[Elecraft] K2 Failure

[Dauer, Edward]

Two things.  First, I want to say again how much I appreciate not just 
this reflector, but more so the people who participate in it.  Last night, 
within 20 minutes I had six replies from, literally, across the globe, 
every one of them encouraging and helpful.  Thanks, guys.

Second, for anyone who likes solving puzzles, I am reprinting below my 
recent off-list message to Don Wilhem, W3FPR, to whom the rig will be 
going for further investigation.  Submit your diagnoses and I’ll let you 
know what Don discovers once he actually has the rig on his bench.  No 
prizes available, but it might be fun . . .   Here’s the latest status 
report:

___________________________________________ 

This morning the symptoms remained:  No LCD light, popping noise in 
speaker, voltages at either side of F1 were 14 with power switch off and 
essentially zero with power switch on.  The same on both sides of D10, and 
the same at the accessory DC power pins.  Resistance from Q7 and Q8 
cathodes to ground was about 2 ohms.

1.  Removed control board and front panel board; tested RF board alone — 
same symptoms (low resistance and apparently a short somewhere on the 13 
VDC line.)  So those boards weren’t the problem

2. Reassembled everything and then removed the heat sink.  Resistance 
measurements at Q7 and Q8 unchanged.  Examined thermal pads, checked 
shoulder washer placement and all associated hardware.  No changes – still 
low resistance from collectors to ground.

3.  Reassembled and tried it again.  No change – no lights, popping noise 
in speaker, voltages as in step 1 above.

4.  As a last ditch effort, decided to change power supplies (had been 
using an MFJ rated at 40 watts.)  Made up a cord to connect K2 to a known 
good supply with a built-in ammeter.  I thought I’d be able to see a 
current spike if there was a short, as F1 opened and closed.  The K2 came 
to life and the current draw was nominal (less than 200 mA.) Switched back 
to the MFJ supply.  That worked too.  Problem solved?  No.  See number 5.

5.  Decided to see if it really was cured.  Connected K2 to a known good 
dummy load, through an SWR meter.  Keyed it with power set to 10 watts – 
but the forward power needle set for full scale barely nudged.  Checked 
the power output as read by the K2’s onboard power meter.  It read 0.5 
watts, no matter where the power control is set.  Confirmed that current 
maximum in the configuration menu was set at 3.5.  It was.  Tried the same 
routine with a known good antenna connected (measured SWR at less than 
1.2:1)  Same results.

I don’t know what all this means, but I am going to take a wild guess.  
Last night something – maybe Q7 or Q8 but maybe something else – developed 
an internal short at exactly the instant I did a direct frequency entry of 
10.000 Mhz (a relay clicks at that point).  This morning, for whatever 
reason, when I changed power supplies, I am guessing that whatever had 
been shorted (or nearly so) burned out, creating an open circuit where 
there had been a low-resistance fault.  Since the rig is producing 0.5 
watts but not more, that makes me think it may be one of the output 
transistors.  Note that, until last night, it happily produced as much as 
15 watts.

In any case, the rig came out of its coma but now it can’t speak. And 
maybe it can’t hear.  Tthe receive function also seems to have been 
degraded – I heard no signals when the antenna was connected – though I 
can check that further by using the K3 to see if anyone is on the bands 
and comparing it to the K2.

So, what do you think is going on?

Ted, KN1CBR