[Elecraft] RE: A Bad Thing Happened To My K2 - Update 1

[Geoffrey Mackenzie-Kennedy]

Mark,

If you have a signal generator, with your K2 switched to 40m in TEST mode 
you could try injecting a 40m signal at different points in the signal path 
between the RF input of the TUF-1 mixer and the antenna connector (antenna 
disconnected), starting at the mixer and working towards the antenna 
connector, to find out where you no longer hear the signal. This would 
narrow down the search for the cause(s) of your problem. If you do not hear 
the 40m signal when the generator is connected to the mixer's input, this 
would indicate that something is wrong with the LO system when the K2 is 
switched to 40m  -but I would think an unlikely cause of your problem.

If you do not have a signal generator, you could try either "scratch" tests 
along the signal path OR use one side of your disconnected antenna's feeder 
connected via a capacitor as a signal source. If you want to use the feeder 
please contact me off-List for some "warnings", also if you want further 
information about "scratch" tests.

The K2 *must* be in TEST mode when these tests are performed to prevent the 
Tx generating any RF if keyed by accident.

In my list of suspects is relay K1 which switches in the 40m bandpass 
filter. Although problems rarely occur with the type of contacts used in 
this relay they do occur, especially if the contact is overstressed during 
some fault condition. Therefore among the tests I would suggest that you 
inject a signal via a 1nF or 10nF capacitor at K1's terminal 3 also 4 also 7 
also 8, or "scratch" them. Alternatively "scratch" the top ends of L1 and L2 
and K1's terminal 7. If all the results of these tests are positive then the 
cause of the problem is somewhere else in the signal path.

Good luck.

73,
Geoff
GM4ESD


Mark Saunders, KJ7BS wrote on Wednesday, September 24, 2008 4:51 AM

> Thanks to everyone for the suggestions.  Here is some more information
> resulting from your suggestions.
>
> I have no measurable RF output on 40m or any meaningful RX on 40m.
>
> The RX/TX works on all other bands including 160m.
>
> I get the same RX/TX results from ANT1 & ANT2 and main.
>
> I get the same RX results on the receive ANT.


Geoffrey Mackenzie-Kennedy wrote on September 23, 2008:

> Don and Mark,
>
> I note that Mark said "virtually no receive on 40m" and not "no
> receive on 40m", from which I would understand that his K2 receiver's
> sensitivity on 40m has become poor and only strong signals can be
> heard. If that is the case then a possible cause of the problem might
> be "dirt", for want of a better word, on the contacts of either K1a or
> K1b which switch the 40m bandpass filter. During receive any
> attenuation of the signal caused by contact resistance here would
> reduce the receiver's sensitivity of course, but during transmit the
> ALC might compensate for the added loss.
>
> While I appreciate that the type of contact used in these relays are
> not supposed to get dirty, they do once in a very blue moon. K1a acted
> up in my K2 which I cured by adding a DC Wet circuit, removed after a
> few months. Cycling the relay K1 on and off many times might be worth
> trying.
>
> What I do not understand is why the problem appeared immediately after
> the reverse polarity episode, unless the spark was caused by static
> that had built up on the antenna and also managed somehow to get to
> K1's contacts after passing through the low pass filters, and without
> zapping the T-R switch diodes