Paul,
Sometimes it is just better to replace D16 and D17 than to trust them to
an ohmmeter reading.
They can be "half bad" - OK at DC, but not at RF. They are 1N5711 diodes.
If you have the latest updates, the oscillation is not a concern unless
you have a T/R switch problem.
If your thermal pads look OK, reuse them. they already have conformed
to the slight contours of the heatsink and PA transistor surfaces. Just
be sure to put then back the same way they were - they usually stick to
the heatsink, so re=positioning them is not a problem.
Measure those 22 ohm resistors to make certain they have not changed
value due to overheating.
73,
Don W3FPR
On 1/5/2019 3:55 PM, Paul Wilton wrote:
Don
Thanks for the helpful pointers.
Although I didn’t build the KPA100, I did have to fix a few items
before it worked. It has got all the updates.
I have checked D16 and D17 - they are OK.
I’ll check for oscillations - should be easy to spot at 8MHz on an SDR
The PA transistors do appear to be tight. However, I have a spare
pair of pads so I’ll replace them since I’ll take to undo the heatsink
to put the resistors back. I’ll also check out the T/R switch when I
recommission everything - I was aware of the changes to the manual thanks.
Hopefully I will be able to do these over the next couple of evenings.
And I’ll pay attention to the 10s max power spec!
Thanks again
73
Paul
M1CNK
On 5 Jan 2019, at 20:12, Don Wilhelm <donw...@embarqmail.com
<mailto:donw...@embarqmail.com>> wrote:
Paul,
I have three possible guesses.
First is that the KPA100 was developing the full power output that it
is capable of (often 180 watts) because the wattmeter diodes D16 and
D17 have been damaged by static (wind, rain, snow, or nearby lightning).
If those diodes are damaged the MCU thinks there is no power output
and drives to it full power capability. If left to continue, the PA
transistors in the base K2 will eventually destroy themselves. The
PA transistors in the KPA100 are quite hardy and seldom need to be
replaced (I have only replaced 1 set in the last 14 years of
repairing K2s).
The 2nd possibility is that there is some oscillation. That can
happen more frequently on 40 meters, and the KPA100 tries to transmit
at about 8 MHz. Be certain your KPA100 is at the latest level. If
you have blue toroid cores at RFC1 and L16 it is the latest. If not
the latest, upgrade it with the KPA100UPKT.
Thirdly, were the PA transistors tight? If not, the PA transistors
will heat the board around the transistors and not be thermally
coupled to the heatsink. Are the thermal pads properly in place?
After initial assembly (and anytime afterwards if the thermal pads
are changed) you must re-tighten the screws after about 10 hours of
operation at high power.
A 4th possibility is that you have a problem in the T/R switch.
After you get it up and running, do the Diode Voltage checks in the
manual. If you have an older manual but have the latest upgrade
installed, use the chart in the KPA100UPKT instructions rather than
what is in your manual.
Note well, the KPA100 is rated for only 10 seconds of keydown power
at 100 watts, so you did exceed its maximum rating.
If you melted the fuseholder without blowing the fuse, you may have
had bad contact from the wire to the fuse. That would also result in
a low voltage to the KPA100 and cause it to draw greater than the
normal amount of current because the K2 controls try to keep the
power constant - less voltage requires more current for the same power.
Hopefully some of that information will answer your question.
Do all initial testing into a dummy load and not an antenna and
bypass the KAT100 by connecting the dummy load (and perhaps a
wattmeter) directly to the KPA100 - remove the coax between the
KPA100 RF out and the KAT100 RF IN jacks.
73,,
Don W3FPR
On 1/5/2019 2:34 PM, Paul Wilton wrote:
I was using my KPA100/KAT100 combination today to check out some
changes to my OCFD antenna. Previously I have had problems with the
balun heating up and the SWR increasing over an extended transmit on
7MHz (eg 50w on an FT8 over). I have made an improvement to my
OCFD antenna by adding another choke to reduce the common mode
current. So in order to test it out, I planned to run 100w CW for
15-30seconds watching the SWR meter on the KAT100 to see if I was
going to get the SWR increase. This was done with a “cold” PA.
So I started the test and unlikely previously, the SWR remained
good. However at about the 30s mark, I saw some smoke coming from
the KPA100 and then it went dead. It also melted my fuseholder in
the PA supply (although didn’t actually blow the fuse).
I have opened up the KPA100 only to find that the two 22R power
resistors around the two final transistors (R38 and R39) have both
unsoldered themselves from the board and were lying in the bottom of
the case!
Clearly it is an easy job just to put them in again but before doing
so I want to try and understand what is happening
So a few questions:
a) I am hoping that my two finals are still OK. Is there a simple
test I can do perform applying power? Using a DMM in resistance
mode, I measure approx 2M between collector and emitter and 11.2R
between base and emitter (other than the missing 22R resistors I
haven’t disconnected anything else).
b) Any idea what could have caused that much heat that the two
resistors unsoldered themselves? It can’t be a DC failure since
there is a capacitor blocking the path in the event of a PA
transistor shorting to ground. I am concerned that there is some
sort of oscillation going on ie the PA is starting to hoot.
c) Any advice on how to proceed?
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