[Premium-Rx] Reaction Instruments 685A BW change

[Peter Gottlieb]

Further adventures with this receiver.

It came with 8 MHz, 2 MHz, 500 kHz and 100 kHz IF bandwidths.  Not so useful, so 
I decided to change things.  The receiver uses "WJ Miniceptor" style IF filters 
with an Fc of 21.4 MHz.  Since I sometimes listen to FM broadcast I picked up a 
300 kHz filter, and for regular voice comms, I settled on a 10 kHz filter.  That 
leaves me with 8 MHz wideband, 300, 100 and 10 kHz.  With a little PCB drilling, 
another filter, and a couple of components there could be a fourth BW I suppose.

The IF filter assembly has to be removed from the radio to get at the solder 
side but the change was easy.  The way it works is that the IF first goes 
through a 8 MHz filter, then after that it is brought to the rear BNC for the 
panadapter, and then to the IF filter board where there are four diode selected 
filters.  One is a straight through, so there are really only three filters 
which may be selected.  I replaced the 500 kHz with the 300 kHz and the 2 MHz 
with the 10 kHz.  While I had the assembly out I sketched a diagram of the three 
boards inside:  600 MHz filter/mixer, switchable IF filter, and variable gain IF 
amp.  If anyone wants pdfs of my sketches let me know.  Nobody seems to have any 
documentation at all on this receiver so I figured I'd start.

So it sort of works.  The 300 kHz works great, the 10 kHz not so much.  The 
output is very low and a bit distorted.  Usable though. That's sort of expected, 
of course, as the discriminator is set up for a 2 MHz BW, right?  So that's my 
next area to get into.

There is no sensing of the filter BW, the displayed settings are in the PROM.  
Fortunately, they are near the beginning and really obvious so I patched the 
code and burned a new PROM.  So at least the displayed values match the filters 
I have installed.

I'm now removing the demodulator assembly and will sketch out a diagram of it so 
I can do the mods to have it work reasonably at 10 kHz.  More on this later.

Peter