K1TTT Technical Reference
>Over th e past several years, I have tried several different PC
>compatible computers in the shack in an attempt to use with my HF
>Station. It seems no matter with PC I have tried, and having tried
>toroid chookes, ac filters, etc. my PC (now a 486 /66) still QRM's the
>heck out of the ham hams with birdies and other annoying noise.
>I know my PC is not a CLass B machine, just a clone. But over the years,
>I have had several clones, real IBM's, Compaqs, and AST's all with
>basically the same result.
>Does anyone out there know of a manufacturer of a PC tower or desktop
>cabinet that has excellent RFI suppression and shielding? If so,
>would be very interested in finding this out.
>Short of spending magabuck for a new class b machine, is there any other
>solution? My pca nd my hf station are totally useless together. What
>does all you guys do?
It may not be your computer. At least not the computer box itself.
One handy tool for finding rf leakage is a 10' piece of rg58 as an
antenna for the main RX, with 3" of the shield pulled back on the
'working end', with heat shrink over the exposed shield and center
conductor. Locate a strong birdie with the case open, then close up the
case and use this rf probe to find the leaky holes.
The first thing you need to do is to unplug everything but the power
cord from the computer, and turn it on. Look for birdies on the bands.
I'll bet that they are greatly reduced.
In short, the first step is to make the PC box itself quiet with
nothing plugged into it but the power cord... If you have birdies, look
inside the power supply - some have Corcom filters on the AC input,
some don't. You may have to supress the PS before anything else. I've
seen the exact replacement RF filter power assemblies at the swap meets
for $5. The common name is a "Corcom" - the name of the most common
manufacturer. Once the power cord is quiet, a piece of copper screening
(from the local hobby store) between the fan body and the metal box
may be required. The back side of the faceplates for the empty drive
bays may have to be covered with copper screening, just as any other
non-shielded RF windows. Then hook up the keyboard. If the trash
level in the RX goes up, work on what was just changed: the keyboard
connection. It might take pulling RG-8 braid over the keyboard cable,
and grounding it on each end. It might even take removing the keyboard
connector from the motherboard, and replacing it with a DB-9 (like a
serial plug) mounted on the cabinet with supression networks between
it's pins and the motherboard. Then cut the DIN plug off the keybaord
cable and put the other half of the DB-9 set on it - using good quality
metal hoods, of course. If you had to change the connectors, don't
forget to cover the keyboard connector hole). Once you can't tell the
RF difference between the PC by itself and the PC-keyboard combination,
you can start on the monitor. Do the same thing here - it might take
shielding the inside of the cabinet with grounded foil and insulating
fish paper over that. One article I read years ago about a monochrome
monitor ended up with replacing the video wire between the picture tube
socket and the circuit board with RG-174. When the PC-kbd-monitor set is
quiet, then plug in the printer. It might take a shielded cable here.
On mine, I ended up making one with shielded wire and metal hoods on
good quality connectors. I also found that of the half-dozen "ground"
pins on the DB-25 parallel socket on the I/O card, only 3 were actually
tied to ground. A jumper on the card fixed that. Then test the system
with both the printer power on & off - my friend's laser printer is
quieter than his old dot-matrix (the laser has more metal in the case).
Years ago you could buy "aquadag" metalic coating in a spray can - it
makes adequate shielding if you can ground it. Shielding the inside of
the printer case may be a bit difficult with the flip-top aver the print
head area... As to modems and TNCs, the route of least effort is to go
internal to the PC case. Unfortunately, external TNCs are more
common than internal. However for RF & shielding purposes, TNCs and
modems can be treated similarly. My first modem radiated badly - it was a
Prometheus with a plastic case. My second (a Hayes 2400 external)
was much better (it had a metal case but perhaps that was a coincidence?).
My current modem is internal - one less cable/antenna. I took the RJ-11 off
the card and replaced it with a shielded RJ-11 with built in RF filters
on the pins - AMP makes it. I changed it before I even installed the card
in teh computer - so I have no idea as to the performance before the mod.
Mice cables can be replaced with shielded wire - but again, before you
do anything, try it with the mouse plugged in and unplugged - you
might discover that there is no difference. RF chokes and bypass caps
on the plug may be enough.
The above is a potentially a lot of work. But it is worth it. My
386 is audible on my scanner on 6m AM from 50 feet away (and my 2m
ssb rig will hear it from further) with the case open,
but with the case closed there are a couple of minor birdies. If I had
shielded the back side of the mini-panel that has the keylock, turbo
switch and reset button those would have probably been eliminated. If I
ever rebuild the system, I'm probably going to replace that plastic
piece with a sheet metal piece with a metal (shielded) push button for
the reset, leave out the turbo switch and lamp, and put mini-rf chokes
on the disk activity leds.
After looking at what I have written, perhaps this would make a good QST
article! "The care and starving of a birdie".
Mike Morris WA6ILQ | This space intentionally left blank.
PO Box 1130 |
Arcadia, CA. 91077 | All opinions must be my own since nobody pays
818-447-7052 evenings | me enough to be their mouthpiece...