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K2 Low Receive Sensitivity on High Power

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Jonathan Taylor, K1RFD

K2 Low Receive Sensitivity on High Power

K2/100 # 2462, with all options except DSP.

I've noticed just recently that when turning the Power above 10 W (and
hearing the relays click), the receiver gain drops 20 or 30 dB. The
receiver returns to normal sensitivity when the Power knob is returned
to a lower setting.

This must have starting happening only recently. The rig is working
fine on transmit at all power levels.

Looking at the schematic, I suspect the the first place to look will be
the T-R switch in the KPA100, but I thought I would post to the list to
see if this is a well-known problem.

Thanks!

Jonathan K1RFD

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Don Wilhelm-4

Re: K2 Low Receive Sensitivity on High Power

Jonathan,

It is not a "well-known problem", but I believe you are correct to
assume the problem source is in the T/R switch. I would suggest that
you check diodes D11, D12 and D14 first - or just replace them, 1N4007s
are plentiful and inexpensive.

73,
Don W3FPR

Jonathan Taylor, K1RFD wrote:

> K2/100 # 2462, with all options except DSP.
>
> I've noticed just recently that when turning the Power above 10 W (and
> hearing the relays click), the receiver gain drops 20 or 30 dB. The
> receiver returns to normal sensitivity when the Power knob is returned
> to a lower setting.
>
> This must have starting happening only recently. The rig is working
> fine on transmit at all power levels.
>
> Looking at the schematic, I suspect the the first place to look will be
> the T-R switch in the KPA100, but I thought I would post to the list to
> see if this is a well-known problem.
>
>

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Jonathan Taylor, K1RFD

Re: K2 Low Receive Sensitivity on High Power - Solved

The problem was with bypass cap C75 in the T-R switch. It had a
resistance of about 1.5 ohms across it. Replacing it solved the
problem. Thanks for the tips.

73
Jonathan K1RFD

Don Wilhelm wrote:

> Jonathan,
>
> It is not a "well-known problem", but I believe you are correct to
> assume the problem source is in the T/R switch. I would suggest that
> you check diodes D11, D12 and D14 first - or just replace them,
> 1N4007s are plentiful and inexpensive.
>
> 73,
> Don W3FPR
>
> Jonathan Taylor, K1RFD wrote:
>> K2/100 # 2462, with all options except DSP.
>>
>> I've noticed just recently that when turning the Power above 10 W
>> (and hearing the relays click), the receiver gain drops 20 or 30 dB.
>> The receiver returns to normal sensitivity when the Power knob is
>> returned to a lower setting.
>>
>> This must have starting happening only recently. The rig is working
>> fine on transmit at all power levels.
>>
>> Looking at the schematic, I suspect the the first place to look will
>> be the T-R switch in the KPA100, but I thought I would post to the
>> list to see if this is a well-known problem.
>>
>
>

Don Wilhelm-4

Re: K2 Low Receive Sensitivity on High Power - Solved

Jonathan,

Good de-bugging. Capacitor failures are not that common and difficult
to find - great troubleshooting job there.

73,
Don W3FPR

Jonathan Taylor, K1RFD wrote:

> The problem was with bypass cap C75 in the T-R switch. It had a
> resistance of about 1.5 ohms across it. Replacing it solved the
> problem. Thanks for the tips.
>
> 73
> Jonathan K1RFD
>
> support this email list: http://www.qsl.net/donate.html
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