> two?  Is there some teaching reading on the web?  Does the passive up 
> conversion on the TR7 avoid issues?  I would think that distortion and 
> mixing would be a problem any time you do not have a filter to cut the 
> signals before you hit an amplifier and or mixer.
>
> I have been hamming for over 25 years, and honestly do not understand 
> all the numbers, but would like a better understanding.
>
>>> Agreed. Without getting into too much technical speak, the R-4C 
>>> lends itself to becoming a top-notch "competition
grade" receiver. 
>>> With the proper roofing filter and the mixer mods, etc., this rig 
>>> will hear stuff that many modern rigs won't. The dynamic range is 
>>> even up there with the Yaesu FT-1000 series.
>>
>
> And speaking of R4B mods, how good of an idea is the crystal filter 
> mod? The one where you sacrifice a T4XB for a IF can and xtal 
> filters.  Is it worth getting a basket case T4XB to perform the 

---