RE: KPA-100 low power problem on 17m..10m
Posted by
Don Wilhelm-3 on
Mar 17, 2006; 10:06pm
URL: http://elecraft.85.s1.nabble.com/KPA-100-low-power-problem-on-17m-10m-tp387696p387698.html
Rich,
Yes, it is easy to scan the KPA100 LPF with the MFJ259 or any other antenna analyzer - and that would be the first step. Remove the AUX RF connector from the K2 RF Board and insert a 51 ohm resistor leads into the connector going to the KPA100 - then connect the analyzer to the SO239 and with the K2 in receive, switch to the band to be scanned and see wnat the analyzer indicates.
I had one KPA100 a while back that displayed the same symptoms - turned out that the builder had put .047 uF (473) caps in the C80 and C81 positions rather than the proper 4700 pf (472) disc capacitors. On another occasion, R98 on the bottom of the K2 RF Board had been changed to too large a value, but that does not usually cause HI-CUR indications.
If you want a sanity check on your wattmeter readings, use the scope (with 10x probe) to measure the peak to peak RF Voltage at your dummy load - a coax TEE can give you access to the center conductor if your dummy load is sealed. The simple formula for power is Vp-p * Vp-p/400 if the dummy load is 50 ohms.
73,
Don W3FPR
> -----Original Message-----
>
> 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.
>
>
>
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