Ted,
The most common cause of low resistance on the collectors of Q7 and Q8
is failure of the thermal pads to insulate the DC voltage.
Take the heatsink off and again measure the resistance between ground
and the collector of Q7/Q8. If it is then OK, simply get new thermal pads.
Yes, F1 trips very quickly. I don't know about the exact time, but that
time is short.
73,
Don W3FPR
On 12/4/2015 10:56 PM, Dauer, Edward wrote:
More on the defunct K2: I measured the DC voltage on both sides of F1,
the self-resetting fuse. With the power switch “off” the voltage is 13.8.
With it on, the voltage is essentially zero on both sides of F1.
Measured the resistance to ground at the collector of Q7 - it’s about 2
ohms, not the >500 it should be. But that doesn’t mean that Q7 or Q8 is
the culprit. A short anywhere on the DC line could cause that reading
anywhere else, yes? So, what’s the best procedure to isolate the problem
— I removed the KNB2 and the KAF2; no change. Tomorrow, in the cold gray
light of dawn, I will pull the front panel board and the control board off
the RF board, to see if that narrows it down. Tonight, I will just have
another glass of wine. That should help.
Anyone know if F1 has a reset time of just under a second? If so, then
the “popping” noise may be F1 trying to reset itself. Which doesn’t help
locate the downstream problem, of course. But it would strongly suggest a
DC short somewhere rather than some configuration glitch, yes?
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