[K3] K3 RFI Problems Sorted (Hopefully)

[graham.wood1]

Hello to all

Originally posted as :- K3 RFI Problems – HELP! on 28th of May

My setup is, K3 (sn:5845), computer Athlon 2.5 GHz running WinXP, and homebrew logging program. Station is in an upstairs room.
Antennas, 40m end fed inv vee matched through tuner on all bands, and 20/15m Bi-square coax fed into shack.
Ground is 1/4 wave radials for each band, strapped together and connected to tuner chassis. The K3 ground, computer chassis and CW/FSK/PTT interface box are also grounded to this point.
The computer is mounted under the operating position, rather noisy fan.

Recently keying the rig from the computer, the K3 would lock up, if I keyed using a paddle and K3 keyer no problem also discovered turning off K3 port on computer(no CAT to K3), no other changes, keyed OK.
Depending on the band, calculator, win media player and internet explorer would suddenly appear whilst keying.

The K3 serial lead used an old serial mouse lead all connections at both ends. I read Jim K9YC's article on RFI and followed his suggestion, made up a new lead using 2 twisted pairs for TxD & RxD, 1 lead from each to ground. The first time I tried this, running 10w, the computer switched off and rebooted. With the K3 serial lead disconnected I could run 100w no problem.

Following advice from Don W3FPR, I decided to rehash the setup.
I removed the ground leads to K3, computer and interface from the ground radial system.
Using coax (inner and outer joined) I connected a lead from the computer chassis to the interface diecast box and on to the K3 ground point.

The two audio leads, ready made screened cable, I twisted together. (Ref Jim K9YC twisted pairs less susceptible to RF).

The audio leads and two serial leads and ground lead are made up into an harness between the computer and K3.

I’ve tested on most bands, running 100w, no problems. Need to try in a contest for prolonged operating period. For the first time since I’ve been using the K3, I can run 100w on PSK, previously turning the power up, it reached 30w and wouldn’t go any higher, assumed RF getting into audio, lived with PSK at 25 watts or less.

I would like to thank, Don W3FPR for taking the time to reply to the original post, and Jim K9YC for his excellent article on RFI problems, both proved very helpful.

Hopefully this post might be of use to others having similar problems.

Cheers Gray G3VIP
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