[Premium-Rx] Racal RA6790/GM ISB installation

[Thomas Adams]

Greetings, Troops.

The night after getting my individual LSB / USB filters for the ISB
installation, I couldn't restrain myself any longer. I'd intended to
wait until my day off to tackle the installation of the ISB board &
filters, but after arriving home from work last night at 2 AM, I 
gave the job a shot.

The board itself is pretty straightforward; you mount the aluminum
panel for it on the 4 screw bosses in the radio, and screw the
board to it. Simple.

The electrical connections are a snap too... provided you have all
the stuff you need. I didn't.

The ribbon cable plugs into the A4 board, no problem. I got one
SMB male / SMB male cable with the board, but I found you need
TWO of them. 

At 2:30 AM, the neighbors must have thought I was nuts... even
more than they did before; I was rummaging around out in the
garage, ripping into assorted electronic hulks searching for
another SMB cable.

No luck, but I found the hulk of an old storage oscilloscope that
had a lot of single ended SMB interconnecting cables in it. A
quick pocket knife slash, and I had two of them. Half an hour 
later, with the aid of a soldering iron and a couple of hunks of
heat shrink tubing, I had the two grafted together in an ugly but 
electrically acceptable manner.

Before connecting the cables tho, it was time to deal with the
filters. 

I ought to take the filter cover off more often; the rubber pressure
strips inside had glued themselves to the tops of the filters,
and when the cover came out all of the filters came out too!

Peeling off the filters & removing the rubber residue with some
lighter fluid, I decided it was also time to make the filter changes
I'd been contemplating for a long time.

Per the manual, the filter #1 socket got the LSB filter. It seemed 
logical to install the USB in socket #2.

My stash of spare filters came out, and we started making the
changes.

Socket #3 is the original 400 Hz filter, for CW & 160 Hz shift
FSK. I'd like something narrower for this slot, say 250 Hz, but I
don't have one yet.

Socket #4 got an oddball filter; 700 Hz. This one is to be used
for 425 Hz FSK reception.

Socket #5 got another oddball, a 1000 Hz filter, for 850 Hz shift
FSK. This one got pulled tho; BITE swept it and it came up as
2700 Hz. First bad filter I've run across. An oddball 1400 Hz
went in instead. The bad 1000 Hz will be gone over with a
spectrum analyzer & tracking generator to find out what the
truth about it is; BITE might have made a mistake (that will
happen occasionally).

Socket #6 got the original 6.8 KHz for AM use.

Socket #7 got the original 16 KHz, which actually BITE tests as
20 KHz..It's been useful at times for FM when feeding the rig
with a tunable VHF front end.

Once the filter issues were dealt with, one more task; reset the
SSB / ISB jumper. It's right next to the filter cover, next to the
FL1 position.

In my rig it wasn't the expected jumper plug, but a soldered
wire instead. TIGHT space for soldering!!!

Connecting the two SMB cables & ribbon cable, I crossed my
fingers, turned on the power, and ran BITE.

I was faintly surprised when BITE finished with no error codes
on the display! ISB was ready to go.

First stop; the 80 metre ham band. In LSB, the signals sounded
good thru the "new" filter. Punching into ISB L, it sounded good 
too.

Next stop; 10,051 KHz (New York Radio / VOLMET weather), to
do a similar check in USB and ISB U mode. Everything checked 
out fine.

I wasn't in the mood to go hunting ISB signals this time of the
night, but the next best thing was 7335 KHz (CHU, Dominion
Observatory time signals from Canada). Contrary to popular
opinion, CHU doesn't run conventional AM. They run what can
only be described as vestigial sideband AM, like a TV signal;
there's a carrier and an upper sideband, but the lower sideband
has been removed, or at least greatly suppressed. WHY they
do that, I have no idea! 

Just as advertised, ISB U gave a strong, clean signal, while 
ISB L gave only some crosstalk and noise. I don't know if the
crosstalk on lower is in my ISB board, or if it's at CHU. In any
case, an alignment job on the ISB board is on the agenda, just
on general principles.

One feature of the ISB mode came as a surprise to me.

It appears that there are two seperate AGC lines in the iSB
board, one for each sideband. It's possible to run, for instance,
ISB L as a manual gain channel, and ISB U as AGC with a
long time constant! 

My RA1772 with ISB can't do that; it's something that only a
microprocessor controlled radio can do, apparently. How
useful it is remains to be seen, but it's a slick little bit of
engineering design in any case! <<grin>>

In SSB mode the new, individual filters aren't quite as good
re. audio recovery as the usually encountered 3.24 KHz 
LSB / USB filter, but they sound pretty decent nonetheless.
The difference in bandwidth is clearly apparent between
the two filter schemes. 


73's,

Tom, W9LBB
Mahon Loomis Memorial Monitoring Station &
Irish Setter Retirement Home,
Sun Prairie, Wisconsin