[Elecraft] KPA-100 low power problem on 17m..10m

[recarter at adelphia.net]

Don
Thanks for the reply.  I have a pretty decent Daiwa power meter.  It tracks very well on the other bands and matches the displayed power output of the K2.  I think the power level displayed by both the K2 and the Daiwa meter are correct.  I do have a decent scope that I could use to verify this. 

My first suspicion would be a bad low-pass filter, but the 20m band works fine whereas the 17M band doesn't.  I suppose I could have a marginal 20/17m low pass filter that is good enough for 20m but too low for 17m.  I only get about 65W on 17M.  I'd also need to have a bad 15/12/10 low pass filter.   I suppose I could have screwed up assembling two filters.  I've been staring at the schematic.  Since it happens at both high-power and low-power, I think it has to be a problem in the bandpass filters.

When I assembled the chokes, I pulled tight on the wire to try to make nice tight cores.  In the process, I broke one of the cores in the 15/12/10M low pass filter.  I superglued it back together.  I'm waiting for a replacement part.  If the problem was restricted to this band, I'd suspect my repair.  After breaking one core, I was much more careful about pulling too hard.  I didn't wind the rest of the cores very tightly at all.  I wonder if they are too loose.

I also have an MFJ analyzer.  I'm thinking of disconnecting the RF cable from the RF board to the KPA100 and hooking it up.  I should be able to switch through the bands and measure the impedance at various frequencies.  My guess is that it will show a bad impedance above 17M, but it won't give me any hint as to why.  I wonder if a stuck relay on one of the other bandpass filters might cause something like this.  I've read though the mail archives and found some suggestions about testing for this.


---- Don Wilhelm <w3fpr at earthlink.net> wrote: 
> Rich,
> 
> First question - is your KPA100 wattmeter properly calibrated?  If not, this can be the single source of your problem.
> 
> Second question - how are you determining power output?  The K2 power display will be from different sources when you add and remove the KPA100.  If you are relying on the K2 display indications in both cases, I again refer you to the first question (because the KPA100 wattmeter is not in the circuit when you remove the KPA100 and the wattmeter may be indicating too low).
> 
> The power output that you report for the base K2 is quite normal, and the power gain of the KPA100 is lower on 17 meters than it is on say 40 meters.
> 
> If you are using an external wattmeter to determine the power let us know that, and if you have a 60 MHz or higher 'scope with a 10x probe that you can use to measure RF Voltages, that would be handy in the absence of a good external wattmeter (you said you already have a good dummy load).
> 
> 73,
> Don W3FPR
> 
> > -----Original Message-----
> > 
> > I just finished the KPA100 for my K2 this week.  The K2 and 
> > associated accessories were purchased within the last month.  
> > Without the KPA100, I get 10W or more on each band.  Most bands 
> > provide about 15W, but 10M barely makes the 10W rated output with 
> > about 10.5W.  I think the K2 is just fine.
> > 
> > I added the KPA100 board this week.  I get full power on 20m and 
> > below, but I only get about 70W on 17M and above.  If I select 
> > 50W with the power control, I get 50W out.  As I increase power, 
> > the amplifier tops out.  I'm connected to a good dummy load.  The 
> > SWR is 1:1.  I'm connected to a good 32A power supply.
> > 
> > If I set the power level for 10W, I can only get about 7W on 
> > these bands.  If I unplug the KPA100 and recheck the K2 
> > alignment, I get more than 10W on all bands as stated above.  It 
> > seems as if the KPA100 attenuates the RF signal at frequencies 
> > above 18mhz.  I've inspected the low-pass filters for poor solder 
> > joints, incorrect number of windings or wrong parts.  Everything 
> > looks good to me.
> > 
>