[ct-user] Problems with CW keying - 9.37

P&V Nesbit P&V Nesbit <pnesbit@melbpc.org.au>
Tue, 05 Jan 1999 07:29:52 +1100

At 00:11 2/01/1999 -0500, Dick WC1M wrote:
>(Re 80m RFI) A little experimentation revealed that the RF was getting
>in through the keyboard. I happened to have an old keyboard of different
>manufacture lying around, and using that one solved the problem.

I had exactly the same problem, so I had a look inside. I found that the
keys are decoded by a diode matrix, none of which have RF suppression
capacitors, and which therefore behave as diode detectors. Although
capacitors could theoretically have been added, the mod would have been
messy, so I opted instead to fit a thin brass shield in the bottom half of
the case. This is grounded to the cable where it enters the keyboard. A
sheet of stick-on plastic film prevents short circuits to the internal PCB.
The mod worked beautifully.

I also found it worthwhile to ground everything together, especially the
PC. The net effect of all the mods was also to reduce the noise on receive.

Peter VK3APN

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