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 <[email protected]> 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? ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
