K2: Very strange power problem has cropped up

[K5WA]

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?  
 
I'm not really an electronics guy so I'm just using the process of
elimination to try and track this down, but I can't imagine that there is
something on the SSB board that can shut the power down completely at both
the QRP and 100W level.  However, I have accidentally discovered that if I
leave the jumper capacitor out of the jumpered condition (SSB board
removed), there is no power, so maybe Q3, C39, D3 or C42 on the SSB board
are the culprits with either a failure or bad solder joint.  That where I
will start looking tomorrow and hopefully you guys will have a suggestion or
two to limit my random hunting process.  ;-)
 
By the way, for the guys asking about the value of used rigs, I was amazed
that my other K2/100 (loaded) went at the Buy-it-Now price on eBay within 12
hours of being listed last Saturday.  I got slightly more than I paid for
the original rig and options, but a couple of bucks less than the current
"new" pricing.  I was very pleased.  I'm expecting my first K3 within "two
weeks" and the second one should show up in March/April.  I'll be selling
this second K2/100 at that time, but only after I track down this recently
discovered problem.
 
Thanks,
Bob K5WA
K2/100 #4687
K2/100 #5119 (just sold)
K3/100 #2XX???
K3/100 #4XX??? maybe?
 
 
 
 
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[Don Wilhelm-4]

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?  
>
>  

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