[Premium-Rx] Racal questions

[Dan Rae]

John Reed wrote:

> I have a Racal RA6793A here that was purchased new by a good friend of 
> mine.  There are several puzzling things about it.  It has the LF 
> option and non-standard roofing filters.  There was a 16 KHz bandwidth 
> filter before the 40.455 MHz first IF amplifier, and an 8 KHz after.  
> This looked backwards to me.  I changed them around and the close in 
> reception is now much better since that IF amplifier isn't looking at 
> as much bandwidth.  Is this a mistake by Racal?  I can't think of any 
> reason for it to be configured that way.

This is the way they usually did it in the VLF versions of these radios, 
this is the standard roofing filter compliment for the VLF / MF / HF 
version, and they are indeed normally installed in that order, with the 
wider one immediately after the filter.   The 16 kHz wide roofing filter 
has much less extreme impedance variations near the center frequency and 
thus provides a much better termination to the first mixer.  I would be 
surprised if there is a major difference in "close in reception" after 
reversing them, but the 3rd order intermod numbers would probably be 
slightly worse.  If the designers had wanted to put a narrower filter 
after the first mixer they could presumably have done it easily 
enough.   There is not enough gain between the two filters to increase 
signal levels to the point where they will cause trouble that early on 
in the IF chain.   I think the main reason for the narrower roofer was 
to get down as far as possible in the VLF range, you will probably see 
on your first mixer board that there is very elaborate filtering of the 
first LO to remove noise close to the first IF, which can get through 
the mixer.

I have an RA6790/GM-3 here, 1992 vintage, where there is also a wider 
filter immediately after the filter, 30 kHz in this case, followed by a 
16 kHz one in the second place.  This does not have significant 
differences in 3rd order IIP from the VLF version that I have at 10 kHz 
spacing, and only slightly worse at 5 kHz.  IMHO most of the 3IP 
products in these radios comes from the second mixer, rather than the 
amplifiers that precede it.

As for the LSB filter question, the link on the A4 board is there in 
plain sight, well marked, and if nobody noticed that it was wrongly set 
for ten or fifteen years then I don't think there's a lot of blame to be 
attributed to Racal :^)    Running BITE at any time would have shown 
that there was an error.
I have seen Racal stuff with manufacturing faults, an op amp inserted 
into plated through holes but not soldered, for example, but I don't see 
how this could have got out of the factory like that...  Maybe someone 
removed the ISB board at some point.

Very nice radio though!

73
Dan
ac6ao / g3ncr