[Elecraft] K2: Very strange power problem has cropped up

[Don Wilhelm]

Bob,

Those DC voltages are not significant to your problem.

The BFO signal is routed through the KSB2 board and for SSB it serves as 
the carrier frequency for the balanced modulator while on CW the 
components around Q3 route the BFO signal from P1 pin 12 , through Q3 
and back out to P1 pin 7.  If the problem occurs on both CW and SSB, it 
is likely associated with these few components, and certainly may be a 
bad solder connection, or even an unsoldered one.

Check C42, D3, RP3, D5, C40, Q3 and C39 as well as P1 pins 12 and 7 on 
the KSB2 board.
There is also a chance that Q1 and Q2 along with their associated 
components could have some effect - check the schematic to identify the 
connected components.

73,
Don W3FPR

Bob Evans wrote:
> My K2/100 has been working great for the year and a half (except for a bad
> power cord once) since I built it, but it just recently presented me with a
> perplexing problem.  Since I don't get on SSB very often, I don't know how
> long this has been an issue, but the symptom is that when I start
> transmitting in SSB, power is OK for about 3-4 seconds and then power shuts
> down to zero on both SSB and CW.  When I remove the SSB board and install
> bypass jumpers (and recommended capacitor), the rig works perfectly in CW.
> When I reinstall the SSB board, same problem; i.e. 3-4 seconds of good SSB
> power and then power shuts down to zero.
>  
> Thinking that the problem must be on the SSB board, I took voltage
> measurements on all the SSB board ICs.  The only voltages from the IC
> voltage table that were off were these:
> U2-5   RX and TX was .47V rather than .6 (probably not significant?)
> U2-2   TX was 5.63V rather than 5.9V
> U2-8   TX was 3.1V rather than 5.4V
> U3-5   RX was .25V rather than .4V
> U3-6   TX was .64 rather than .8-1.2V
> U4-3   RX was .47V rather than .6V
>  
> Anybody care to venture a guess on what these symptoms could mean?  
>
>