[Premium-Rx] Fw: Racal genius

[Michael O'Beirne]

----- Original Message ----- 
From: "Michael O'Beirne" <michaelob666 at ntlworld.com>
To: "Charles P. Steinmetz" <charles_steinmetz at lavabit.com>
Sent: Thursday, August 16, 2012 8:37 PM
Subject: Re: [Premium-Rx] Racal genius


> Dear Charles
>
> I can't speak for the 6790 but there are two sensible mods for the similar 
> RA1792 which may be helpful.
>
> The first is to change the absurdly wide 40.455 MHz crystal filter for 
> something much narrower, but obviously with the same impedance.  G3YFK 
> located an excellent manufacturer in Belgrade.  He sent them his filter 
> for a comparison and measurement and they produced an excellent one-off 
> filter with a -6dB bandwidth of around 6kHz.
>
> Sadly the aforesaid factory disappeared during the Bosnian conflict, 
> though it may be that other manufacturers may be around who could help.
>
> The other mod is to replace the 455 kHz ceramic filter (FL1 on Board A3), 
> which has a BW of 35kHz, (ahead of the main IF crystal filters) with a 
> much narrower one.  You may need to make a small adaptor board for this. 
> I have a 6 kHz BW one and a mini board but have yet to fit it.  The 
> problem is that the BITE will be fooled and will display a fault even 
> though there is none.
>
> As regards the IF filter leakage what make of diode would be better than 
> the existing ones?  Also, will duplicating the output switching on the 
> input side of the filters make much difference?  There is not a lot of 
> room to achieve this on the PCB.
>
> 73s
> Michael
> G8MOB
>
> ----- Original Message ----- 
> From: "Charles P. Steinmetz" <charles_steinmetz at lavabit.com>
> To: <Premium-Rx at mailman.qth.net>
> Sent: Wednesday, August 15, 2012 9:48 PM
> Subject: Re: [Premium-Rx] Racal genius
>
>
>> Tisha wrote:
>>
>>>Yes, it is a sin but whenever I see a parted out 6790 on eBay I almost
>>>cannot help myself and if the bandpass filter is the part for sale I will
>>>spring $30 for it. I have added that filter to many other receivers (even 
>>>a
>>>venerable Hammarlund SP-600) to reduce out of band interference. It is a
>>>great design.
>>
>> In the 6790 itself, the IF filter switching circuitry leaks pretty
>> horribly -- a consequence of feeding all of the filter inputs in
>> parallel and switching only their outputs with diodes.  If you have
>> the 16 kHz filter installed (most of the ones I've measured are more
>> like 20 kHz at 6 dB), it spoils the ultimate out-of-band attenuation
>> of the narrower filters from their cutoffs up to 25 kHz or more.  The
>> simple solution is to remove the "16 kHz" filter.  Presumably the
>> narrower filters all leak too, when you are using an even narrower
>> filter, but I have never been bothered by it.  However, my widest
>> filter is 6 kHz.  If you have an 8 or 10 or 12 kHz filter installed,
>> that might also exhibit the problem.
>>
>> It has long been known that if you are going to switch only one end
>> of your IF filters, it should be the input end, for this very
>> reason.  For a radio of this stature, it is (IMO) an unpardonable sin
>> not to switch both ends, and even worse to switch only the wrong
>> end.  All you would need to modify a 6790 to switch both ends is a
>> handful of cheap components.  If I were doing this, I would change
>> the existing diodes to something more suitable than the installed
>> 1N916s, and use them for the input side, as well.
>>
>> I never bothered to modify the filter switching because removing the
>> "16 kHz" filter works for me, and I really don't much like the
>> performance of the radio even after this change (to say nothing of
>> the hateful ergonomics).  The only reason I've kept mine is because
>> it is one of the ultra-rare 2174B models (factory equipped for
>> LF/VLF/ELF, using 5 or 6 modules that are unique to this model, as
>> well as the normal complement of modules from the 2174/2174A).  Only
>> 200 2174Bs were made, according to Gary W., and only a few seem to
>> have escaped from the original owner (the USAF).
>>
>> Best regards,
>>
>> Charles
>>
>>
>>
>>
>>
>>
>>
>>
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>